The Moon

For die-hard stargazers, Full Moon is the worst time to observe the sky. A full or nearly full Moon washes out the fainter objects in the sky, especially deep-sky objects. Even the Moon itself gives up detail less readily when it is nearly full. The glare is so intense that looking at the full Moon through a large telescope can be painful and will temporarily destroy night vision. Polarizing filters are useful for cutting down on the glare.

For a beginning astronomer with binoculars or a small telescope, however, the Moon at any phase is the first object to observe. It is easily found, easily identified, and will show incredible detail in any optical instrument.

In general, the best time to observe the Moon is when it is less than half lit, although fuller phases do offer some interesting views. When the Moon is a crescent, one can often see the outline of the rest of the lunar disk. This is called "the Old Moon in the New Moon's arms", or "Earthshine". The latter name refers to the fact that a portion of the Moon which is not receiving direct sunlight is being dimly lit by light reflected off the Earth. Thus, we see the dim outline of the full disk. The line which separates the Moon's brightly sunlit portion from the rest of the disk is called the terminator. This line divides night and day on the Moon. On Earth, shadows are long when the Sun is low in the sky, and the same effect is apparent along the Moon's terminator. Craters and mountains along the terminator cast long shadows and are thrown into sharp relief as seen from Earth. The southern portion of the Moon is especially rich in craters, so this part of the terminator is quite fascinating. When the Moon is a thin crescent, one can often see a mountain or two poking up from the unlit portion, catching the Sun's rays while the landscape around it remains dark.

At Full Moon, there is no terminator. The Moon can be uncomfortably bright in a telescope when it is near full, although a filter will cut down on the glare. Several craters are most impressive at Full Moon, because of their extensive ray systems. The craters Tycho and Copernicus in particular have a network of white lines extending outward in all directions from their rims. These rays are ejecta thrown out by the meteorite impacts that formed the craters.

The Moon offers hundreds of landmarks for amateurs to explore. Lunar maps and atlases are available at some bookstores and science stores and from Sky Publishing.

As mentioned before, some faint astronomical objects are diminished in appearance when a bright Moon is in the sky. The five brightest planets, on the other hand, are bright and can be easily viewed even when the moon is nearby.
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