Log, Aug 18, 2006

64 people. Simply put, this was the finest viewing night I've seen at Frosty Drew Observatory in years. It was a dark moonless night, with stable dry air and a telescope that operated well once we took a few minutes to get a great two star alignment. We were splitting double stars as small as 0.8 arc seconds with clear dark lanes between the stars. M13 knocked my socks off. It looked like an etching rather than a telescopic view. The Milky Way was bright and contrasty. You could see the arms of the galaxy easily. I could spot M31 in Andromeda without difficulty.

One gentleman brought his own laser pointer. I noticed him pointing at what I took to be a plane and shouted to him. Dumb me! It was the International Space Station - something we can safely point at. We saw quite a few late Perseid meteors and at least three other satellites.

We went to a great number of planetary nebulae, globular clusters, double stars and emission nebulae. I frankly lost track of which ones we saw but they included M8, M20, M22, M54 a bunch of NGC designations and our old friend Jupiter.

I might have stayed late, except that I had to be at FDO by 8:30 to do my part in the Frosty Drew Open House. After the morning clouds dissipated, we had visitors looking at the Moon.

Can you say "planets" boys and girls?

By now, many of you have heard the tentative new definition of a planet that will be voted upon (up or down) by the entire International Astronomical Union [IAU] in the near future. The new definition will define a total of at least 12 and potentially as many as 43 objects in the Solar System as planets. The certain 12 are the rocky planets [Mercury, Venus, Earth and Mars], the gas giants [Jupiter, Saturn, Uranus and Neptune], the asteroid Ceres and the [tentitively named] plutons [Pluto, Charon, Sedna and UB313 (sometimes called Xena)]. Another 31 objects are potentially large enough to become planets (several asteroids and a large number of plutons). We need much better images of them to be certain.

My guess is that they will "approve" the new definition because the IAU members are fed up with the debate. The proposed IAU committee definition states that a planet must be a world which directly orbits a star. A world is any body [except a star or former star] large enough for its gravity to pull itself into an approximate spherical shape. A star is any spherical body which produces energy from the fusion of hydrogen into helium.

I really have no objection to the definition of a world or the definition of a star. We wouldn't want a definition of a planet to include really small bodies. The spherical shape rule is probably reasonable. The definition of a star is also under review as the status of objects called brown dwarves is debated. However the IAU committee has come up with a really weird definition of which body a world orbits if there are more than two bodies being considered.

The obvious situation is where a large world orbits a star and where in turn a smaller world orbits the larger world. We say that the smaller world is a satellite of the larger moon or more colloquially that the smaller world is a moon of the larger world. However there are cases where it is not at all evident if the smaller world is really in orbit about the Sun and secondarily in orbit about the larger world or not. These aren't hypothetical cases; two planets in the Solar System have attendant worlds where the smaller world is more in orbit about the Sun than about the planet. One of these systems is Pluto and Charon but the other such system is much closer to home - in fact it is home - the Earth and Moon.

The gravity of the Earth on the Moon is only about a third as strong as the gravity of the Sun on the Moon. The real orbit of the Moon is primarily solar with perturbations (tugs back and forth) provided by the much smaller Earth gravity. If the rather logical definition of what orbits what were based on gravity, the Moon would be reclassified as a co-planet of the Earth. However the IAU didn't want the problem of explaining why the Moon wasn't really a moon!

Way out beyond Neptune, the Sun's gravity tapers off to about 1/1000th the force its has near the Earth. A jet plane taking off from Pluto would escape the Solar System forever. There is just barely enough gravity to hold a cloud of icy objects called plutons. Two of the plutons - Pluto and Charon spin about each other almost touching. The gravity between them is many times the gravity they feel from the Sun.

The IAU committee wanted to leave the Moon defined as a moon, no matter what either physics or common sense said about the situation. What the IAU committee needed was a definition of "in orbit about" which would say that the Moon was in orbit about the Earth in spite of the fact that the Sun pulls the Moon three times as fiercely as does the Earth. Here is where the IAU committee goes into a kind of a scientific never never land.

Instead of asking which body [the Sun or the Earth] exerts the greater gravity on the Moon they decided the following: For any two bodies, there is a point between them called the barycenter [effectively the center of gravity of the pair]. Suppose you call the mass of the larger body M and the mass of the smaller body m. Similarly call the distance from the center of the larger body to the barycenter D and similarly the distance for the smaller body d. D and d can be calculated from the equation D*M = d*m and the fact that D+d adds up to the distance between the two bodies. Clear so far? [I doubt it for most people!] Now it gets really weird. If the barycenter lies within the larger body, then the IAU committee says the smaller body orbits the larger body rather than the Sun no matter what the ratio of gravity fields say.

OK gang, I am not a member of the IAU and I don't get a vote. What is more, a single group needs to be in charge of naming and that group is the IAU. No matter how odd the definition for what is a planet, I will go along with it - probably unhappily. However, I positively absolutely totally will not take a ride on a space craft with a definition of an orbit based on barycenters! I like living too much and gravity rules the orbits of spacecraft, planets and moons not barycenters!

-Les Coleman

Leslie Coleman
Leslie Coleman
Entry Date:
Aug 18, 2006
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Leslie Coleman's Log
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