[tz] Time for a Lunar time zone?

Clive D.W. Feather clive at davros.org
Thu Mar 2 09:56:54 UTC 2023

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 at davros.org     | it will get its revenge.
Web: http://www.davros.org  |   - Henry Spencer
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