The Moon can be both a bane and a blessing to the amateur astronomer. Moonlight renders faint deep sky objects invisible, and it washes out even the brightest ones so they are less attractive than when they are seen in a dark sky. For these reasons, many stargazers try to get out under the stars when the Moon is not up.
However, the Moon is an excellent telescope or binocular target in its own right. As our planet’s only natural satellite, it lies an average of 240,000 miles from Earth—about 100 times closer than Venus at its closest approach to Earth. Because the Moon lies so close to Earth, backyard telescopes can resolve details on its surface that are just a couple miles across.
Observing the Moon is quite easy, but there are a few things you can do that will enhance your lunar observing experience. First, tame the Moon’s brightness to a manageable level. The Moon is not bright enough to damage your eyes, but it is uncomfortably bright when viewed after dark with an unfiltered telescope. One easy way to remedy this problem is to use a lunar filter, which is just a neutral-colored filter that blocks most of the light coming through the telescope. However, you can also solve the problem by observing the Moon during twilight, before your eyes are adapted to the dark.
Just as importantly, look at the Moon on calm nights when the atmosphere is steady. Astronomers call these conditions good “seeing.” The Moon will look sharp on a night of good seeing, but it will look blurred or “watery” on a night of poor seeing, as if you were looking through shimmering heat waves. This effect is particularly pronounced at high magnification.
It is also important to allow your telescope some time to adjust to the outdoor temperature if the outdoor temperature is much different from the temperature at which the telescope was stored. If you look at the Moon immediately after taking a telescope from a warm building into the cold outdoors, the temperature difference around the telescope will have the same effect on your view as a turbulent atmosphere.
You can get a fair idea of how good the seeing is by checking whether the stars are twinkling. If you have good vision and can’t tell that the stars are twinkling, the seeing is good. If the stars are twinkling strongly, then the seeing is terrible, and you are better off looking at the Moon with binoculars or your unaided eyes until the atmosphere calms down.
Finally, you will typically see the most detail on the Moon by looking at surface features near the terminator, the line between the illuminated and dark areas. Near the terminator, sunlight is striking the Moon at a shallow angle, creating long shadows which increase the contrast of the surface features. By observing the Moon at various times throughout the lunar cycle, you can get the best possible view of different areas of the lunar surface.
