View Full Version : Questionable criterion for calling Earth and Moon a double planet

2015-Jul-22, 02:22 PM
It has been stated that the Moon's motion relative to the Sun is always concave toward the Sun, and that it is the only known natural satellite that does that. Some people use that as a criterion for calling the Earth/Moon system a double planet. I disagree. If we could substitute a vanishingly small satellite for the Moon, it would follow virtually the same orbit, but by no stretch of the imagination would I call that combination a double planet. Conversely, two planet-sized bodies of equal mass in a tighter pair, such that their barycentric orbits would be convex toward the Sun part of the time, would be a perfectly good example to call a double planet.

Comments would be most welcome.

2015-Jul-22, 02:38 PM
I agree that concave is not a good criterion, but that comes as a result from Asimov's definition which uses relative gravitational pulls between the three bodies.

The shape of the orbit around the sun will be dependent on it's orbital speed around the main body and the orbital speed around the sun. When the moon was formed, it was in a much tighter/faster orbit. I'm not sure, but I think it was fast enough to produce convex portions of the orbit.

I think that the barycenter definition is not only simpler, but easier to determine when discovering a pair of objects.

2015-Jul-22, 04:59 PM
I agree that it is a specious part of a definition for double planet. Looking at the other extreme, if you had two Earth-sized objects in a 48-hour co-orbit, they would clearly be a double planet, but would neither have concave to the Sun orbits. I think we can ignore it unless someone shows up trying to defend it.

2015-Jul-23, 02:35 PM
I'd say if the common barycenter of the bodies is outside both of them, it's a double planet. If it's inside one, it's a planet and moon.

2015-Jul-23, 03:17 PM
Yes this seems a much better a precise way of describing such.