Paul Eggert via tz said:
On 2023-03-01 04:18, Tim McBrayer via tz wrote:
On the lighter side; we may need to consider going interplanetary.
Is it interplanetary if it's just the Moon? I had thought that since the barycenter of the Earth???Moon system is under the surface of the Earth, the Moon doesn't count as a separate planet.
This is arguable. The barycentre is under the surface, yes, but if the Earth was denser it would be in the same place but that place would be outside it. Similarly, the Sun-Jupiter barycentre is outside the sun even though the barycentres for the other planets is inside. It seems an arbitrary definition. Also note that, as the moon drifts away from the earth under tidal braking, the barycentre will move outside the Earth. Does it become a separate planet at that point? An alternative approach is to look at the shape of the orbit. A typical satellite's orbit (say Titan) is always concave relative to the planet it orbits but sometimes convex relative to the sun. But our moon's orbit is different: it is always concave relative to the sun but, I think, can be convex relative to the earth. This is because the solar gravitational pull on the moon is more than twice the earth's pull. So the point at which the pulls are equal might make a reasonable definition. -- Clive D.W. Feather | If you lie to the compiler, Email: clive@davros.org | it will get its revenge. Web: http://www.davros.org | - Henry Spencer Mobile: +44 7973 377646