The "What is a moon?" definition thread

[Ara Pacis]

OK, here it is. What's a moon? Is there only one moon, The Moon. Should the question be "What is a Natural Satellite (http://en.wikipedia.org/wiki/Natural_satellite)? Should it be defined based on size, composition, or origin, longevity, parent object?

How big does a moon need to be? Large enough to be detectable is a start, but is it a definition? If there is a size limit, where should it be set?

Is composition important? Can it be its own life bearing world and still be a moon or should it be limited to airless worlds? Should it be similar or dissimilar to the parent planet.

Is the genesis, or origin, of the natural satellite important? Does co-formation make it a moon or a binary planet? Should a moon come from the planet originally or can it be captured? Is capture the only way to get a moon and any other genesis is something else entirely?

Should it be based on longevity? The earth's biggest natural satellite, sometimes called Luna, has been orbiting the earth for a period less than the earth's existence. On the other hand, scientists have detected several small asteroids that orbit the earth in corkscrew orbits for years at a time - Are those objects natural satellites? Are they moons? Are they both?

Should it be based on what it is orbiting? A planet is a natural satellite of a star or stellar remnant. What about natural satellites orbiting Almost Stars, popularly known as brown dwarfs? Some objects orbit the star as primary with periods or influence by a larger mass. Is it a moon if it is co-orbital or does it need to be beholden to the larger mass? Does a moon fall in towards the parent planet or does it move away?

I'm not expressing any preference, yet. What's your definition?

[Tinaa]

Moved to questions and answers

[Jeff Root]

Anything that is orbiting a planet.

-- Jeff, in Minneapolis

[Saluki]

Anything that is orbiting a planet.

-- Jeff, in Minneapolis

Including discarded rocket stages? What about dust specs? Are the rings of Saturn really millions of moons?

[Jeff Root]

Anything that is orbiting a planet.
Including discarded rocket stages?
Sure.



What about dust specs?
I guess so. I can't think of any reason not to.



Are the rings of Saturn really millions of moons?
What do you mean by "really"?

Assuming the word "millions" was arbitrarily chosen to
represent the uncountably vast number of objects comprising
Saturn's rings, if you had asked, "Are the rings of Saturn
composed of an uncountably vast number of moons?" I would
answer, "Yes".

-- Jeff, in Minneapolis

[Saluki]

How about human waste discharged from the shuttle, ISS and other manned vessels?

[tdvance]

I'm not sure what the official definition of a moon is, but I would propose: a body held together by its own gravity in orbit around a non-stellar body.

Under that definition, the earth has three moons--the one we all know, and two "temporary" moons....captured asteroids that are not in long-term stable orbits around the Earth.

See http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm?list185042

Todd

[Saluki]

I would argue that the definition should follow these rules:

1. Man-made objects are excluded. If we put it there, it is an artificial satellite, not a Moon.

2. Only objects with orbits that are stable for a reasonably long term (actual length of time is debatable, but many times longer than a Human life) are included.

3. Above a certain size. The size cut-off is debatable, but I would certainly not include anything so small that it is not held together by its own gravity. When I think of a moon, I think of something large enough that a small group of people could land on it in a space ship, and not worry about significantly altering the moon's orbit.

[korjik]

A natural object held together by its own gravity that orbits a planet.

[Saluki]

I just thought of #4.

4. Objects orbiting in similar orbits with many other similar objects are excluded from the definition of the moon. If Ceres is not a planet, then a big ring particle is not a moon.

[Ara Pacis]

I would argue that the definition should follow these rules:

1. Man-made objects are excluded. If we put it there, it is an artificial satellite, not a Moon.

2. Only objects with orbits that are stable for a reasonably long term (actual length of time is debatable, but many times longer than a Human life) are included.

3. Above a certain size. The size cut-off is debatable, but I would certainly not include anything so small that it is not held together by its own gravity. When I think of a moon, I think of something large enough that a small group of people could land on it in a space ship, and not worry about significantly altering the moon's orbit.

very anthropocentric

[tdvance]

I think i'd better add the requirement that both the moon and planet be substellar. Otherwise, one could consider the Magellanic clouds to be moons.

[Saluki]

very anthropocentric

What is your point? All of scientific jargon is arbitrarily selected for the convienience of the organism using the word. If it were a dog defining the term, it would probably relate to the suitability of the object for leg heisting.

[tony873004]

I'm not sure what the official definition of a moon is, but I would propose: a body held together by its own gravity in orbit around a non-stellar body.

Under that definition, the earth has three moons--the one we all know, and two "temporary" moons....captured asteroids that are not in long-term stable orbits around the Earth.

See http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm?list185042

Todd
Those temporary "moons" shouldn't be counted at all. They "orbit" Earth in the sense that they travel around Earth in a rotating frame of reference, but they are not gravitationally bound to Earth. If the Sun suddenly disappeared, Earth would retain the Moon as it drifted through interstellar space. The temporary "moons" would immediately depart on their own seperate trajectories throug interstellar space.

[Jeff Root]

I'm not sure what the official definition of a moon is
There is no "official" definition.



I would propose: a body held together by its own gravity in
orbit around a non-stellar body.

Under that definition, the earth has three moons--the one we
all know, and two "temporary" moons....captured asteroids that
are not in long-term stable orbits around the Earth.

I agree with Tony. The other two bodies you refer to are not
"captured" and are not orbiting Earth. Thus, not moons.

In another thread (I forget which) in the last couple of days,
someone said that the Moon does not orbit the Earth. (Meaning
that it orbits the Sun but not the Earth.) That, too, is wrong.
The Moon unambiguously orbits the Earth.

-- Jeff, in Minneapolis

[Jeff Root]

I would argue that the definition should follow these rules:

1. Man-made objects are excluded. If we put it there, it is an
artificial satellite, not a Moon.
What was the first use of the term "satellite" in astronomy?
The use which established the astronomical meaning of the term,
and which it retains to this day?

"Satellite" is the term Galileo applied to the four objects
he saw orbiting Jupiter: The moons Io, Europa, Ganymede, and
Callisto. Moons. Satellite = Moon.

Luna is the only known natural satellite of Earth. Thousands
of small, artificial moons are currently orbiting the Earth.



2. Only objects with orbits that are stable for a reasonably
long term (actual length of time is debatable, but many times
longer than a Human life) are included.
Why?



3. Above a certain size. The size cut-off is debatable, but I
would certainly not include anything so small that it is not
held together by its own gravity.
Why?



When I think of a moon, I think of something large enough that
a small group of people could land on it in a space ship, and
not worry about significantly altering the moon's orbit.
When I think of a moon, I think of something orbiting a planet.

-- Jeff, in Minneapolis

[Saluki]

What was the first use of the term "satellite" in astronomy?
The use which established the astronomical meaning of the term,
and which it retains to this day?

"Satellite" is the term Galileo applied to the four objects
he saw orbiting Jupiter: The moons Io, Europa, Ganymede, and
Callisto. Moons. Satellite = Moon.

Absolutely correct, but moon ≠ artificial satellite. At least not in any scientific usage I have ever seen.


Luna is the only known natural satellite of Earth. Thousands
of small, artificial moons are currently orbiting the Earth.

I suppose I can allow that with the word "artificial" inserted. Again, moon ≠ artificial satellite, but perhaps artificial moon = artificial satellite.



Why?

Why not? Word definitions are based on usage. An object that temporarily orbits, and then flys off somewhere else is not typically called a moon.



Why?

Again, why not? We do not commonly call random space debris moons. Look at rings again as an example. The small moons responsible for the gaps are called moons ("sheppard moons", in fact), but the actual particles that make up the rings are not.


When I think of a moon, I think of something orbiting a planet.

-- Jeff, in Minneapolis[/quote]

Fair enough.

[Kaptain K]

The Moon unambiguously orbits the Earth.
Only from a geocentric point of view. It would be more accurate to say that the Earth and the Moon share an orbit around the Sun. The Moon's path is always concave to the Sun, as is the Earth's. This is not true of any other moon in the solar system!

[Jeff Root]

2. Only objects with orbits that are stable for a reasonably
long term (actual length of time is debatable, but many times
longer than a Human life) are included.
Why?
Why not?
But why?



Word definitions are based on usage. An object that temporarily
orbits, and then flys off somewhere else is not typically called
a moon.
No? What is it typically called?

What precedent of usage are you trying to adhere to?





3. Above a certain size. The size cut-off is debatable, but I
would certainly not include anything so small that it is not
held together by its own gravity.
Why?
Again, why not?
And again, why?



We do not commonly call random space debris moons.
Why not?

I don't know what "random space debris" is. What distinguishes
it from non-random space debris? What distinguishes it from
random space non-debris?

Do we commonly call random space debris (whatever it is) anything?
If so, what do we call it?



Look at rings again as an example. The small moons responsible
for the gaps are called moons ("sheppard moons", in fact), but
the actual particles that make up the rings are not.
I called them moons 14 posts back in this very thread.
Obviously they are called "moons" at least occasionally.

The correct spelling is "shepherd".

-- Jeff, in Minneapolis

[Halcyon Dayz]

Only from a geocentric point of view. It would be more accurate to say that the Earth and the Moon share an orbit around the Sun. The Moon's path is always concave to the Sun, as is the Earth's. This is not true of any other moon in the solar system!
Maybe that is because the Moon is the moon closest to the Sun.

