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The edge of the Moon is a little rough. The mountains and highlands tower
dramatically above the lunar maria and basins. Although it's only a small
correction --six miles difference in altitude on the Moon corresponds to
about 5 seconds of arc at the Moon's mean distance as seen from here on
Earth-- I have often thought that it might be worth trying to correct for
it.

I used the data from the Clementine lunar orbiter mission as processed by
the USGS ( http://pubs.usgs.gov/of/2006/1367/ ) and created smoothed limb
profiles for each degree in the full range of lunar libration in ecliptic
latitude and longitude. Just eyeballing the profiles, about 75% of the time,
the correction for the lunar limb is less than two seconds of arc. The rest
of the time, the correction is rarely above three seconds of arc. This might
shift a marginal lunar distance, e.g. from 30d 20.1' to 30d 20.2' but most
of the time, this will be insignificant. It should be said though that the
lunar distance tables that have been published (and are published by some
people, including me, today) assume that the Moon's limb is a perfect
circle. So if you see a predicted geocentric lunar distance quoted to an
exact number of degrees, minutes, and seconds, bear in mind that the seconds
should probably be marked with a +/-2. For the future, it might be worth
incorporating limb corrections into these tables, to eliminate one small
source of error.

The reason the limb is still relatively smooth even though there are big
differences in altitude on the Moon appears to be similar to the smoothing
of the horizon at sea in the presence of large swells. The foreground peaks
tend to fill in the troughs of waves behind them lined up near the horizon.
On the Moon, the only major exceptions to this pattern occur where a large
depression is located close to the mean limb of the Moon. Mare Orientale is
a good example of this.

By the way, there are examples of limb profiles published for solar eclipses
that you can find on the web. Go to google images and type in 'lunar limb
profile'. These profiles are useful for verifying the general appearance of
limb profiles for lunar distances but they're not directly applicable. Limb
profiles for eclipses are designed to show very fine detail in the limb
(where a last bit of the Sun's disk might peak through a lunar valley
creating the phenomenon of Bailly's beads) while for lunar distance
observations you would want something smoothed over several degrees of the
Moon's limb.

 -FER
www.HistoricalAtlas.com/lunars


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