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New Moon of Jupiter Discovered
xihr writes "Astronomers have discovered a new moon of Jupiter, bringing its known retinue of satellites to a whopping 40. The new moon, designated S/2002 J1, is only 3 km wide, and has a highly inclined and eccentric orbit.
Astronomy.com has the story."
I personally wouldn't think of this as a moon, i'd think it was a part of the jupiters rings
Actually, many astronomers consider the Earth-Moon system to be a double planet.
Il n'y a pas de Planet B.
Here we need a good definition of 'orbit' - if, at any time, an object's orbit brings it *away* from the center of mass of its solar system, and towards its planetary primary, it's in orbit around that planetary primary, and not its star. This means, incidentally, that the Earth's Moon is not a moon - it's another planet that happens to co-orbit the sun within the same boundry space as the Earth, and the two planets perturb each other's orbits.
This seems to be a bad and ambiguous definition and an erroneous interpretation of that definition.
1) It's circular. You definite orbit in terms of orbit.
2) The Earth is, at some point in its orbit, moving closer to the Moon and away from the sun, ergo it is the Moon's moon. And vice versa.
3) The Earth is, at some point in its orbit, moving closer to Neptune and away from the Sun, ergo it is Neptune's moon.
Surely the simple criterion is along the lines:
1) if we can see A moving around B and C, and we were to remove the influence of C, and then A continues around B, A is orbiting B -- if they would just fly off to infinity (or otherwise approximate a hyperbolic orbit) then A is not orbiting B.
2) if A and B satisfy condition 1, then we say A orbits B if A is (some degree of our choosing) less massive than B. If they are within (some degree of our choosing) we say they orbit each other.