Satellite (http://en.wikipedia.org/wiki/Satellite): "A satellite is any object that orbits another object" and "The general criterion for an object to be a satellite is that the center of mass of the two objects is inside the primary object."

The Moon is Earth's natural satellite (http://en.wikipedia.org/wiki/Natural_satellite).
According to Wiki: "A natural satellite is a non-man-made object that orbits a planet. It is commonly referred to as a moon (not capitalized)."
It also includes those orbiting minor planets.

But, if only for purely practical reasons, we can not include every dust and ice particle that happens to orbit a planet. Those would be satellites but not moons.

For me a moon is something I could walk on.

[Saluki]

I called them moons 14 posts back in this very thread.
Obviously they are called "moons" at least occasionally.



Ahh, we are now at the root of the matter. They are moons because Jeff calls them moons. I see now. Thank you for clarifying.

[jseefcoot]

If you take what people are repeating the most, the definition of a moon could be: A naturally occuring celestial body, large enough to be held together by its own gravity, that is gravitationally bound to another in a stable orbit.

But I wonder if a distinction should be made between our Moon (or Luna) and other satellites. . . .similar to the debate over whether to call Pluto a planet or not. How important is such a specific definition when our knowledge base of space and astronomy outgrows itself constantly? Often, as soon as we define something, we learn something new that challenges that definition. To me it's pure semantics, and in a country where a house can burn up while it burns down, and where maters, tuhmatuhs and tomatoes are all the same thing, it doesn't make much difference how we define it, as long as we all know what we're talking about.

[Jeff Root]

The Moon unambiguously orbits the Earth.
Only from a geocentric point of view.
The "point of view" is irrelevant.



It would be more accurate to say that the Earth and the Moon
share an orbit around the Sun.
It is accurate to say that, but not more accurate.

The Earth orbits the Sun. The Moon orbits the Sun. The Moon
orbits the Earth. The Earth-Moon system orbits the Sun.

With somewhet less clarity, we could also say:

The Sun orbits the Earth-Moon system. The Sun orbits the Earth.
The Earth orbits the Moon. The Sun orbits the Moon.

Although those orbits are far smaller than the first four.



The Moon's path is always concave to the Sun, as is the Earth's.
This is not true of any other moon in the solar system!
That is an interesting bit of trivia, but it doesn't have any
impact on the fact that the Moon orbits the Earth. If the Sun
were to instantaneously blink out of existence, although the
variation over time of the Moon's orbit around the Earth would
be altered, the orbit itself would not. The sidereal period
would be the same, the apogee and perigee would be the same.

You read or heard somewhere that the Moon is in orbit around
the Sun, not the Earth. Whoever wrote or said that was wrong.

-- Jeff, in Minneapolis

[Ara Pacis]

Some people want the term "moon" (non-capitalized) to mean a natural satellite. Some further assign or assume mass or size limits for that term. Some people think that Moon (capitalized) is the proper name of Earth's neighbor. I'd be willing to call earth's moon Luna and leave the word moon as a generic noun if we can keep using geology/geography, perigee/apogee, and other geo-derived words generically. As we explore new worlds I don't want to have to memorize some strange new science discipline names like areology, selenography, and perijove.

Also, did y'all see the post in Astronomy about a new theory for moon formation? The moons of Jupiter and Saturn suggest a standard formation limit of .01% of the parent planet's mass. Triton is over this limit is thought to be captured instead. Luna is 1% of Earth's mass, roughly 100 times more massive than the formation limit calls for. Should this datum support the issue that Luna is not a moon but a binary planet with Earth?

[tony873004]

...The Earth orbits the Sun. The Moon orbits the Sun. The Moon orbits the Earth...
Interesting trivia: Although this is true for Earth's moon, this isn't true for a lot of the moons of the Solar System.

Jupiter's moon, Io spends about 3/4 of its orbit around Jupiter with a velocity relative to the Sun that exceeds solar escape velocity for that distance. For most parts of its orbit, if Jupiter suddenly disappeared, it would leave Io on a hyperbolic solar trajectory.

But when Io is inbetween the Sun and Jupiter, and for ~1/4 of its orbit centered around that point, Io's velocity relative to the Sun is below solar escape velocity. If Jupiter disappeared during this quarter of Io's orbit, Io would remain bound to the Sun.

One could argue that Jupiter is constantly ejecting and capturing Io from the Solar System.

Could one make the argument that sometimes Io is not part of our solar system as its solar trajectory is hyperbolic?

The same holds true for the other 3 Galilean moons, and a lot of the other moons of the outer planets, but as distance from the planet increases, they spend a larger percent of their orbit below solar escape velocity.

[Halcyon Dayz]

Some people think that Moon (capitalized) is the proper name of Earth's neighbor.
The proper name of Earth's neighbour is Maan (http://nl.wikipedia.org/wiki/Maan).
The rest of the world is just being stubborn.
Luna is just something the Romans came up with.


Luna is 1% of Earth's mass, roughly 100 times more massive than the formation limit calls for. Should this datum support the issue that Luna is not a moon but a binary planet with Earth?
Since it originated the word, the Moon (http://dictionary.reference.com/browse/moon) is by definition a moon.
The datum does support the notion that our Moon was formed differently then other moons though.

ADD: If it turns out that the Earth and Moon are binary planets, then the term moon
should be reserved for the smaller element of such a system.
Natural satellites should then be called just that.

[Van Rijn]

The proper name of Earth's neighbour is Maan (http://nl.wikipedia.org/wiki/Maan).
The rest of the world is just being stubborn.
Luna is just something the Romans came up with.


Sounds good to me, if it will stop this silly bickering.

Though I prefer Selene . . . :razz:


For a "moon," I'd go with: Natural satellite of a planet, with some minimum size, not necessarily held together significantly by gravity (at the lower end of the scale it really doesn't take much of an impact to knock bits off, and if there is rotation . . .), but structurally stable. We're starting to run into the same problems with "moons" or satellites as we are with KBOs - as the detection capabilities grow, the number of "moons" keep increasing. At some point you have to decide on the dividing line between orbital debris and natural satellites.

[Jeff Root]

But, if only for purely practical reasons, we can not include
every dust and ice particle that happens to orbit a planet.
What practical reasons? I'm not aware of any practical reason
not to call every dust and ice particle that happens to orbit
a planet "a moon".



Those would be satellites but not moons.
Why? How is a satellite different from a moon? To me they are
synonymous terms. They became synonymous when it was understood
that the satellites of Jupiter are analogous to Earth's Moon, and
the term "moons" was applied to them by English-speaking people.

Some 350 years later, the terms "moon" and "satellite" were both
applied to the objects put into orbit by the Soviet Union and the
United States. I don't recall anyone objecting to that use of
the terms at the time. (Okay-- I probably wouldn't have known
if there were any objections, because I was just a little kid.)

-- Jeff, in Minneapolis

[Jeff Root]

Some people want the term "moon" (non-capitalized) to mean a
natural satellite.
I would go along with that as the preferred meaning, but there
is significant historical precedent (perhaps beginning with
the 1869 story "The Brick Moon" by Edward Everett Hale) for
applying the term "moon" to artificial satellites, as well.
Sputnik, Vanguard, and Explorer were frequently described as
"artificial moons". But I can accept that that usage is no
longer current.



Some further assign or assume mass or size limits for that term.
Without explaining why.



Some people think that Moon (capitalized) is the proper name of
Earth's neighbor.
People whose first language is English, yes.



Also, did y'all see the post in Astronomy about a new theory for
moon formation? The moons of Jupiter and Saturn suggest a standard
formation limit of .01% of the parent planet's mass. Triton is
over this limit is thought to be captured instead. Luna is 1% of
Earth's mass, roughly 100 times more massive than the formation
limit calls for. Should this datum support the issue that Luna is
not a moon but a binary planet with Earth?
I'd call that a total non sequitur. I have no problem with
referring to the Moon as a planet, but since it orbits another
planet, it is a moon. That's completely unambiguous. You can
call the Earth-Moon system a "binary planet" if you like (I'm
not sure what it means, but it does seem descriptive), but use
of that term has no bearing on the fact that the Moon is a moon.

-- Jeff, in Minneapolis

[Jeff Root]

One could argue that Jupiter is constantly ejecting and capturing
Io from the Solar System.

Could one make the argument that sometimes Io is not part of our
solar system as its solar trajectory is hyperbolic?
But it isn't. Jupiter is part of the Solar System, and would
have to magically vanish in order for Io to be on a hyperbolic
escape trajectory from the Solar System. And since Jupiter
is what causes Io to have the high speed that it has, if
Jupiter disappeared, Io could not be on a hyperbolic escape
trajectory at all. Nomatter how you analyze it, Io is
gravitationally bound to the Solar System.

-- Jeff, in Minneapolis

[Jeff Root]

At some point you have to decide on the dividing line between
orbital debris and natural satellites.
Why?

-- Jeff, in Minneapolis

[Jeff Root]

I called them moons 14 posts back in this very thread.
Obviously they are called "moons" at least occasionally.
Ahh, we are now at the root of the matter. They are moons because
Jeff calls them moons. I see now. Thank you for clarifying.
You claimed that the particles that make up the rings of Saturn
are not called moons. I showed that to be wrong by citing an
instance of my calling them moons, earlier in the thread.

