Observing the Moon: Oh No...A Full Moon!

Jack Kramer

Picture this: it's a beautifully clear night and you're dying to observe. As you step outside, you realize that glow isn't just the normal light pollution...the almost full Moon is out. Drat! (...or a worse epithet.)

Other than observing the planets, on such a desperate night you might actually want to take a look at the Moon. Yes, I'm serious - the full Moon. The various lunar details take on a different dimension under full-face illumination. As soon as you turn your scope on the Moon, you'll notice those craters that stand out brilliantly white. Under direct illumination, the floors of these craters have such a high albedo because the material itself is newer and more reflective than the average lunar surface. Most lunar "soil" is actually about the color of an asphalt driveway. An example of a bright feature is the crater Aristarchus, which first appears near the limb when the Moon is approaching full phase. The following CCD image from Michael Purcell includes the area around Aristarchus, with lunar north "up". Even though the image wasn't taken near full Moon, the crater shows up brightly and hints at the spectacle to be seen with a higher angle of illumination. Besides its renown as the brightest object on the lunar surface, the thing that makes it especially prominent is that the crater is setting out pretty much by itself on the north edge of Oceanus Procellarum. Aristarchus is one of those objects that's also recognizable even when it's in the dark area beyond the terminator -- it can be seen illuminated by earthshine when the Moon's a thin crescent.

The less prominent crater adjacent to Aristarchus is Herodotus; both are of almost identical size - about 40 km. While Aristarchus is a relatively new crater, Herodotus predates the surrounding maria, as evidenced by its smooth floor that was flooded by lava. Directly to the north lies the narrow, winding, deep feature known as Schroeter's Valley, which stands out well when the Moon is about 12 days old (12 days past the new phase). Under full illumination, the valley is difficult to detect, while the area around it shows up slightly darker than the surrounding maria. The valley itself is a channel created by a lava flow. Nearby is the horseshoe- shaped crater Prinz, which illustrates how lava overspilled it and all but obliterated the crater except for the highest part of its wall.

Another interesting feature of Aristarchus is the ejecta around it. This is the material that was thrown out at the time of the impact that formed the crater. The material shows up brightly around the crater, but on one side it's particularly long and seems comma-shaped. What sort of impact would cause the ejecta to be distributed in this manner?

This brings up another interesting aspect of the full Moon. The rays emanating from certain craters show up remarkably; some of them extend for long distances over the lunar surface. In certain cases, the rays are double -- parallel lines of ejecta. Sometimes the rays cross each other. Can you determine which is the more recent one, overlaying the other? You might try to judge the angle of impact based on the orientations of these rays. From which direction would the meteor be coming?

This whole area is one of the most active in terms of Lunar Transient Phenomena -- random brightenings that may indicate gas being expelled or even random vulcanism. In fact, the Apollo astronauts noted the highest levels of radon gas emissions in this area. Sometimes slight color differences between areas may be noted, as the orange-colored glassy granules catch and reflect sunlight at various angles.

If the Moon appears too bright, you might want to use a neutral-density filter, often referred-to as a "Moon filter". However, I've observed all these features well under high magnification in a 3" refractor without a filter.

Okay, so the full Moon may not be your very favorite object to observe, but doesn't it seem that the clearest nights always occur around this time of the lunar month? If you need an astronomy fix, the full Moon can be a satisfying diversion.

Published in the July 1995 issue of the NightTimes