1. Established Member
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Feb 2004
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177

## Planetary Alignment

I was talking to a friend tonight, and among other things, he asked me if I could find out when (if?) all 9 planets (OK, 10, if we include Sedna )would be in alignment (from the point of view of looking down from solar north). Anybody know/know how to work it out? I've got a link to NASA's Solar System simulator, but that involves 'stepping through' time until I see that an alignment has ocurred. Anyone happen to know if/when this will happen?

2. Hi Andrew.

I googled "planetary alignment" and came up the BA's website 3rd from the top, and a nasa website second from the top. The nasa website also had a link going to the BA's website.

Boy does Phil ever get a lot of publicity.

I have a feeling we'll have to wait a very long time for all 9+ of them to line up though. :-?

Here's a link to a website that tries to figure it out for us.

http://www.lcsd.gov.hk/CE/Museum/Spa...arsystem_2.htm

As the ks in both equations above must be equal, k will be the LCM of 36966 and 36672, i.e. 225,936,192. It means the 225,936,192th oppositions between the Earth and Neptune. For these four planets, it will take 227 million years for them to align. For the Nine Planets, the time for them to align will be even longer than the age of the Solar System!
Here we assume planets move in circular motion at constant speed, and the synodic periods given above is just an approximate value. Actually the planetary motion is full of small variants, and the synodic period values are irrational number. The quotient of two irrational numbers cannot be written as a fraction. Therefore, the Nine Planets simply do not align.

3. dvb:
Here's a link to a website that tries to figure it out for us.

http://www.lcsd.gov.hk/CE/Museum/Spa...arsystem_2.htm
The site looks like it has some good info, but it makes some basic errors.
As the ks in both equations above must be equal, k will be the LCM of 36966 and 36672, i.e. 225,936,192. It means the 225,936,192th oppositions between the Earth and Neptune. For these four planets, it will take 227 million years for them to align. For the Nine Planets, the time for them to align will be even longer than the age of the Solar System!
Notice, that is assuming precise alignment, which is probably not really necessary for purposes of the OP. And even if it were, it assumes that the planets are aligned right now, and it calculates it will then take 227 million years to re-align. In other words, they could align tomorrow, if they were aligned 227 million years ago.
Here we assume planets move in circular motion at constant speed, and the synodic periods given above is just an approximate value. Actually the planetary motion is full of small variants, and the synodic period values are irrational number. The quotient of two irrational numbers cannot be written as a fraction. Therefore, the Nine Planets simply do not align.
That assumption of circular motion means that in real life the time of the alignment is not going to be the same as the calculation. And there is no way to tell if they are rational or they aren't rational, contrary to what that website says.

4. I vote the answer is never.

Pluto is in a 3:2 sync with Neptune, the closest they ever get is 17 AU apart and they'll never collide even though Pluto's orbit seems to cross Neptune's as seen in 2 dimensions.

Someone can correct me if I am wrong, but due to the inclination of Pluto's orbit I doubt they would ever be close together in they sky as viewed from Earth. And if we look at them in 3D we would see that they would never line up with the other planets when they are on the same side of the sun.

http://www.nineplanets.org/plutodyn.html

5. Originally Posted by aurora
I vote the answer is never.

Pluto is in a 3:2 sync with Neptune, the closest they ever get is 17 AU apart and they'll never collide even though Pluto's orbit seems to cross Neptune's as seen in 2 dimensions.

Someone can correct me if I am wrong, but due to the inclination of Pluto's orbit I doubt they would ever be close together in they sky as viewed from Earth. And if we look at them in 3D we would see that they would never line up with the other planets when they are on the same side of the sun.
But the OP is asking "from the point of view of looking down from solar north"--in that 2D sense then.

6. In addition to milli's clarification, Pluto and Neptune *can* line up (2-dimensionally) with the sun. The three to two resonance means that every two orbits Pluto makes, Neptune makes 3. They line up at least once in those 2 plutonian orbits.

7. Newbie
Join Date
Jul 2004
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9
Well when they launched the voyager probes the planets were "kinda" in a line, so as the voyager 2 probe left the solar system it would fly by each of the planets on its way out.

--- = Voyager path

E-------M----------------J-------------------S-------U---------N--------P----

At least that is what my Science teach told me several years ago..

Hard to explain, hope it made sense. Maybe another way to say it.

The planets would all arrive at a point near voyagers path out of the solar system at about the same time as voyager passed the orbit of that planet.

Yruc

8. Through some googling, I found this response to planetary alignments and the odds of all nine planets and the moon lining up.

The probability of all nine planets and the Moon aligning perfectly is 1 in 86 billion-trillion-trillion-trillion years. Our solar system has only been around for 4.6 billion years and its total life is only 10 billion years. The chances are: a perfect alignment won't happen during the lifetime of our Moon, Sun, and planets.
I couldn't help but notice there were links to the BA's site for reference at the end of the article.

9. Originally Posted by Yruc
Well when they launched the voyager probes the planets were "kinda" in a line, so as the voyager 2 probe left the solar system it would fly by each of the planets on its way out.

--- = Voyager path

E-------M----------------J-------------------S-------U---------N--------P----

At least that is what my Science teach told me several years ago..

Hard to explain, hope it made sense. Maybe another way to say it.

The planets would all arrive at a point near voyagers path out of the solar system at about the same time as voyager passed the orbit of that planet.

Yruc
Mars and Pluto shouldn't be in the diagram, and the remaining planets are in more of a curve than a straight line. See here. But you've got the concept down.

10. Established Member
Join Date
Feb 2004
Posts
177
So, the consensus appears to be, never, or very close to it.

(BTW, my degree of accuracy would be, roughly, that a planet could be as far out of alignment, in degrees, as they are off the plane of the solar equator. or the average orbital plane of the solar system, if that's significantly different)

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