I made no claim that my calling them "moons" makes them moons.

-- Jeff, in Minneapolis

[Ara Pacis]

Jeff, You won't get much argument from me for the first parts, but I don't think my last bit was a non sequitur --I think it's relative to the OP's question "What is a moon?"

If the Moon is a different type of object than Ganymede, Triton, Phobos and others then the generic term moon may not apply to them. Then again, those three objects seem to typify two or three other types of natural satellite classes --in situ formation, captured ice body, captured asteroid. There seem to be two sides to the argument, generallity versus specificity. Generally, anything that orbits something that is not a star is a moon. Specifically, If we want the Moon to typify a class of objects called moons then those objects should bear a close resemblance to their namesake. They may also be a third side that wants to draw a line between, that excludes man-made or temporary satellites but include natural objects that are not necessarily similar in genesis, composition, mass ratio, etc.

[Van Rijn]

Why?

-- Jeff, in Minneapolis

Well, why don't we take it to the logical extreme. Let's take every molecule in free orbit and call it a moon. Just a tad impractical. This is the same issue we are getting into with KBOs and the definition of "planet." Should all the KBOs be classified as planets, only 2003 UB313 and Pluto, or should there be some other criteria? There are a lot of KBOs. At some point, it makes sense to make a distinction for classification purposes and practicality. Yes, any definition of "planet" will be somewhat arbitrary, but then that's a given - it's a human invention of terminology.

The same follows for moon. It is a bit pointless to classify every speck of dust, every chunk of ice in Saturn's rings as a "moon."

Anyway, I'm really curious why you seem to be getting so worked up over this, Jeff. You act like there can only be one answer to a fairly esotoric question, and don't seem to want to accept that other people may have other points of view.

[Jeff Root]

Well, why don't we take it to the logical extreme. Let's take every
molecule in free orbit and call it a moon. Just a tad impractical.
What would be impractical about it?



This is the same issue we are getting into with KBOs and the
definition of "planet."
Obviously.



Should all the KBOs be classified as planets, only 2003 UB313 and
Pluto, or should there be some other criteria? There are a lot of
KBOs. At some point, it makes sense to make a distinction for
classification purposes and practicality.
Can you explain how "practicality" comes into this?



Yes, any definition of "planet" will be somewhat arbitrary, but
then that's a given - it's a human invention of terminology.

The same follows for moon. It is a bit pointless to classify
every speck of dust, every chunk of ice in Saturn's rings as a
"moon."
Why?



Anyway, I'm really curious why you seem to be getting so worked
up over this, Jeff.
I don't think I'm getting worked up. It's just a topic I've
thought about before and am competent to give opinions on, so
I'm taking advantage of the opportunity to spout off.



You act like there can only be one answer to a fairly esotoric
question, and don't seem to want to accept that other people
may have other points of view.
So far I have mostly been asking other people to explain their
points of view, and they are not doing so.

I'm not certain I grok what you mean by "esoteric" in this
case, but I think the question is pretty simple, and I think
the answer can be equally simple. Nobody has yet shown any
reason why a simple definition of "moon" is inadequate.

-- Jeff, in Minneapolis

[hhEb09'1]

That is an interesting bit of trivia, but it doesn't have any
impact on the fact that the Moon orbits the Earth. If the Sun
were to instantaneously blink out of existence, although the
variation over time of the Moon's orbit around the Earth would
be altered, the orbit itself would not. The sidereal period
would be the same, the apogee and perigee would be the same.

You read or heard somewhere that the Moon is in orbit around
the Sun, not the Earth. Whoever wrote or said that was wrong.Issac Asimov. One of his main points was that the force of the Sun's gravity on the moon is twice the force of the Earth's gravity. But as Van Rijn says, it's not a matter of being right or wrong, it's just a different point of view.

[Jeff Root]

I don't think my last bit was a non sequitur --I think it's
relative to the OP's question "What is a moon?"
For reference, I was talking about this:



Also, did y'all see the post in Astronomy about a new theory for
moon formation? The moons of Jupiter and Saturn suggest a standard
formation limit of .01% of the parent planet's mass. Triton is
over this limit is thought to be captured instead. Luna is 1% of
Earth's mass, roughly 100 times more massive than the formation
limit calls for. Should this datum support the issue that Luna is
not a moon but a binary planet with Earth?



If the Moon is a different type of object than Ganymede, Triton,
Phobos and others then the generic term moon may not apply to
them.
Okay.



Then again, those three objects seem to typify two or three
other types of natural satellite classes --in situ formation,
captured ice body, captured asteroid.
I don't see any reason to distinguish capturing a body made
mostly of ice from capturing a body made mostly of not-ice.

Nor, for the purpose here, do I see any reason to distinguish a
body made mostly of ice from a body made mostly of not-ice.

Nor -- again for the purpose here -- to distinguish a body
formed in situ from a captured body.



If we want the Moon to typify a class of objects called moons
then those objects should bear a close resemblance to their
namesake.
I shoulda become a biologist. I have a penchant for classifying
things. Probably everyone who engages in this argument has a
penchant for classifying things.

The Ford Model A, VW Beetle, and Rolls Royce Phantom are all
called "cars". They are all called "automobiles". And they
are all called "passenger vehicles". How much do they resemble
one another? In what ways do they differ? Are the differences
important in classifying what they "are", or what we want to
call them?

-- Jeff, in Minneapolis

[Jeff Root]

You read or heard somewhere that the Moon is in orbit around
the Sun, not the Earth. Whoever wrote or said that was wrong.
Isaac Asimov. One of his main points was that the force of the Sun's
gravity on the moon is twice the force of the Earth's gravity.
It is more than three decades since I read that essay. I don't
question that he said the force of the Sun's gravity on the Moon
is twice the force of the Earth's gravity, but I do question that
he said the Moon is not in orbit around the Earth. I will not
believe it without seeing it in ink on paper. In which book was
the essay included?

The Moon is gravitationally bound to the Earth. That is the
essential fact which requires the Moon to either be orbiting
the Earth or sitting on it's surface.



But as Van Rijn says, it's not a matter of being right or wrong,
it's just a different point of view.
Do you think the value of pi should be adjusted slightly upward
next year? Seems like a good idea, considering the volitility
in volume-to-surface area ratio lately. Some people think the
value should be lowered! That makes no sense to me.

-- Jeff, in Minneapolis

[hhEb09'1]

Do you think the value of pi should be adjusted slightly upward
next year? Seems like a good idea, considering the volitility
in volume-to-surface area ratio lately. Some people think the
value should be lowered! That makes no sense to me.Me either! :)

[Kaptain K]

Jeff Root has defined "Moon" and that settles it. Everybody who disagrees with Jeff Root, even Isaac Asimov, is wrong. End of discussion.

[aurora]

Originally Posted by Van Rijn
Well, why don't we take it to the logical extreme. Let's take every
molecule in free orbit and call it a moon. Just a tad impractical.

What would be impractical about it?



Well, for starters, we would have to name and log every molecule. :sick:

[Jeff Root]

Jeff Root has defined "Moon" and that settles it.
It's odd that you would think that, when I don't.



Everybody who disagrees with Jeff Root, even Isaac Asimov,
is wrong. End of discussion.
How can it be the end of the discussion? Nobody has produced
any evidence that Isaac Asimov's and my views are in conflict.
I do not believe Asimov ever wrote that the Moon does not orbit
the Earth. I expect that he never wrote such a thing. Gimme
a reference to a book or essay title.

In addition to that, please explain in your own words how you
think it is possible that the Moon does not orbit the Earth.
The Moon is gravitationally bound to the Earth. It cannot
escape Earth without the addition of kinetic energy, and it
cannot fall to Earth without the removal of kinetic energy.
So it is constrained to orbit the Earth until such energy is
added or removed.

I'm also waiting to hear from others what is "impractical"
about calling any object in orbit around a planet "a moon".
I'll give them time. I'm not in a hurry. But until they
explain what they meant by those assertions, the discussion
is not over.

-- Jeff, in Minneapolis

[aurora]

In addition to that, please explain in your own words how you
think it is possible that the Moon does not orbit the Earth.
The Moon is gravitationally bound to the Earth. It cannot
escape Earth without the addition of kinetic energy, and it
cannot fall to Earth without the removal of kinetic energy.
So it is constrained to orbit the Earth until such energy is
added or removed.


I consider that the Moon orbits the Earth. But it is also true that everything you stated about the Earth-Moon system also applies to the Earth-Sun. The Moon is graviationally bound to the Sun. The Moon cannot escape the sun without the addition of energy. The Moon cannot fall into the Sun without the removal of energy. So it is constrained to orbit the Sun until such energy is added or removed.

Also, you might be surprised to learn that the Earth's moon never moves backwards in relation to the orbit of the Sun. I've seen really cool animations of the Moon's path around the sun, a search of the web would likely turn one up.



I'm also waiting to hear from others what is "impractical"
about calling any object in orbit around a planet "a moon".
I'll give them time. I'm not in a hurry. But until they
explain what they meant by those assertions, the discussion
is not over.

