Fight for Pluto !, A Campaign to Reverse the Unjust Demotion |
Fight for Pluto !, A Campaign to Reverse the Unjust Demotion |
Aug 24 2006, 08:24 PM
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Member Group: Members Posts: 548 Joined: 19-March 05 From: Princeton, NJ, USA Member No.: 212 |
Dear Friends,
Today I am extremely dissapointed that the Pluto Demoters have triumphed. I respect their opinion, but disagree with it. I strongly agree with Alan Stern's statement calling it "absurd" that only 424 astronomers were allowed to vote, out of some 10,000 professional astronomers around the globe. This tiny group is clearly not at all representative by mathematics alone. I believe we should formulate a plan to overturn this unjust decision and return Pluto to full planetary status, and as the first member of a third catagory of planets, Xena being number two. Thus a total of 10 Planets in our Solar System Please respond if you agree that Pluto should be restored as a planet. ken Ken Kremer Amateur Astronomers Association of Princeton Program Chairman |
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Aug 25 2006, 11:23 AM
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Member Group: Members Posts: 307 Joined: 16-March 05 Member No.: 198 |
Too many of the elements in the IAU's definition are inherently flawed. Curiously that includes the very one that many people may have the least problem with: ""A planet is a celestial body that (a) is in orbit around the Sun".
Just exactly how do you determine that body goes round the Sun? In many cases the answer is doubtless obvious. However, as with so much else in the universe there are always exceptions. One particularly bizarre example of such an exception is object J002E3, which paid a visit to our vicinity a few years ago. It is believed to be the spent 3rd stage of one of the Apollo missions. Its orbit is such that although it usually goes round the Sun it can perodically be captured by the Earth, orbit it for a while, then escape back into solar orbit again. A more pertinent example of an exception, though, is Earth's own Moon. Apparently the Sun's gravitation field has a stronger pull on the Moon than the Earth's own field. (For more check out here and here; for a more explicitly worked out version of the maths check out here; and there's an interesting discussion of the problem, together with lots of maths, here; while this page puts a more democratic perspective on the issue: "the moon co-orbits the sun with the earth".) Since the IAU chose not to define the yardstick(s) on which it determines when a body is in orbit around the Sun (and when it isn't), there seems no reason the balance of gravitation forces could not be used as that yardstick. Yet were that yardstick applied to the Moon it could be used as the basis for arguing that since the Sun has a stronger pull on the Moon than the Earth's own gravitation field, the Moon is therefore (technically) in effect going round the Sun rather than around the Earth and thus (technically) not a moon under the IAU resolution but either a planet or a dwarf planet (depending on the other elements in the definitions). ====== Stephen |
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Aug 25 2006, 08:26 PM
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Senior Member Group: Members Posts: 2492 Joined: 15-January 05 From: center Italy Member No.: 150 |
advice: this post is OT here! sorry for this...
A more pertinent example of an exception, though, is Earth's own Moon. Apparently the Sun's gravitation field has a stronger pull on the Moon than the Earth's own field. (For more check out here and here; for a more explicitly worked out version of the maths check out here; and there's an interesting discussion of the problem, together with lots of maths, here; while this page puts a more democratic perspective on the issue: "the moon co-orbits the sun with the earth".) Stephen, I must confess that I ignored that ratio between the gravity force from Earth and from Sun is below 1 only for our moon! Your links are very interesting and, apparently, all figures are right but... while math is right, there is a big flaw in the physics of all of them! In fact, they forget that Earth-Moon system is not inertial! You cannot ignore that the Earth-Moon baricenter is moving in a circular trajectory, so there are other forces that must be considered in the game, especially the centrifugal force, which is equal to Sun gravity in the baricenter (but, obviously, points in the opposite direction). The proof of how wrong are conclusions arising from application of inertial rules to such a rotating system is the question posed in the math forum you posted: the asking guy calculated that the equilibrium point between Sun and Earth gravity is about 258000 Km from Earth, so he wonder how the Moon can still orbiting the Earth from a larger distance. But it is well known that the equilibrium (Lagrange) points are located 1.5 million Km from the Earth, 6 times away and well behind Moon orbit. This result can be ontained only considerning also centrifugal force arising from the rotation around the Sun. In conclusion, when two body are gravitationally linked (and Earth-Moon are linked without any doubt), you must first consider their movement around their common baricenter and, before to consider Sun attraction, you must subctract the motion of the baricenter around the Sun. All residual forces are very small and can be considered tidal forces or second-order rotational effects... -------------------- I always think before posting! - Marco -
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