Math Is Fun Forum

U7109

Real Member

Registered: 2007-03-19

Posts: 485

The Nine Planets

This website is one I have bookmarked myself. I'm quite into Astronomy, and this website provides information and links. The link is http://seds.lpl.arizona.edu/nineplanets/nineplanets/ if you're intrested. And here's a part of an article on Neptune:

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws.  It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.

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"Reality leaves a lot to the imagination," ~ John Lennon

Sekky

Member

Registered: 2007-01-12

Posts: 181

Re: The Nine Planets

There was a similar problem with Neptune's orbit when pluto was predicted, except when they found pluto it turned out after extensive observation that it wasn't in fact the "planet" affecting neptune in the way they had predicted, but pluto was classified a planet far too soon. As it happens, pluto is just one of many small objects in its belt.

Zach

Member

Registered: 2005-03-23

Posts: 2,075

Re: The Nine Planets

Eight Planets. There are eight planets.

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Boy let me tell you what:
I bet you didn't know it, but I'm a fiddle player too.
And if you'd care to take a dare, I'll make a bet with you.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: The Nine Planets

Something confused me about how they demoted Pluto.
If I remember, they changed the definition of a planet to be something like:

• A planet is sufficiently large such that it assumes uniform shape under its own gravity (ie. it's a sphere).

• A planet orbits around a star.

• A planet has swept all other bodies out of its orbit, either by disrupting their path or colliding with them.

The third point is the one they added, which allowed them to say that Pluto wasn't a planet because its orbit overlaps with Neptune's. But surely if that's the case, then Neptune shouldn't be a planet either?

It's very possible that I'm misremembering something here, so please do correct me if you know better.

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Why did the vector cross the road?
It wanted to be normal.

Devantè

Real Member

Registered: 2006-07-14

Posts: 6,400

Re: The Nine Planets

Eight Planets. There are eight planets.

No, there isn't. There are more than eight. 'planet' has subcategories, one of them being 'Dwarf Planets'. So there are, at least, more than 10 planets.

George,Y

Member

Registered: 2006-03-12

Posts: 1,379

Re: The Nine Planets

Eight big planets.

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X'(y-Xβ)=0

Sekky

Member

Registered: 2007-01-12

Posts: 181

Re: The Nine Planets

• A planet has swept all other bodies out of its orbit, either by disrupting their path or colliding with them.

The third point is the one they added, which allowed them to say that Pluto wasn't a planet because its orbit overlaps with Neptune's. But surely if that's the case, then Neptune shouldn't be a planet either?

It's very possible that I'm misremembering something here, so please do correct me if you know better.

No, the difference is that they have different orbits. Pluto is one of thousands of objects in its orbit, and it's smaller than a lot of them.

Ricky

Moderator

Registered: 2005-12-04

Posts: 3,791

Re: The Nine Planets

A similar story happened Mercury, where there were discrepancies in it's orbit.  However, this could not just be solved by "inventing" a body of mass.  Instead, it took Einstein to rework our fundamental understanding of gravity.  Then, a soldier in WWI, while in the trenches, worked our the calculations for Mercury's orbit, and found it to behave exactly as it should under Einstein's concept of gravity.

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"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

U7109

Real Member

Registered: 2007-03-19

Posts: 485

Re: The Nine Planets

I think there's thirteen planets. Eight big ones, three dwarf ones. The new dwarf planets are Eris, and I've forgotten the name of the other. Also, Pluto has three moons now; Charon, Nix and Hydra.

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"Reality leaves a lot to the imagination," ~ John Lennon

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: The Nine Planets

I think there's thirteen planets. Eight big ones, three dwarf ones.

That would mean that there were 11.

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Why did the vector cross the road?
It wanted to be normal.