BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Lunar Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in July, 1993.]

Even though many amateur astronomers consider the Moon to be a nuisance to "serious" observing, it can be a very interesting target for an informed, open minded, observer. Our Moon is the closest astronomical object (approx. 240,000 miles on average), and the brightest besides the sun. It is also the only one we've actually visited, other than by proxy. The large day to day changes in the Moon make it especially interesting to those of us with short attention spans. It's unfortunate that the moon is so often ignored as an amateur astronomy subject.

Almost any equipment is suitable for some type of lunar observation. In fact, there are many interesting observations that can be made with the unaided eye. Nearly everyone notices the changing phases of the Moon. With only a little extra attention, one can appreciate the geometric relationship among the Sun, Earth, and Moon that gives rise to the particular phase you are seeing, and can note that if you were on the Moon the phase of the Earth would appear exactly opposite that of the Moon seen from the Earth. Sharp eyed observers can try to spot the earliest crescent Moon following the New Moon. Such observations have great historical significance in establishing the dates of certain religious holidays. Seeing the oldest waning crescent can be equally challenging.

Other phenomena involving the movement and position of the Moon can also be appreciated sans optics. The librational position of the Moon can be seen to vary over the course of each lunation (a complete sequence of phases, i.e. a lunar month). Libration is the apparent tilting or rocking of the Moon, due to its elliptical orbit and other effect, that allows us to actually see more than half of the Moon's surface. Even crude sketches of the naked eye Moon will reveal the librations. It can also be interesting and instructive to follow the rising and setting times and azimuths of the Moon from night to night. On average, the Moon rises about 50 minutes later each night, but this "retardation" varies considerably depending on the position of the moon in its orbit, from as little as 10 minutes to as much as 90 minutes. Some fairly sophisticated mental gymnastics are required to rationalize these variations intuitively. Associated with these variations is the relationship between the Moon's orbit and the ecliptic, which gives rise to variations in the altitude of the culminating Moon. You may have noticed that the Moon is usually higher in the sky during the Winter than during the Summer. There is a reason for this (I'm not going to tell you what it is!). [The answer to this "excercise for the reader" is as follows: The Moon more-or-less follows the ecliptic during its monthly movements, as the Sun does annually. Thus, the Sun, seen during the day (obviously), is high in the sky during the Summer months and low in the Winter. Since the Moon is usually observed at night (at least it's more often noticed then), the situation is reversed: the Moon, particularly when nearly full, appears generally higher in the sky during the Winter and lower during the Summer. --G.B.]

Of course eclipses of the Moon are easily observed unaided, but so much has been written on the subject that I'll not belabor it here. A distantly related phenomenon is "Earthshine," which is caused by the light from a nearly "full" Earth illuminating the otherwise dark portion of a crescent Moon. The brightness of the Earthshine is related to a number of factors, not the least of which is the weather on the Earth at the time! There are a number of lunar features visible to the unaided eye. There are over 25 named maria on the Moon, along with several "lakes," "marshes," and "bays." How many can you see? Can you pick out any of them when illuminated only by Earthshine? How about with binoculars?

There are of course a multitude of interesting lunar feature accessible to amateur telescopes, including craters of every size and description (Do you really believe they're all meteoric and none are volcanic?), crater chains (smart meteors!), mountains and mountain chains, rills, grabens, scarps, clefts, and ray systems. Most features are best seen when they are close to the "terminator," the line that divides the illuminated and unilluminated portions of the visible surface (No, it's got nothing to do with Arnold!). There the light of the Sun arrives at a low angle and topographic features stand out in striking relief. Some lunar domes, small rises in otherwise level areas of the lunar surface, can only be seen within one or two hours of lunar sunrise or sunset because they are so low. An exception to the foregoing are the ray systems, which are best seen around full phase. Rays are believed to be debris from meteoric impacts. Of course, a good map makes exploring the lunar surface easier and more enjoyable. (See book recommendation below.)

The appearance of most lunar features varies considerably under different lighting conditions. If you sketch a feature near the terminator on a particular night, then locate that feature the next night, you will see drastic changes in its appearance. On later nights you may not even be able to locate the feature because of the lighting differences. Carefully sketching can also reveal appearance changes cause by librational variations. If you're not artistically inclined (Actually, sketching features on the Moon requires more patience than skill.), the Moon is an easy target for photography. It is bright enough that good photos can be taken with simple arrangements, and usually no guiding is required. Comparison of photos taken at different times can be very instructive.

Those of us with an inclination to sleuthing can investigate the infamous "TLPs" or Transient Lunar Phenomena. These are suspected occurrences of fogs, outgassings, glows, color changes, and various other questionable events that are claimed to have been observed from time to time. Much has been written about them, but very little definitive evidence has been accumulated. A well documented TLP event could win you fame and fortune in astronomical circles!

Lunar occultations provide interesting observations that relate the Moon to the rest of the sky. When the Moon occults (passes in front of) a bright star, the star disappears from view instantaneously. Through a telescope, the abruptness of the event is startling. This occurs because the Moon has no atmosphere to fade out the star's light, and the very distant star has no appreciable diameter, so it is obscured all at once. During grazing occultations the star passes behind mountain peaks along the limb (edge) of the Moon, and is seen to blink off and on as the lunar mountains move in front of it. Serious occultation timing groups can produce accurate estimates of mountain heights from carefully recorded grazing occultation observations.

There are many books available on the subject of lunar observing, lunar geology, lunar exploration, and so forth. The one I have and will recommend (I've not really evaluated many) is The Moon Observers Handbook by Fred W. Price (Cambridge University Press, 1988). It contains many excellent maps organized by feature type, and by phase (i.e. terminator position). Another worthwhile book is A Portfolio of Lunar Drawings by Harold Hill (Cambridge University Press, 1991). This book is an absolutely stupendous accumulation of Hill's pen and ink sketches of lunar features. It's great for cloudy night astronomy.