-- Jeff, in Minneapolis

We already pointed one out. If your definition of "object" is anything from a molecule on up in size, then that is certainly impractical.

Did you see my previous post?

[hhEb09'1]

Also, you might be surprised to learn that the Earth's moon never moves backwards in relation to the orbit of the Sun. I've seen really cool animations of the Moon's path around the sun, a search of the web would likely turn one up.Kaptain K's point was, not only is that true, but the path of the moon around the sun does not even go concave away away from the sun.

[Halcyon Dayz]

If you called both Callisto, and a speck of dust orbiting Jupiter, moons, you
would still need qualifiers to tell them apart.
On one you could actually land, the other is not even a navigational hazard.

A moon is something of substance and significance.
(Just look at one.)
If the term moon only relates to orbital characteristics, it becomes useless.
We don't call meteoroids planets either.

[Van Rijn]

So far I have mostly been asking other people to explain their
points of view, and they are not doing so.


That's funny. I thought that was exactly what we were doing, but every time we do, you just respond with "Why? Why? Why?" It reminds me of discussions with a small child.



I'm not certain I grok what you mean by "esoteric" in this
case, but I think the question is pretty simple, and I think
the answer can be equally simple. Nobody has yet shown any
reason why a simple definition of "moon" is inadequate.

-- Jeff, in Minneapolis

Then you apparently haven't been reading the posts.

[Van Rijn]

We already pointed one out. If your definition of "object" is anything from a molecule on up in size, then that is certainly impractical.

Did you see my previous post?

And I pointed out the KBO versus planet issue. By the same logic that Jeff is using, we should classify every speck of dust orbiting the sun as a planet. For the purposes of classification, it makes sense to separate them. There is a group of KBOs. There is a group of asteroids. There is dust, etc.

[Saluki]

The Ford Model A, VW Beetle, and Rolls Royce Phantom are all called "cars". They are all called "automobiles". And they
are all called "passenger vehicles". How much do they resemble
one another?

These examples are akin to comparing Luna to Ganymede to Titan. Your definition would require a comparison between Titan, a weather satellite, and an orbiting dust particle. A better comparison would be an LEM, a Piper Cub, and a Schwinn bicycle. All transport humans, but the similarity ends there.

[Jeff Root]

I consider that the Moon orbits the Earth. But it is also true
that everything you stated about the Earth-Moon system also
applies to the Earth-Sun. The Moon is graviationally bound to
the Sun. The Moon cannot escape the sun without the addition
of energy. The Moon cannot fall into the Sun without the
removal of energy. So it is constrained to orbit the Sun until
such energy is added or removed.
I said that myself, yesterday in post #23:



The Earth orbits the Sun. The Moon orbits the Sun. The Moon
orbits the Earth. The Earth-Moon system orbits the Sun.



Also, you might be surprised to learn that the Earth's moon
never moves backwards in relation to the orbit of the Sun.
I've seen really cool animations of the Moon's path around
the sun, a search of the web would likely turn one up.
I've known that for at least three decades, possibly four.
Several times over the years I've drawn a large circle to
represent Earth's orbit, and then tried to accurately trace
the path of the Moon along it. It requires a really big sheet
of paper and a well-sharpened pencil.




I'm also waiting to hear from others what is "impractical"
about calling any object in orbit around a planet "a moon".
We already pointed one out. If your definition of "object"
is anything from a molecule on up in size, then that is
certainly impractical.

Did you see my previous post?
Ah -- yes. This:



Well, for starters, we would have to name and log every molecule.
I took that to be obviously facetious.

In the totally implausible case that it was not intended
facetiously -- Why would calling anything orbiting a planet a
"moon" require naming and logging every molecule?

-- Jeff, in Minneapolis

[Jeff Root]

So far I have mostly been asking other people to explain their
points of view, and they are not doing so.
That's funny. I thought that was exactly what we were doing,
but every time we do, you just respond with "Why? Why? Why?"
You made this assertion:


At some point you have to decide on the dividing line between
orbital debris and natural satellites.
But you did not say why one would have to do that. So I asked.
You replied:



Well, why don't we take it to the logical extreme. Let's take every
molecule in free orbit and call it a moon. Just a tad impractical.
But you didn't say why that would be impractical. So I asked.
You haven't replied yet.

You also asserted:


It is a bit pointless to classify every speck of dust, every
chunk of ice in Saturn's rings as a "moon."
But you didn't say why it is pointless. So I asked.
You haven't replied yet.

Am I supposed to guess what you had in mind?

I haven't the faintest idea why you think that calling any
object orbiting a planet a "moon" of that planet would be
impractical or pointless.

Please humor me and explain what you meant.

-- Jeff, in Minneapolis

[Van Rijn]

You also asserted:

It is a bit pointless to classify every speck of dust, every
chunk of ice in Saturn's rings as a "moon."
But you didn't say why it is pointless. So I asked.
You haven't replied yet.

Am I supposed to guess what you had in mind?


With all the comments and examples, why would you need to guess? If anybody but Jeff is having difficulty with this concept please let me know.



I haven't the faintest idea why you think that calling any
object orbiting a planet a "moon" of that planet would be
impractical or pointless.

Please humor me and explain what you meant.

-- Jeff, in Minneapolis

See here:

http://www.bautforum.com/showpost.php?p=763797&postcount=47

Following your logic, why don't we call everything in orbit about the sun a "planet"?

If you can't understand why people, as matter of practicality, classify things I really don't think I can help you.

[Van Rijn]

I took that to be obviously facetious.


Why?



In the totally implausible case that it was not intended
facetiously -- Why would calling anything orbiting a planet a
"moon" require naming and logging every molecule?

-- Jeff, in Minneapolis

What is the point of classifying them as moons if you have no intention of identifying them?

[hhEb09'1]

I do not believe Asimov ever wrote that the Moon does not orbit
the Earth. I expect that he never wrote such a thing. Gimme
a reference to a book or essay title.I don't really know for sure what he said either. A little googling around came up with a few websites that seem to attribute the convexity notion about the lunar orbit to the article Just Mooning Around from his collection Of Time, and Space, and Other Things (Avon, 1975). However, this webpage (http://slashdot.org/science/00/01/25/0826212.shtml) say they paraphrase something from his Asimov on Astronomy when they say "Interestingly enough, Luna is not a true satellite of ours. Our moon isn't really a moon! We're more like a double planet." and "Double planets are coorbital. They each have their own orbit around the sun."

Anybody have copies?

[Jeff Root]

if you call both Callisto and a speck of dust orbiting Jupiter
moons, you would still need qualifiers to tell them apart.
Obviously. Same as any other two things. So?

The qualifiers are "Callisto" and "a speck of dust".

If you call both Abraham Lincoln and Lucy Liu "people",
you would still need qualifiers to tell them apart.

The qualifiers are "Abraham Lincoln" and "Lucy Liu".

If you call both Betelgeuse and PSR J1748-2446ad "stars",
you would still need qualifiers to tell them apart.

The qualifiers are "Betelgeuse" and "PSR J1748-2446ad".

Pretty straightforward.



On one you could actually land, the other is not even a
navigational hazard.
Okay. How is that relevant?



A moon is something of substance and significance.
That statement is rather vague.



(Just look at one.)
I have only seen six moons. The others are apparently too
insubstantial or insignificant to be visible with any telescope
I have used. What am I to make of that fact?



If the term moon only relates to orbital characteristics, it
becomes useless.
It does? It is useless to say that something is in orbit
around a planet?

A moon of Neptune was discovered a month after the planet was
discovered. Cassini is currently an artificial moon of Saturn.
MESSENGER will become the first known moon of Mercury in 2011.

Those seem like useful statements to me.



We don't call meteoroids planets either.
Yes we do. More often, though, we call them "meteoroids".
They are among the things studied by the Minor Planet Center.

-- Jeff, in Minneapolis

[Van Rijn]

Okay. How is that relevant?


It is criteria for his definition of "moon" and it seems quite reasonable to me. You've already stated that there is no official definition of "moon" and you have denied being hung up on your definition. So why do you complain every single time someone gives you different criteria?




A moon of Neptune was discovered a month after the planet was
discovered. Cassini is currently an artificial moon of Saturn.
MESSENGER will become the first known moon of Mercury in 2011.

Those seem like useful statements to me.


Why is calling them "moons" a useful statement as opposed to "artificial satellites" or "orbiting spacecraft"?



Yes we do. More often, though, we call them "meteoroids".
They are among the things studied by the Minor Planet Center.

-- Jeff, in Minneapolis

Oh, please. As with "moon" there is no official definition of "planet" - yet. However, the proposals for an official definition certainly do not include meteoroids.

Now, we've answered your questions. Will Jeff Root start answering his questions, for instance, here (http://www.bautforum.com/showpost.php?p=763850&postcount=52)?

[Jeff Root]

See here:

http://www.bautforum.com/showpost.ph...7&postcount=47
Okay, you posted that after my post you are replying to here.



And I pointed out the KBO versus planet issue.
Which I acknowledged. You pointed it out in post 34, and said:



At some point, it makes sense to make a distinction for
classification purposes and practicality.
I asked:



Can you explain how "practicality" comes into this?
Instead of answering, you complained about my asking "Why?"



