Comment Re:The strangest moon in the solar system is ours. (Score 1) 141
The relative sizes of the Earth/Moon system is a total anomaly,
Is it? Moon diameter is 0.2724 that of Earth ; Charon's diameter is 0.5050 that of Pluto. For masses the corresponding ratios are 0.0122 (1/81) and 0.1160 (1/9). so, is the Moon a "total anomaly"?
so much so that it is very very close to the point where you have to call them a double planet rather than a planet and moon.
I've been taking an interest in astronomy for 40 odd years now, and I don't know what the point where I'd "have to" call a system a "double planet" is. I don't recall ever seeing the term defined in the astronomical literature. Even Wikipedia puts it as no stronger than an "informal term".
What would be the benefit of such a term? Once you acknowledge that two objects are in an orbital relationship, you need to calculate various properties (mutual eccentricities, velocities, masses, periods), but these are going to be essentially the same calculations whether you're looking at Pluto-Charon or Pluto-Nix (or is it Hydra that's the smallest known component of that system?). It's only when systems are close enough that they become tidally-locked that you get something new happening. Until the atmospheres meet. It's the same situation for multiple stars too - until you have to account for mass transfers from one to the other, then you're still looking at a Keplerian system.