Of all the celestial sights that pass across the sky, none is more inspiring or universally appealing than our planet's lone natural satellite, the Moon. Remember the rush of excitement that you felt when you first peered at the rugged lunar surface through a telescope or binoculars? (If you haven't, you'll be amazed.) The first view of its broad plains, coarse mountain ranges, deep valleys, and countless craters is a memory cherished by stargazers everywhere.
A New View Every Night
Since the Moon orbits our planet in the same time that it takes to rotate once on its own axis, one side of the Moon perpetually faces Earth. Though the face may be the same, its appearance changes dramatically during its 27.3-day orbital period, as sunlight strikes it from different angles as seen from our standpoint. Due to the sunlight's changing angle, the Moon presents a slightly different perspective every night as it passes from phase to phase. No other object in the sky holds that distinction. (Note that it is actually 29.5 days from New Moon to New Moon; the added time is due to Earth's motion around the Sun.)
The Moon is the ideal target for all amateur astronomers. It is bright and large enough to show amazing surface detail, regardless of the type or size of telescopic equipment, and can be viewed just as successfully from the center of a city as from the rural countryside. But bear in mind that some phases are more conducive to Moon-watching than others.
The Best Times to View It
Perhaps the most widespread belief is that the Full Moon phase is the best for viewing, but nothing could be further from the truth. Since the Sun is shining directly on the Earth-facing side of the Moon at this phase, there are no shadows to give the lunar surface texture and relief. In addition, the Full Moon is so bright that it can overwhelm the observer's eye. Although no permanent eye damage will result, the Full Moon is uncomfortable to look at even with the naked eye. Instead, the best time to view the waxing Moon is a few nights after New Moon (when the Moon is a thin crescent), up until two or three nights after First Quarter (First Quarter is when half of the visible disk is illuminated). The waning Moon puts on its best show from just before Last Quarter to the New Moon phase. These phases show finer detail because of the Sun's lower elevation in the lunar sky.
Using a Moon Filter Improves the View
No matter what the phase of the Moon is, the view is almost always better through a lunar filter. It screws into the barrel of a telescope eyepiece and cuts the bright glare, making for more comfortable observing and bringing out more surface detail. Some lunar filters, called variable polarizing filters, act something like a dimmer switch, permitting adjustment of the brightness to your liking.
Notable Surface Features
The Moon is dominated by large, flat plains known as maria; the singular is mare (meaning "sea"), which is pronounced (MAH-ray). Maria were first thought to be large bodies of water. In reality, the maria are ancient basins flooded by long-solidified lava created some three billion years ago when the Moon was still volcanically active. All are relatively free of craters except for a few scars from impacts that have occurred since. Their romantic-sounding names, such as the Sea of Crises, Sea of Fertility, Sea of Serenity, Ocean of Storms, and the Sea of Tranquility, are believed to date back to the mid-17th century.
Surrounding the maria are the lunar highlands, dominated by nearly uncountable craters that measure up to several hundred miles across. Most are believed to have been created when debris from the formation of the solar system collided with the young Moon, leaving a permanent record of the barrage on its surface. Some of the more spectacular lunar craters include Tycho, Copernicus, Kepler, Clavius, Plato, and Archimedes, all named for figures of historical stature. Tycho, Copernicus, and Kepler are especially noteworthy, as each displays a broad pattern of bright rays radiating outward. These are particularly impressive during the Moon's gibbous phases (between Quarter and Full), when the Sun appears high in the lunar sky. The Moon also has several noteworthy mountain ranges, such as the Alps and Apennines, as well as straight cliffs, towering ridges, broad valleys, and small, sinuous rilles.
Focus on the Terminator Region
The greatest amount of detail is visible along the Moon's terminator, the line separating the lighted area of the lunar disk from the darkened portion. It is here that the Sun's light strikes the Moon as the narrowest angle. This casts the longest shadows, increasing contrast of lunar features and showing the greatest three-dimensional relief. Sometimes you will notice a bright "island" surrounded by darkness on the dark side of the terminator. That's a high peak, tall enough to still catch the light of the setting Sun, while the lower terrain around it does not.
A Great Target for Telescopes or Binoculars!
So, the next time the Moon is riding high in the sky, take time to visit our nearest neighbor in space. A binocular provides a terrific view; use a tripod or brace it against something to hold it steady. If you have a telescope, begin with a low-power eyepiece. Slowly scan across the lunar disk and try to imagine the emotion that the astronauts must have felt as they orbited that alien world, a world so close to our own, yet so astonishingly hostile and different — "magnificent desolation," as Edwin Aldrin put it during his and Neil Armstrong's historic visit on Apollo 11 in 1969. Then, switch to higher powers for close-up studies of specific areas and features. Get a lunar map or lunar atlas to identify specific craters and features.
An amazing world, our Moon, so rich in detail and so easy to see.
Checklist of Observable Features
1) Maria — Once thought to be oceans of water, these "seas" are actually vast plains of hardened lava. In some of them you will see giant ripples.
2) Craters — Like snowflakes, no two appear exactly alike. In the center of some larger craters, look for peaks formed from the upsurging of molten rock at the impact point. Look for small craterlets inside craters, too.
3) Crater Rays — Long, bright "splash marks" radiating from a few craters, such as Copernicus and Tycho. Best observed at Full or Gibbous phases.
4) Mountains — Several major mountain ranges scar the lunar surface. Check out the largest one, the Apennines, in the southern half of the Moon's disk along the vertical centerline. You can't miss it!
5) Domes — These small, low mounds often have a tiny craterlet in the middle and tend to cluster in groups.
6) Rilles — Filamentous faults and channels, some of which were once meandering rivers of flowing lava.