By the same logic that Jeff is using, we should classify every
speck of dust orbiting the sun as a planet.
Please note that it is Saluki, aurora, you and most recently,
Halcyon Dayz who are suggesting that I'm talking about things
like dust specks and molecules. I merely assented by saying
"Sure." and "I guess so. I can't think of any reason not to."
when you asked if I would use the term "moon" for these absurd
examples.

My own words, in answer to the question, "What is a moon?" were:



Anything that is orbiting a planet.
That still appears to be an adequate description, and closely
corresponds to common usage of the last 50 years or longer.



For the purposes of classification, it makes sense to separate
them. There is a group of KBOs. There is a group of asteroids.
There is dust, etc.
That is something of an answer. But there is no need to alter
meanings of currently-used terms to distinguish between classes
of objects. If you mean "KBOs", say "KBOs". If you mean
"asteoroids", say "asteoroids". If you mean "dust", say "dust".
If you mean "a moon 12 metres in diameter", say "a moon 12 metres
in diameter". Simple, straightforward, easy for anyone to
understand without complex definitions or long explanations.

Here is a web page classifying all Solar System moons from
100 km diameter up to the diameter of Titan into three groups,
by diameter -- Large, medium, and small:

http://www.freemars.org/jeff/planets/moons1b.htm

No changes to existing terminology were needed.

-- Jeff, in Minneapolis

[Halcyon Dayz]

If you call both Callisto and a speck of dust orbiting Jupiter, moons,
you would still need qualifiers to tell them apart.
Obviously. Same as any other two things. So?

The qualifiers are "Callisto" and "a speck of dust".

If you call both Abraham Lincoln and Lucy Liu "people",
you would still need qualifiers to tell them apart.

The qualifiers are "Abraham Lincoln" and "Lucy Liu".
[... ]
Pretty straightforward.
Doesn't work if I want to talk about presidents or actors.
Having words like president and actor would be convenient.




On one you could actually land, the other is not even a navigational hazard.
Okay. How is that relevant?
It is relevant to me.
Somebody who uses words.
I have no use for words that do not say what I need them to say.




A moon is something of substance and significance.
That statement is rather vague.
Well, make a suggestion.




(Just look at one.)
I have only seen six moons. The others are apparently too insubstantial or insignificant to be visible with any telescope I have used. What am I to make of that fact?
Are you familiar with the inverse-square law (http://en.wikipedia.org/wiki/Inverse-square_law)?
[Father Ted]These (toy) cows are small. The ones in the field are far away.[/Father Ted]




If the term moon only relates to orbital characteristics, it becomes useless.
It does? It is useless to say that something is in orbit around a planet?

A moon of Neptune was discovered a month after the planet was discovered.
Cassini is currently an artificial moon of Saturn.
MESSENGER will become the first known moon of Mercury in 2011.

Those seem like useful statements to me.
We already have a word for that.

I prefer to use the word moon for something that has more than just one
characteristic in common with the object that originated the word, and
was the only moon humankind was aware of for 99.8% of its existence.




We don't call meteoroids planets either.
Yes we do. More often, though, we call them "meteoroids".
They are among the things studied by the Minor Planet Center.
'We' do?
Maybe you do. And it makes you quite exceptional.
I'm certain the Minor Planet Center doesn't use the term planet for every object it tracks.

[Jeff Root]

Am I supposed to guess what you had in mind?
With all the comments and examples, why would you need to guess?
Because most of your comments have not been informative.
You did not introduce any "examples" of anything with the word
"example", and I can't guess what you are referring to.

You commented in your first post in the thread (post #27):



We're starting to run into the same problems with "moons" or
satellites as we are with KBOs - as the detection capabilities
grow, the number of "moons" keep increasing. At some point you
have to decide on the dividing line between orbital debris and
natural satellites.
You didn't explain why an increasing number of moons is a
problem. You still haven't.

You didn't explain why you have to decide on a dividing line,
even after I asked you. And you still haven't.

What you did say in your second post (#34) was:



Well, why don't we take it to the logical extreme. Let's take every
molecule in free orbit and call it a moon. Just a tad impractical.
Aside from the fact that this "logical extreme" is silly, you
did not explain why calling "every molecule in free orbit" a
"moon" is impractical. I see nothing impractical about it.
I asked you to explain why it is impractical, but instead you
complained about my asking "Why" too much. You could just
answer the question. You still haven't.

Along the same line, you continued in post #34:



This is the same issue we are getting into with KBOs and the
definition of "planet." Should all the KBOs be classified as
planets, only 2003 UB313 and Pluto, or should there be some
other criteria? There are a lot of KBOs. At some point, it makes
sense to make a distinction for classification purposes and
practicality.
The statement "it makes sense to make a distinction for
classification purposes" is a substantive explanation, if
rather limited. Thank you for that. But you did not explain
how "practicality" is a factor, even after I asked you to
explain it, and you still haven't.

Continuing in post #34, you said:



It is a bit pointless to classify every speck of dust, every
chunk of ice in Saturn's rings as a "moon."
Again you failed to explain why it is pointless, even after I
asked you to explain it, and you still haven't.

In post #47, a reply to aurora, you said:



And I pointed out the KBO versus planet issue. By the same logic
that Jeff is using, we should classify every speck of dust orbiting
the sun as a planet. For the purposes of classification, it makes
sense to separate them. There is a group of KBOs. There is a group
of asteroids. There is dust, etc.
You repeated the desirability of having various classifications,
and elaborated a bit. That is fine, but it doesn't explain any
of your unexplained assertions quoted above which I asked you
to explain.

That is essentially everything you posted in the thread, up to
the post I'm now replying to.

-- Jeff, in Minneapolis

[Jeff Root]

I haven't the faintest idea why you think that calling any
object orbiting a planet a "moon" of that planet would be
impractical or pointless.

Please humor me and explain what you meant.
See here:

http://www.bautforum.com/showpost.ph...7&postcount=47
As I said in my previous post, that expressed the desirability
of having various classifications. It says nothing whatsoever
about why you think that calling any object orbiting a planet
a "moon" of that planet would be impractical or pointless.



Following your logic, why don't we call everything in orbit
about the sun a "planet"?
I will not object to that. It matches the ancient usage of
the term, and pretty closely matches modern usage.



If you can't understand why people, as matter of practicality,
classify things I really don't think I can help you.
You might notice that in an earlier post I said I have a
penchant for classifying things. I find it extremely useful.
I do it all the time. I think most everyone does.

You led me to believe that you were saying that there is
something impractical about calling any object orbiting a
planet "a moon":



Just a tad impractical.
When I asked you why, you did not correct me and tell me that
what you said was not what you meant. I asked you to explain
why you think that calling any object orbiting a planet a "moon"
of that planet would be impractical or pointless. You have not
answered that question. Instead you changed the subject to why
classification has value.

-- Jeff, in Minneapolis

[Van Rijn]

You didn't explain why an increasing number of moons is a
problem. You still haven't.

[snip]
You didn't explain why you have to decide on a dividing line, even after I asked you. And you still haven't.


To repeat: Practicality, classification. If you don't understand those terms, look them up. I provided the planet versus KBO example. I really don't care if you agree or not. The question was how we would define "moon." You gave your definition, I gave mine, and I told you my reasons. We're talking about opinions about an unofficial term. You are no more right than I or anyone else here.



Aside from the fact that this "logical extreme" is silly, you
did not explain why calling "every molecule in free orbit" a
"moon" is impractical.


Given your definition ("Anything that is orbiting a planet"), why is it silly? What criteria are you using to make an exception to your definition?



I see nothing impractical about it.
I asked you to explain why it is impractical, but instead you
complained about my asking "Why" too much. You could just answer the question. You still haven't.


You were given reasons why it is impractical and pointless, but you ignored them. And this is where I stop playing your game. Clearly, all answers will be ignored, classified as too vague, or questioned to the point of ridiculousness unless they are identical to yours.

So have your last word, because I'm sure you'll insist on it.

[Van Rijn]

I don't really know for sure what he said either. A little googling around came up with a few websites that seem to attribute the convexity notion about the lunar orbit to the article Just Mooning Around from his collection Of Time, and Space, and Other Things (Avon, 1975). However, this webpage (http://slashdot.org/science/00/01/25/0826212.shtml) say they paraphrase something from his Asimov on Astronomy when they say "Interestingly enough, Luna is not a true satellite of ours. Our moon isn't really a moon! We're more like a double planet." and "Double planets are coorbital. They each have their own orbit around the sun."

Anybody have copies?

I have the paperback version of "Of Time, Space and Other Things." First printing for the paperback version is 1975, but the articles go back to '59. Quite reasonably, in "Just Mooning Around" he uses the term "satellite" except when referring to the moon. From the chapter, page 97:

"WHAT THE BLAZES IS OUR OWN MOON DOING SO FAR OUT THERE?
It is too far out to be a true satellite of the Earth, if we go by my beautiful chain of reasoning- which is too beautiful for me to abandon."

and on page 98

"We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth."

so anyway, he does call the Earth-Moon system a double planet. Whether I agree with his logic is another matter.

