Why is it that mercury and venus have no moon? I find that strange. Denis.
Established Member
Why is it that mercury and venus have no moon? I find that strange. Denis.
Member
Is it venus and mercury are the oddballs? I think the more pertinent question is why does Earth have such a large satellite? I personally think only the gas giants are "supposed" to have moons. A freak collision early in the solar system's formative days probably gave Earth it's moon; mars has two tiny captured asteroids for moons.
Order of Kilopi
Correct, infact it is thought that an object about the size of Mars collided with earth to give it it's moon.Originally Posted by desertmonk
I actually think of planets with moons to be oddballs, and the ones without them to not be, but it is all in how you look at it...
Established Member
Mercury and Venus are very close to the Sun and have very small Hill Spheres. Compared to the other planets, there is not as much space around them where an object can exist in a stable orbit for extended periods of time. Earth's Hill Sphere is a little larger, but not large enough to support multiple moons.
Order of Kilopi
The rotation of Venus and Mercury is also very slow, so that (IIRC) the synchronous orbital radius is larger than the Hill radius. So any satellites would have found themselves in tidally decaying orbits (like Phobos), and would have ended up crashing into the parent planet.
Grant Hutchison
Established Member
what Grant hutchison said is correct. Quite franky there are couple ways to get moons make them..... early molten blob days, get hit, protuding lump that gets detached from high rotation speeds. Or capture them.
I would like to add that there are several documented astroids with there own satalites, so based on some definition there own moon. many of the astroids are not spheres or eliptical but quite irregular. I don't know if they have decaying orbits if anyone knows the answer let me know.
So it appears to be luck why you have moons but wether you can keep them is propably another matter
Order of Kilopi
Even More Importantly ...
Our, Own Moon, Is VERY Close, To The Edge, of The Stable Region ...
It's Actually, More Co-Orbital, than Anything Else!
Order of Kilopi
It is co-orbital. Unlike every other moon in the solar system, with the possible exceptin of Charon, our moon never backtracks in its orbit about the sun. That and that it is actually attracted to the sun twice as powerfully than it is to Earth means that in reality the moon is orbit the sun in the same space with are, esentially as sibbling planets. The gravity of both distort their orbits as they do, the moon obviously the greater since the Earth is more massive. In such it swings outside the Earth's orbit and then crosses back over and comes inside the Earth's orbit then crosses back outside. As it does the shortened or lengthened paths means that it either catches up and passs us crossing ahead, or slips back and falls behind us crossing behind. This means that to our stationary frame of reference the moon appears to orbit us.
Oh and a question I've been meaning to ask for years (well perhaps not quite years, but still) ZaphodBeeblebrox, do you have to make your post so darn illegable? Your punctation and capitalization make them all but impossible to read.
Order of Kilopi
It's Been, Almost 2 years ...
And, Yes I Do ...
This, Is The Way, I Talk, Besides, It Makes Sure, People Read My Posts, VERY Carefully!
Established Member
Lmao
Order of Kilopi
Really? EVERY other moon backtracks? I find it hard to believe that say, Phoebe, which takes 550 days to orbit Saturn, ever moves backward in relation to Sun.
BTW, the fact that Phoebe's orbit is retrograde is irrelevant. It simply means that backtracking (assuming it occures) happens when Saturn is between Sun and Phoebe, instead of Phoebe being between Sun and Saturn.
Established Member
I just imagine zaphod talking like stevie on malcolm in the middle.
Established Member
There's actually lots of moons that don't backtrack, especially the outer moons of the outer planets.
Earth is also co-orbital with the asteroids Cruithne and 2002AA29. But there's a big difference between them and the Moon. If the Sun suddenly disappeared, Earth, Cruithne, and 2002AA29 would fly off into intergalactic space, never to see each other again. The Moon, however, would continue to orbit the Earth. So it is correct to say that the Moon orbits the Earth and is not simply co-orbital.
