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From: dsmith@hplabsc.UUCP (David Smith)
Newsgroups: sci.astro
Subject: Re: Moon crater viewing
Message-ID: <778@hplabsc.UUCP>
Date: Fri, 24-Oct-86 12:46:37 EST
Article-I.D.: hplabsc.778
Posted: Fri Oct 24 12:46:37 1986
Date-Received: Sun, 26-Oct-86 02:32:43 EST
References: <90@ritcv.UUCP> <394@uwmacc.UUCP>
Distribution: net.astro
Organization: Hewlett-Packard Laboratories
Lines: 32
Summary: 2 libration sources

In article <394@uwmacc.UUCP>, demillo@uwmacc.UUCP (Rob DeMillo) writes:
> In article <90@ritcv.UUCP> jaw7509@ritcv.UUCP (John White) writes:
> >
> >...remember reading that at a certain time, on a certain day, once a year,
> >the top edge of a crater on the moon's dark side is visible.
>
> The lunar revolutions are not *exactly* resonant locked with the earth's.
> The result is that about an additional 5% of the moon (2.5% on the east
> limb, and 2.5% on the west limb) will periodically be shown throughout the
> year. Although this is not much, it does allow for certain craters and
> mountain ranges to "peep thru" to viewer's on the earth. 
> 
> Since the change is, of course, gradually taking place as the year passes,
> I doubt that this "mystery crater" *pops* into view on a given night/time...

Lunar libration has two causes that I am aware of:

1. Its elliptical orbit causes its angular velocity about Earth to vary 
    over a monthly period, whereas its rotation around its own axis is
    uniform.  This causes it to appear to wag from side to side, showing
    a little more on the east, then a little more on the west.
2. Its rotational axis is not normal to its geocentric orbit plane.
    Therefore, over a month's time, it will nod its poles alternately
    toward and away from Earth.

At the time of the Apollo 11 flight, I got a poster of the moon from a
bank.  It showed the limits of earthbound observation, and stated that
libration causes 59% of the lunar surface to be visible from Earth.

So what are the yearly cycles that two posters have referred to?

				David Smith