[Jeff Root]

Well, why don't we take it to the logical extreme. Let's take every
molecule in free orbit and call it a moon. Just a tad impractical.
What would be impractical about it?
Well, for starters, we would have to name and log every molecule.
I took that to be obviously facetious.
Why?
Obviously, using the term "moon" to refer to "anything that is
orbiting a planet" does not set any requirement to name and log
each individual object. In particular, it does not set any
requirement to name and log each individual molecule.

Analogously, using the term "carbon" to refer to "any atom that
has six protons in its nucleus" does not set a requirement to
name and tag each individual carbon atom. Using the term
"beetle" to refer to a specified class of insects does not set
a requirement to name and tag each individual beetle. Using
the term "meteoroid" to refer to a specified class of objects
in Space does not set a requirement to name and tag each
individual meteoroid.

Obviously. I have to believe that aurora was being facetious.
It would obviously be absurd to think that using the term "moon"
to refer to "anything that is orbiting a planet" requires us to
find every particle in Space, give it its own name, and record
its location and characteristics. Obviously.




Why would calling anything orbiting a planet a
"moon" require naming and logging every molecule?
What is the point of classifying them as moons if you have no
intention of identifying them?
By "identifying them" do you mean naming and logging each
individual molecule, as aurora said and as I repeated in the
quote you were replying to? If so, the point of classifying
anything orbiting a planet as a "moon" is to have a name with
which to refer to members of the class of things orbiting
planets.

What is the point of classifying beetles as beetles if I have
no intention of identifying each individual beetle with its
own name and recording its location and characteristics in a
log? So that I have a word by which to refer to them.

Obviously.

-- Jeff, in Minneapolis

[JMV]

What is the point of classifying beetles as beetles if I have
no intention of identifying each individual beetle with its
own name and recording its location and characteristics in a
log? So that I have a word by which to refer to them.
Why would you want to classify them as beetles when you can already refer to them as insects?

[Ara Pacis]

Hey, I started this thread. No reason for anyone hog it all.

I think Jeff is a generalist in the extreme. If I understand correctly, he thinks moon, as term, should encompase anything that satisfies a single criterion: it orbits around a body that orbits a stellar object (star). I think he is using it as synomymous with satellite and does not want to limit the extent of that classification.

I think everyone else has something more specific in mind. The difference between Jeff and everyone else seems to be that he is using moon as a higher order of classification.
Jeff: Star-->Planet-->Moon
Others: Star-->Planet-->Satellite-->Natural Satellite-->moon

The issue is that the others may not realize that they are creating these hierarchical orders in their description and that seems to be the misunderstanding. (There might even be more orders of classification in there that are assumed but not mentioned like a mass distinction after Natural Satellite.) I think everyone else is using a non-hierarchical system where everything that comes after Planet are subdivisions of that level, instead of sub-classifications:
Star-->Planet-->moon/gas/meteor/dust/asteroids(if there are any, not counting those that only orbit temporarily in corkscrew orbits)/maybe others

I like my classifications to use a logical hierarchy. The system I just showed is like having a Colonel leading Privates instead of using ranked subgroups. The problem with having a one-term-catch-all hierarchy is that one minor change to the definition erases a whole class of objects that are now in scientific limbo. Jeff wants to be inclusive and everyone else wants to be exclusive.

I think there is a place for everything and everything should be in its place. I disagree with all y'all. I don't think moon is synonymous with satellite. People use the term moon as satellite as an analogy descriptor, but would describe an object like the Moon/Luna and not an object like the ISS, space dust, space gas, or small meteors if pressed. I do not think that most people would automatically exclude asteroids that orbit the earth permanently. Therefore, I don't think they would necessarily exclude, out of hand, asteroids that orbit the earth temporarily. From an Earth POV they orbit earth while they are here.

I am tempted to go towards the science side and not categorize them even as temporary moons, but I think that leads to a level of specificity that may lead to even more fractionalization of the term moon. Once we get to size and origin limits for large objects exclusion may become a matter of degree--anything short of eternal is temporary. If a large enough impacter were to strike Luna and it left orbit with the earth (immediately or eventually) then it would cease to be a moon. How is that different from the corkscrew asteroids?

Current science suggests that there are three types of natural satellite formation. Formation during or after the proto-planetary phase from dust and gas. Capture of an object after both the object (which may have been a planet on its own) and the planet have formed. Formation of an object after a cataclysmic collision of two objects (both of which may have been planets on their own). Are all three of these natural satellite scenarios equal? Should the results of each process be categorized as similar or distinct?

One could argue that the large range of object sizes and compositions means they should all be grouped together since they are too dissimilar to be subcategorized. But some might say that those differences (like in Io, Ganymede, Europa, and Callisto) might be a natural spectrum of chemistry types due to their genesis, not in spite of it. I haven't read the recently released theory about gas giant moon formation, but if it or a later theory reveals a natural and logical sequence of moon types from that genesis type, then it might be suitable to subcategorize them accordingly.

Capturing an object could lead to general inclusion of many objects as moons that have no other similarity. There might be no rhyme or reason to it and any inclusion or exclusion will probably be based on arbitrary limits like size, composition, location or duration. Phobos is in the process of being destroyed so referring to asteroid capture as a moon-forming process is incidental. Triton, despite its size, appears to have a similar doom ahead of it.

Impact theory seems to use both mechanisms of capture and protoplanetary formation. However, I think the physics is actually different than capture because the size and velocities of both objects suggests capture was not actually possible. Similarly, formation/accretion from protoplanetary debris may not be accurate since both objects were fully formed, if still in the near molten stage. Thus both objects (Earth and Theia) were planets at the time of their collision. Even if they were in the same orbital path (with Theia at a trojan point) that should not disqualify the Mars-sized Theia from planethood (unless we plan to demote Mars, Mercury, and Pluto) or call it some sort of companion or moon at that point (unless we want to start calling all trojans and all companions, like Cruithne, moons). Call it a companion planet or, as I like to call it, a twin planet. It's still a planet. According to the Giant Impact theory the Moon/Luna was created from the ejecta from both objects. It wasn't just a splash-and-mash of a small amount of ejecta, but it was a merge-and-purge (this reminds me of some forms of unicellular sexual reproduction). The resulting objects are combinations of both. Theia/Luna/Moon has part of earth in it and the Earth has part of Theia in it --a remnant of Earth is orbiting earth and a remnant of Theia is orbiting Theia. For me, that seems to come as close to a mathematical/algebraic expession of Double Planet as is physically possible.

Of course, this line of thinking leads to the incongruity that the Moon is not a moon. Or, that the definition of Moon is Double Planet Minor Object (including Charon in this group) and that Phobos and Ganymede represent not moons but yet to be defined classes. Several Double Planet definitions include the Earth-Luna system, some planet defintions include Luna, and the most recent "moon" formation theory excludes it. However, it seems obvious that most people will not go for this idea, despite the logic and evidence behind it. So, perhaps the definition of moon can remain as a large natural satellite with three subclasses: GIR (Giant Impact Remnant); FIP or FIS (Formed in Place or Formed In Situ), and; CPO (Captured Planetary Object or Captured Primary Object (since it was a former primary orbiter)).

[aurora]

I've known that for at least three decades, possibly four.
Several times over the years I've drawn a large circle to
represent Earth's orbit, and then tried to accurately trace
the path of the Moon along it. It requires a really big sheet
of paper and a well-sharpened pencil.


Then I am rather confused as to why you asked for an explanation of how it could be considered that the moon orbits the sun rather than the Earth.

Why did you ask for that definition? You, by your own statement, already knew the answer.

[Ara Pacis]

Did I kill the thread?

[neilzero]

Hi ara: It is unlikely you killed this thread. I agree we should leave definition sufficiently fuzzy, to avoid frequent redefining. There is no president (that I know of) for having separate definitions for satellite and moon. Neither term seems appropriate for pea size and smaller objects, weather natural or man made, but perhaps that is just me. Neil

[Ara Pacis]

I am wondering... what is the actual likelihood that gas and dust would orbit a planet anyways? The magnetic field, gravity well, and solar wind would make gas and dust fall in or fly away I would think.

[Jeff Root]

I'm picking up at the point where I left off Saturday morning.





On one you could actually land, the other is not even a
navigational hazard.
Okay. How is that relevant?
It is criteria for his definition of "moon"
I understood that. My question was: "How are the facts that on
one you could land, while the other is not even a navigational
hazard, relevant criteria for a definition of 'moon'"?



You've already stated that there is no official definition of
"moon" and you have denied being hung up on your definition.
So why do you complain every single time someone gives you
different criteria?
I will complain when criteria appear to be irrelevant, in
conflict with existing usage, or pose potential problems.

Mostly I have been asking for you to justify your criteria,
not complaining about them. What I have repeatedly complained
about is your failure to provide that justification, while
telling me that it has already been provided. It hasn't.




A moon of Neptune was discovered a month after the planet was
discovered. Cassini is currently an artificial moon of Saturn.
MESSENGER will become the first known moon of Mercury in 2011.

Those seem like useful statements to me.
Why is calling them "moons" a useful statement as opposed to
"artificial satellites" or "orbiting spacecraft"?
I prefer the terms you give. However, "moon" has been used
as an English-language synonym for "satellite" for hundreds
of years, and has been applied to artificial satellites at
least as far back as 1869. I wish to maintain continuity
with that usage.

