Robert A. Garfinkle - Luna Cognita: A Comprehensive Observer’s Handbook of the Known Moon

The Moon is more a creature of the Earth than of the heavens. Our satellite probably came from, or had a common origin with, our own planet. In terms of the details visible on it, the Moon is a hundred times closer to us than Venus, the nearest planet, at its closest approach. All this means is that observing the Moon is unlike observing any other celestial object. In particular, it is the only extraterrestrial body (besides, marginally, Mars and Mercury) that shows relief features to an earthbound telescope.

The Moon has a few other peculiarities. It has no appreciable atmosphere, so there are no clouds or haze to obscure our view; sadly, likewise no variable cloud features. The Moons colors are subtle; they exist, but certainly are not as evident as those seen on Mars or the giant planets. Also, the Moon is the only body with one hemisphere permanently turned toward the Earth; no matter how long we watch, we see only a little more than half of our satellite. However, even this 59-percent fraction of the Moon has an area nearly as large as that of North America; and in this giant expanse, we can, under good conditions, see detail as small as a kilometer across with an ordinary telescope.

Finally, lunar features appear changeless to us. Perhaps a very patient observer would see an occasional meteoroid-impact flash on the Moons nightside. Sometimes other lunar transient phenomena (LTP) are reported, but even a dedicated observer might never see one in his or her lifetime. What does change, and never exactly repeats, is the combination of lighting and orientation that pertains to each moment of observation. Indeed, it is the constant change of solar illumination and libration (orientation of the Moon with respect to the observer) that reveals new aspects to even a familiar feature each time one returns to it.

So it is not surprising that lunar observers are not entirely like other astronomers, even the planetary scientists who specialize in solar system objects. The Moon is the only body where one can clearly see terrain (in amature instruments), so if you enjoy such things as satellite views of Earth, or simply looking out of airplane windows at the landscape below, you may well be a person who would enjoy studying the landscape of the Moon.

Each feature, or portion, of the observable part of the Moon experiences four lighting conditions each month. One is night, better described as earthlight time, when one can watch for several types of LTP, including recording tonal changes with CCD imaging. The next condition is that of low solar illumination, shortly after local lunar sunrise, or shortly before sunset. At such times the Sun is only a few degrees above the lunar surface, its low rays bringing out subtle forms of relief such as lunar basins, ghost craters, domes and wrinkle ridges. The third condition, is characterized by a medium solar angle; as an example, except near the lunar poles, theLunar Orbiterframes were taken with a solar altitude of 16 at their centers. Here, the more spectacular forms of lunar relief are seen at their best, with enough shadow to show their form, but not so much shadow as to drown them in darkness. The Moons mountain ranges, and all but its most dilapidated craters, are best seen under these conditions. Finally, there is the lunar midday, with a high sun for all but the polar regions. This book describes several types of features best, or only, seen under a high sun; dark halo craters, bright spots, ray systems, and banded craters. Likewise, a high sun angle makes for the best conditions for viewing or imaging the subtle colors of lunar regions; indeed, using narrow-band filters, color differences give clues as to the mineral composition of the Moons surface.

As becomes evident during author Garfinkles fifteen-chapter crater-hopping tour is that, for most of the lunation, at any time one can see areas under all four lighting conditions. Naturally, one must bear in mind that to truly know a feature one must see it under both lunar morning (pre-full) and lunar evening (post-full) lighting. Full moon itself is a special time when shadow detail is present only at one extreme limb, but when a galaxy of tonal features covers the rest of the disk.

Other special moments occur when favorable combinations of lighting and libration turn one of the Moons marginal areas unusually far toward us. It is by using such rare opportunities that we can add 9 percent to the usual 50 percent of the earthside hemisphere.