Great Time to Observe the Moon and Its 'Seas'
The moon's first quarter phase is the best time of the month to observe its craters and "seas."
Credit: Starry Night Software [Full Story]

A moon in the first quarter phase, such as we will have over the next few nights, is the best time to gaze the lunar surface with binoculars or a small telescope.

At first quarter, the sunlight is falling on the moon from its right side (left side as seen from the Southern Hemisphere). Along the line dividing day and night on the moon, called the terminator, the shadows are long, and the slightest surface features stand out in stark contrast.

This moon graphic shows popular lunar targets for moongazing during the moon's first quarter phase. [How Moon Phases Work]

Begin your study of the moon by noting the largest features, the dark plains of ancient lava known by the Latin name "maria," singular "mare." Early astronomers were unaware that the moon had no atmosphere and thought these broad lifeless plains were seas and oceans.

At first quarter, three maria dominate the moon's surface. From north to south these are the Mare Serenitatis (Sea of Serenity), the Mare Tranquillitatis (Sea of Tranquility), and the Mare Nectaris (Sea of Nectar). Between the first two and the edge of the moon is the smaller circular Mare Crisium (Sea of Crises).

Notice how the maria tend to be concentrated on the northern half of the moon, while the southern half is mostly highlands, pockmarked all over with craters. Long ago in the north, lava flows from the huge maria swamped the ancient craters. Here and there you may see their ghosts, however.

All of the larger craters on the moon are named, mostly the names of ancient astronomers. At first quarter, one of the most spectacular craters is Maurolycus.

This crater, 71 miles (114 km) in diameter and nearly 3 miles (4.8 km) deep, is named for a 16th century Italian mathematician who opposed the theories of Copernicus. The people who named the moon's craters carefully placed Maurolycus and Copernicus on opposite sides of the moon's disk to keep these two from arguing.

At the far northern edge of the first quarter moon is an interesting pair of craters named for the two mythical strongmen, Hercules and Atlas. Hercules is 43 miles (69 km) in diameter and has a large crater in its floor. Atlas is 54 miles (87 km) in diameter and has a flat floor crisscrossed by so-called rilles ? the remnants of collapsed tubes of lava.

One of the most interesting things to do while observing the first quarter moon with a small telescope is to concentrate on a particular crater and watch in real time while the sun rises over it.

At first the crater will be engulfed in shadow. Then a beam of sunlight will illuminate the central peak and the far wall. Gradually, as the sun rises, more and more of the crater floor will be revealed.

If you return to an area you studied carefully a night later, you will be surprised to find almost nothing visible.

The topography of the moon is, in reality, very slight, and only reveals its details when lit by the glancing rays of the rising or setting sun. That is why the first quarter moon is so appealing to amateur astronomers.

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.