My examples could equally well have used the term "satellite",
but this thread is about the term "moon", not "satellite", so
that is the term I used.





We don't call meteoroids planets either.
Yes we do. More often, though, we call them "meteoroids".
They are among the things studied by the Minor Planet Center.
Oh, please. As with "moon" there is no official definition of
"planet" - yet. However, the proposals for an official definition
certainly do not include meteoroids.
Which does not contradict what I said.

-- Jeff, in Minneapolis

[Jeff Root]

If you call both Callisto and a speck of dust orbiting Jupiter, moons,
you would still need qualifiers to tell them apart.
Obviously. Same as any other two things. So?

The qualifiers are "Callisto" and "a speck of dust".

If you call both Abraham Lincoln and Lucy Liu "people",
you would still need qualifiers to tell them apart.

The qualifiers are "Abraham Lincoln" and "Lucy Liu".
[... ]
Pretty straightforward.
Doesn't work if I want to talk about presidents or actors.
Whatever you meant by that, it is nonsense.



Having words like president and actor would be convenient.
We do have those words and use them where appropriate. If we
didn't have them, we could invent them.

We also have such words as "speck", "dust", "meteoroid", "iron",
"olivine", "chondrite", "carbonaceous", "pallasite", "large",
"small", "diameter", "metre", "mass", "kilogram", and so forth
which can be used as qualifiers. They work. New words can be
invented when needed.





On one you could actually land, the other is not even a
navigational hazard.
Okay. How is that relevant?
It is relevant to me.
I understood that. How is it relevant?



Somebody who uses words.
I have no use for words that do not say what I need them to say.
That shouldn't be a problem. Don't use those words!





A moon is something of substance and significance.
That statement is rather vague.
Well, make a suggestion.
I was hoping you would clarify what you meant by those words,
rather than leaving it to me to clarify what you meant by them.
The range of possible interpretations is enormous. If a moon
is something of substance, then perhaps it is made of atoms,
rather than fields, ideas, or emotions. If a moon is something
of significance, then maybe it has an entry in the Encyclopedia
Brittanica. Or it can be found on a map. Or it's big enough
to feel if you roll it between your fingers. I dunno.




I have only seen six moons. The others are apparently too
insubstantial or insignificant to be visible with any telescope
I have used. What am I to make of that fact?
Are you familiar with the inverse-square law?
Yes. It says that distant moons are insignificant, no
matter how substantial they may be.





If the term moon only relates to orbital characteristics, it
becomes useless.
It does? It is useless to say that something is in orbit around
a planet?

A moon of Neptune was discovered a month after the planet was
discovered. Cassini is currently an artificial moon of Saturn.
MESSENGER will become the first known moon of Mercury in 2011.

Those seem like useful statements to me.
We already have a word for that.

I prefer to use the word moon for something that has more than
just one characteristic in common with the object that originated
the word,
Could you explain why? It appears to be the defining
characteristic. Attempts to define something by multiple
characteristics generally cause problems when it turns out
that not all the defined things have all the characteristics.



and was the only moon humankind was aware of for 99.8% of its
existence.
So you prefer the English word "moon" revert to its meaning of
almost 400 years ago, and again refer only to Earth's Moon?

That's fine, but I asked why you said that if the term "moon"
only relates to orbital characteristics, it becomes useless.
I gave some examples of its use, in which only the orbital
characteristics are essential. You haven't explained how it
can be useless despite having been used that way for so long.





We don't call meteoroids planets either.
Yes we do. More often, though, we call them "meteoroids".
They are among the things studied by the Minor Planet Center.
'We' do?
Maybe you do. And it makes you quite exceptional.
I'm not going to make any assertion about whether or not I have
ever referred to meteoroids as "planets", because I don't know
off hand whether I have or not.

Brian Marsden, director of the Minor Planet Center, has a
rather surprising personal definition of the term "meteoroid".
He told me in August 2001:



The International Astronomical Union has (so far) carefully
refrained from making distinctions on the basis of size--and
rightly so.

When the term "meteoroid" was first adopted, 40 years ago, it
seemed to me that the intent was that it should refer to a natural
object while it was in the earth's atmosphere--the object that
produces the meteor light phenomenon and that might actually land
and become a meteorite. Since it changes name between atmosphere
and ground, there is no reason why it should not also change name
between space and atmosphere. Outside the atmosphere the body
is presumably either an asteroid or a comet--or a fragment of an
asteroid or a comet.
Don't be too sure what is exceptional and what is not.



I'm certain the Minor Planet Center doesn't use the term planet
for every object it tracks.
I'll just quote that as an interesting statement.

-- Jeff, in Minneapolis

[Jeff Root]

You didn't explain why an increasing number of moons is a
problem. You still haven't.
[snip]
You didn't explain why you have to decide on a dividing line,
even after I asked you. And you still haven't.
To repeat: Practicality, classification. If you don't
understand those terms, look them up.
I understand the meanings of the terms "practical" and
"classification".

You still have not explained why an increasing number of moons
is a problem. You have instead evaded explaining it several
times.

You still have not explained why you have to decide on a
dividing line. You have instead evaded explaining it several
times.



I provided the planet versus KBO example. I really don't care if
you agree or not.
There was no assertion in that "example" with which to agree or
disagree.



The question was how we would define "moon." You gave your
definition, I gave mine, and I told you my reasons.
This is where we disagree. You have not given your reasons.
You think that you have expressed them, but you have not.



We're talking about opinions about an unofficial term. You are
no more right than I or anyone else here.
Some of what we are saying is opinion, some is factual.

For example, you said:



Let's take every molecule in free orbit and call it a moon.
Just a tad impractical.
Ignoring the question of why anyone would want to do such a
thing, you have not explained what about it is impractical.

The word "impractical" means: "Possible in theory but so
difficult or problematic to put into practice as to not be
worth doing."

We may agree or disagree that what you are referring to is
impractical, but you still have not explained what it is that
you are referring to.

Just write a sentence or a paragraph or however much it takes
to explain what it is you think is impractical, and why you
think it is impractical.




Aside from the fact that this "logical extreme" is silly, you
did not explain why calling "every molecule in free orbit" a
"moon" is impractical.
Given your definition ("Anything that is orbiting a planet"), why
is it silly?
1) Individual molecules cannot be detected except when they are
inside some kind of microscope.
2) As Ara Pacis points out in a later post, individual molecules
cannot orbit planets because they are blown away by stellar winds
or trapped in planetary magnetic or electrostatic fields.



What criteria are you using to make an exception to
your definition?
I'm not making an exception.




I see nothing impractical about it.
I asked you to explain why it is impractical, but instead you
complained about my asking "Why" too much. You could just answer
the question. You still haven't.
You were given reasons why it is impractical and pointless, but you
ignored them.
The only "reason" I was given that I at first ignored was just
a remark intended for its humor: That it would be necessary to
name and log every molecule. You, yourself, have not stated
any reason.



And this is where I stop playing your game. Clearly, all answers
will be ignored, classified as too vague, or questioned to the
point of ridiculousness unless they are identical to yours.

So have your last word, because I'm sure you'll insist on it.
Nothing is stopping you from responding to me. I am and have
been primarily asking questions, rather than making assertions.
You think my questions have been answered, but they have not.

-- Jeff, in Minneapolis

[Jeff Root]

I have the paperback version of "Of Time, Space and Other Things."
First printing for the paperback version is 1975, but the articles
go back to '59. Quite reasonably, in "Just Mooning Around" he uses
the term "satellite" except when referring to the moon. From the
chapter, page 97:

"WHAT THE BLAZES IS OUR OWN MOON DOING SO FAR OUT THERE?
It is too far out to be a true satellite of the Earth, if we go
by my beautiful chain of reasoning- which is too beautiful for
me to abandon."

and on page 98

"We might look upon the Moon, then, as neither a true satellite
of the Earth nor a captured one, but as a planet in its own right,
moving about the Sun in careful step with the Earth."

so anyway, he does call the Earth-Moon system a double planet.
Whether I agree with his logic is another matter.
Thanks. That may be the first Asimov essay collection I read,
somewhere around sixth grade. I haven't seen it since.

The notion of "if we go by my beautiful chain of reasoning- which
is too beautiful for me to abandon." is one he used elesewhere.
Generally it describes sound logic based on flimsy assumptions --
flimsy because it is not clear what the correct assumptions are,
so he has used assumptions that he likes and which are convenient
for the purpose at hand.

The Moon is gravitationally bound to the Earth, which tells us
that the Moon orbits the Earth, and is thus a moon of Earth.

That the Earth-Moon system may be considered a double planet
is interesting, but not especially relevant to the question of
whether the Moon is a moon.

-- Jeff, in Minneapolis

[Jeff Root]

What is the point of classifying beetles as beetles if I have
no intention of identifying each individual beetle with its
own name and recording its location and characteristics in a
log? So that I have a word by which to refer to them.
Why would you want to classify them as beetles when you can
already refer to them as insects?
Excellent rhetorical question!

I would want to classify beetles as beetles because if I
refer to them as insects, the common meaning of the word
"insects" would be lost, confusing everyone.

Similarly, I am loathe to constrict the meaning of the term
"moon" to fit a mess of arbitrary requirements, and lose the
meaning it has had for nearly 400 years.

