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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: January 30 Earthrise
Message-ID: <325@utastro.UUCP>
Date: Thu, 30-Jan-86 02:00:32 EST
Article-I.D.: utastro.325
Posted: Thu Jan 30 02:00:32 1986
Date-Received: Sat, 1-Feb-86 01:25:10 EST
Organization: U. Texas, Astronomy, Austin, TX
Lines: 35

There's no real "earthrise," as seen from the moon.  We'll tell you why
not, after this.

January 30  Earthrise

Earthrise seen from the moon.  Sounds beautiful, doesn't it?  But the
fact is that there is no earthrise seen from the moon.  It just doesn't
happen.

From the side of the moon that faces Earth, our planet hangs nearly
motionless in the lunar sky.  It doesn't come up, or go down -- no
earthrise, and no earthset either, for that matter.

On Earth, things rise and set because Earth rotates on its axis once
every 24 hours.  The moon also rotates.  But it completes a single spin
in 27 days -- the same amount of time it takes to complete one orbit
around the Earth.  The result is that we always see the same face of
the moon -- the nearside.  The farside of the moon was seen by
earthlings only since the dawn of the space age.

Likewise, any beings on the farside of the moon would never see the
Earth.  Earth never appears in the sky from that side of the moon.

From the moon's nearside, you'd see the Earth in a particular place in
your sky -- a place that wouldn't change -- or at least not enough to
notice.  Again, it wouldn't change because that side of the moon is
permanently locked toward the Earth.

So you'd never see an earthrise from the moon.  But you'd see the Earth
change its phase -- from a crescent Earth to full and back again -- all
taking place before your eyes over the course of about a month -- as
the moon moves in orbit around Earth.

Script by Deborah Byrd.
(c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin