Observing the Full Moon

During full Moon its craters, mountains, and other surface features appear muted because the high Sun casts no shadows. But other features now stand out; note the rayed crater Tycho near the Moon's lower limb.
Courtesy Gary Seronik.
Question: When do many amateur astronomers leave their telescopes indoors?

Answer: the nights around the time of full Moon. After all, bright moonlight washes out faint galaxies and nebulae, and the Moon itself is too glaring to be observed . . . right?

If you heard yourself say this, it’s time to take your telescope outside during the next full Moon and begin exploring a world largely overlooked by amateur astronomers. There are numerous fascinating features to observe during this or any phase when you aim your telescope far from the terminator (the dividing line between lunar day and night). The Moon is very bright when it’s full. If your magnified view of the lunar surface is a bit too dazzling for comfortable observing, use a Moon filter to cut the brilliance without eliminating detail.

Because there are no shadows at full Moon, the dark and light areas you see are variations in the albedo (reflectivity) of different parts of the Moon. The biggest albedo difference is between the dark maria (the lunar “seas”) and the light highlands. This is due to the compositions of the two surfaces. The maria are made of basaltic lava flows, much like those in Hawaii and Iceland. They contain iron, titanium and other dark metals. The highlands of the Moon are dominated by a bright, aluminum-rich rock called anorthosite. When you observe these light and dark regions, you’re seeing rocks that resulted from fundamentally different processes of formation.

Hues, Patches, and Rays

Click on the image to see a map of the Moon showing some of the major surface markings visible through binoculars and small telescopes. North is up and lunar east (celestial west) is to the right.
Sky & Telescope illustration; photo courtesy Lick Observatory.
If you look closely (even with your naked eye) at the maria, you can see that they’re not all the same — Procellarum and Imbrium on the western half of the Moon are lighter than the majority of the lunar seas on the Moon's eastern hemisphere. A telescope shows the most conspicuous tonal contrast in mare darkness along the southern shoreline of Mare Serenitatis. The middle of the mare is lighter than the edges, which are noticeably darker. The difference is due to the age and composition of the lavas that comprise these two regions. The rocks found near the middle of Mare Serenitatis have 1–3 percent titanium oxide and are about 3.0 billion years old, while the dark border rocks contain less than 1 percent titanium oxide and are up to 500 million years older.

Spotting the dark volcanic patches in Alphonsus at full Moon is easier than finding Alphonsus itself. Click on the image to see a pair of comparison photos, then view this section of the Moon a day or two before full to get a sense of the region.
Courtesy Gary Seronik.
Another type of dark marking visible near full Moon are small patches of volcanic ash — great examples are observable on the floors of Alphonsus and Atlas. At the center of each dark spot is a small volcanic crater. Larger, less well-defined ash deposits are visible on the east side of Sinus Aestuum, northwest of the crater Sulpicius Gallus on the western edge of Mare Serenitatis, and at the Apollo 17 landing site on the eastern edge of Mare Serenitatis.

Rays are the most conspicuous bright deposits on the Moon. They're obvious because they contain bits of bright highland rocks excavated on impact from beneath the newly formed crater. When the Moon is full, the great crater Tycho is one of the most noticeable lunar features (see the photo on the previous page). It's circled by a gray halo extending out from the rim about one crater radius.

The great crater Copernicus, an outstanding sight when near the terminator, takes on a different appearance when the Sun nearly overhead. Kepler, another rayed crater, is visible to the lower left.
Courtesy Gary Seronik.
The rays of Tycho stream as much as 1,900 kilometers to the northeast. It's uncertain if the ray that crosses the center of Serenitatis is from Tycho. If so it's the longest ray on the Moon. Compare this with Copernicus's rays, which extend 1,200 km in all directions. Other bright ray systems are centered on the craters Proclus, Byrgius A, and Kepler. Less conspicuous rays extend from Aristarchus, Anaxagoras, and Langrenus — can you see all of them?

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