-- Jeff, in Minneapolis

[Jeff Root]

I think Jeff is a generalist in the extreme. If I understand
correctly, he thinks moon, as term, should encompase anything
that satisfies a single criterion: it orbits around a body that
orbits a stellar object (star). I think he is using it as
synomymous with satellite and does not want to limit the
extent of that classification.
Yes.



I think everyone else has something more specific in mind. The
difference between Jeff and everyone else seems to be that he is
using moon as a higher order of classification.
Jeff: Star-->Planet-->Moon
Others: Star-->Planet-->Satellite-->Natural Satellite-->moon
Right, as far as my view is concerned.

-- Jeff, in Minneapolis

[Jeff Root]

Several times over the years I've drawn a large circle to
represent Earth's orbit, and then tried to accurately trace
the path of the Moon along it. It requires a really big sheet
of paper and a well-sharpened pencil.
Then I am rather confused as to why you asked for an explanation
of how it could be considered that the moon orbits the sun rather
than the Earth.
The facts that the Moon is gravitationally bound to the Sun
and that its orbit is everywhere concave to the Sun have no
bearing on the fact that it is gravitationally bound to Earth.
Earth is the Moon's "primary". The Moon orbits Earth. That is
not "kinda-sorta orbits". It is definite and unambiguous.

A moon can serve two masters.



Why did you ask for that definition? You, by your own statement,
already knew the answer.
I asked, as you said in your first sentence, how it could be
considered that the Moon does not orbit the Earth. Apparently
the answer is that you think the fact that the Moon's orbit is
everywhere concave to the Sun somehow implies that it is not
really orbiting Earth.

-- Jeff, in Minneapolis

[Jeff Root]

what is the actual likelihood that gas and dust would orbit a
planet anyways?
For all gas and very small dust particles, some planets at a
wide range of distances from any star -- zero.



The magnetic field, gravity well, and solar wind would make
gas and dust fall in or fly away I would think.
Yes. Solar wind blows away individual molecules and the
smallest dust particles. Ionized particles are trapped in
magnetic fields or electric fields.

Larger particles (roughly between 0.0001 mm and 10 mm) are
affected by the Poynting-Robertson effect, caused by light
pressure from a star. The particle is slowed in its orbit
until it hits a planet or gets close enough to the star to
be vaporized and blown away. These particles can orbit,
but not for long.

-- Jeff, in Minneapolis

[hhEb09'1]

The facts that the Moon is gravitationally bound to the Sun
and that its orbit is everywhere concave to the Sun have no
bearing on the fact that it is gravitationally bound to Earth.
Earth is the Moon's "primary". The Moon orbits Earth. That is
not "kinda-sorta orbits". It is definite and unambiguous.I think another way of paraphrasing that Asimov argument is that the Earth is not the Moon's primary. When you say it is primary, what is your definition of primary?

[Jeff Root]

I think another way of paraphrasing that Asimov argument is that
the Earth is not the Moon's primary. When you say it is primary,
what is your definition of primary?
Pretty much as described in the quotes below.

I say that in the Earth-Moon system, Earth is the primary body
and the Moon is a secondary body. In the Sun-Earth-Moon system,
the Sun is the primary and either Earth or the whole Earth-Moon
system may be considered a secondary.

Analogous to "Solar System", I might call the Earth-Moon system
the "Terrestrial System", because Earth is the primary body of
the system.

The Facts on File Dictionary of Astronomy (4th edition, 2000)
entry for "primary body" says that it is "The celestial body
nearest to the center of mass of the system. The primary is
the most massive body in the system. All members of a system
orbit a common center of mass. Earth is the Moon's primary."

This Glossary of Astronomical Terms has a similar description:

http://www.wro.org/ras/glossary/n-r.htm



PRIMARY BODY: The object which is the gravitational center of
the system. The sun is the primary body for the Solar System and
the earth is the primary body for almost all satellite operations.
This snippet is from a student-built website called 'The Physics
Factbook', a collection of miscellaneous related quantities and
info gathered from other reference sources. The page is titled
'Diameter of the Earth's Orbit':

http://hypertextbook.com/facts/2004/CherisseBarnes.shtml



The path of the Earth around the sun is called the Earth's orbit.
The orbit lies on an imaginary flat surface that cuts through the
sun. This surface is the earth's orbital plane. Astronomers call
an orbit the path of any object whose motion is controlled by the
gravitational pull of another object. They call the more massive
of the two objects the primary and the less massive one the
secondary. The moon is a secondary that revolves around the
earth, its primary.
-- Jeff, in Minneapolis

[Ara Pacis]

Going by those definitions, is there any possibility of a situation called Double Planet or will one object always be the primary mass and the other a natural satellite or moon? If so, what do you consider the demarcation?

[hhEb09'1]

Yes, the problem with all those definitions, is the Earth is considered the primary only if restricted to the two body Earth/Moon system. Even the quotes say that the Sun is the primary in the Sun/Earth/Moon system. I don't see a contradiction to the Asimov argument.

[Ara Pacis]

Did the moon's orbit go back on itself when it first reformed after the giant impact? If so then the idea of convexity with regards to the sun may only be a recent development or a temporary situation. Would this mean that moon-planet system can become a double planet system and vice-versa?

[hhEb09'1]

Here's an interesting little factoid that may have been mentioned before, but which I just checked on the back of an envelope (so, you all may want to double check :) ): If Jupiter winked away, it's possible that its four Galilean moons would have solar escape velocity. That's not true of the Earth/Moon system. If the Earth winked away, the moon would stay.

[tony873004]

Here's an interesting little factoid that may have been mentioned before, but which I just checked on the back of an envelope (so, you all may want to double check :) ): If Jupiter winked away, it's possible that its four Galilean moons would have solar escape velocity. That's not true of the Earth/Moon system. If the Earth winked away, the moon would stay.

Post #25 in this thread. Beat u to it :)

[dirty_g]

This whole thing is rather easy. The smaller object orbiting the bigger object is it's moon! The moon would most probably be considered a small planet by anybody on Venus (if it was habitable) if for example it was there and the earth was not. Obviously you get all techinal on Kuiper belt objects but it seems quite obvious to me that it's very easy to define a moon.

[Jeff Root]

Going by those definitions, is there any possibility of a
situation called Double Planet or will one object always be the
primary mass and the other a natural satellite or moon? If so,
what do you consider the demarcation?
What gives you the idea that there is a demarcation?

"Double planet" is an obvious parallel to double star, of which
scads are known. Double planets may also be common. Earth-Moon
and Pluto-Charon are the best candidates for this description
in our Solar System, and I don't think I'll object to describing
either of those systems as a "double planet".

I'd say that a "double planet" is a pair of planets of similar
mass which orbit one another. I would not attempt to define
"similar". This definition allows you to hyperjump into an
unknown planetary system, spot a pair of bodies close to each
other, and immediately refer to them as a double planet without
having to observe them for a month to determine their orbital
period and masses first. :)

-- Jeff, in Minneapolis

[Ara Pacis]

Some people want a demarcation because they can not countenance the idea that a moon can also be a planet. I don't have a problem with Luna being a planet as well as a moon, but I went round and round with someone about it in a couple threads a few months ago.

[hhEb09'1]

Post #25 in this thread. Beat u to it ToSeeked! (http://www.bautforum.com/showthread.php?p=763127#post763127) :)

[aurora]

A moon can serve two masters.


Yes, clearly ours does. And every statement you have made about the Moon-Earth system also applies to the Moon-Sun system.

Apparently you knew the answer but either thought no one else did, or else just asked the question for kicks.

[Jeff Root]

Apparently you knew the answer but either thought no one else
did, or else just asked the question for kicks.
No, I had no idea why anyone would assert that the Moon does
not orbit the Earth. It has been too long since I read the
essay by Isaac Asimov. I didn't remember that he went so far
as to suggest that the Moon is not a true satellite of Earth.
I probably didn't take the idea seriously at the time, any
more than I do now.

-- Jeff, in Minneapolis

[Ara Pacis]

I think another reason that people say the moon belongs to the sun is the plane of the moon's orbit does not move along the plane of the earth's rotation. I don't know if this is true, but I remember reading it once.

[hhEb09'1]

I think another reason that people say the moon belongs to the sun is the plane of the moon's orbit does not move along the plane of the earth's rotation. I don't know if this is true, but I remember reading it once.The plane of the moon's orbit is five degrees off the plane of the sun's path--but that's actually the earth's orbit. The earth is tilted 23 degrees with respect to the sun's path.

[Ara Pacis]

Lemme see if I follow. The moon's orbit is about 5 degrees from the plane of the ecliptic (suns path/earth's orbit) but the earth's axis of rotation is inclined 23 degrees from the ecliptic. The argument is: If the moon belonged to the earth the transfer of momentum should have caused the moon to orbit nearer the plane of the earth's equator. Since it does not then the moon is independent from the earth and more reliant on the sun making the earth-moon system a double planet system. Right?

[hhEb09'1]

Lemme see if I follow. I don't know anybody who makes that argument, so I'm not sure. I thought you were the one presenting the argument :)

[Ara Pacis]

I remember reading it on BABB a long time ago, but can't remember the particulars.