Here's some pics of the paths of the planets and their moon systems as they orbit the Sun. The view is from an infinite perspective from above the solar system. Notice how the inner moons of the outer planets do backtrack on themselves, but their outer moons, as well as Earth's Moon, Mars' moons and Charon do not.
http://orbitsimulator.com/BA/moons/earth.GIF
http://orbitsimulator.com/BA/moons/mars.GIF
http://orbitsimulator.com/BA/moons/jupiter.GIF
http://orbitsimulator.com/BA/moons/saturn.GIF
http://orbitsimulator.com/BA/moons/uranus.GIF
http://orbitsimulator.com/BA/moons/neptune.GIF
http://orbitsimulator.com/BA/moons/pluto.GIF
Order of Kilopi
I don't know, but I am going to challenge that.
If the moon is orbiting around the earth toward the direction of the sun, and is ahead of earth to the sun (at this point the moon is closer to the sun) and the ripple in space/time hit the moon first, would it not throw it out of orbit of the earth because of the time it took to reach earth after reaching the moon? Seems like it would to me....
Order of Kilopi
Eh, I Don't, Have Asthma ...
More Like ...
Captain KIRK!
Established Member
It would be less than 2 seconds lag between the two, it might very slightly alter the orbit of the moon but I don't think it would escape.I don't know, but I am going to challenge that.
If the moon is orbiting around the earth toward the direction of the sun, and is ahead of earth to the sun (at this point the moon is closer to the sun) and the ripple in space/time hit the moon first, would it not throw it out of orbit of the earth because of the time it took to reach earth after reaching the moon? Seems like it would to me....
Established Member
Fair enough.
I can simulate it to verify that the Moon remains in orbit around the Earth assuming gravity were instant. But your point about the ripple is interesting. Gravity would "turn off" for the Moon first. Or last, depending on where it is.
How much affect will this have on the Moon's orbital velocity relative to Earth? Since your question goes beyond the capability of my simulator, which assumes instant gravity, I'll have to do the math. Acceleration due to the Sun's gravity can be expressed by:
a = GM/r^2
a = 6.67e-11NM^2 kg^-2 * 1.989e30kg / 150,000,000,000^2 m^2
a = 0.006 m/s^2
Since light travels at about 300,000,000 m/s and the moon is about 384,000,000 m from Earth, the Earth will still be accelerating for 384/300 seconds, or about 1.3 seconds after the disappearance of the Sun, while the Moon is now free of the Sun's gravitational acceleration.
0.006 m/s^2 * 1.3 s = 0.008 m/s.
The Moon's orbital velocity around the Earth is about 1 km / s, and escape velocity is about sqrt(2) * 1 km/s, or about 1.4 km/s. The 8 millimeters / second difference will be insignificant.
Order of Kilopi
Thank you tony for the math.
But the effect would still be enough to change the orbit, even though it is a slight diffrence...quite interesting.
Order of Kilopi
wait, you guys are talking about making the sun disappear cimpletely and instantaneously! Seems like there would be some fudging with physics inherent in that.Thank you tony for the math.
But the effect would still be enough to change the orbit, even though it is a slight diffrence...quite interesting.
Not only would the moon still "orbit" the earth, I'm not sure I understand tony873004's claim that Cruithne definitely would not. Do you mean, because of the other planets's influences? That it would orbit other planets rather than Earth?
Any object that has relative velocity less than escape velocity is not going to escape, right? It would continue to orbit, until it succumbs to atmospheric (or lithospheric) friction.
Established Member
Of course it's fudging with physics to instantly eliminate the Sun. But the hypothetical situation is just to illustrate that the Moon doesn't need the Sun's influence to remain paired with Earth. Cruithne does.
Cruithne's velocity relative to Earth far exceeds Earth's escape velocity at Cruithne's distance. If the Sun disappeared, Cruithne would not orbit anything. It would wander the galaxy by itself for eternity.
Established Member
Yeah that's what I kind of pictured too when I was reading it lol. I knew a kid once who would breathe at the wrong times when he was talking, made it really hard to understand what he was saying.
Posting Permissions
Reply With Quote