A guide to the moon’s craters, seas, and ghostly shine

The following excerpt comes from space scientist Maggie Aderin-Pocock’s The Book of the Moon: A Guide to Our Closest Neighbor, published by Abrams Image.

The High Seas

When observing the Moon with the naked eye, the easiest things to spot are the lunar maria. These dark lunar “seas” are quite visible and show up in good contrast to the lighter highland, or terra, areas. The lunar maria were mainly named in 1651 by the Italian priest/astronomer Giambattista Riccioli, whose lunar naming system has been taken as standard. Another astronomer, Johannes Hevelius, suggested a different naming system at the same time, but Riccioli’s system stuck, probably due to the evocative and romantic language he used to describe the seas and oceans.

Some of the maria that stick out from the crowd are:

Sea of Rains (Mare Imbrium): This is one of the larger maria, but it is not quite an ocean. The sea itself has a diameter of 991 miles. Current research suggests that it was made after a cataclysmic impact with a small protoplanet in the Moon’s past.

Sea of Serenity (Mare Serenitatis): This mare has a diameter of 419 miles and is one of the locations of a mascon. Both the Soviet Luna 21 and the U.S. Apollo 17 landed in close proximity to this mare.

Sea of Crises (Mare Crisium): Some 345 miles in diameter, the strangely named Sea of Crises is another location where a mascon has been found. The Soviet space probe Luna 15 crash-landed at this site in 1969, and the Luna 24 mission returned a sample of lunar regolith from this location in 1976.

Sea of Fertility (Mare Fecunditatis): This mare is 522 miles in diameter but so far has been found to be mascon-free. The Soviet Luna 16 returned the first lunar sample from here in 1976.

Sea of Nectar (Mare Nectaris): Only 211 miles in diameter, the Sea of Nectar is one of the smaller maria, but it is darker in color, which makes it easier to spot with the naked eye.

Sea of Clouds (Mare Nubium): Another evocatively named mare, this one is 444 miles in diameter. Scientists from Spain observed and recorded an impact in this crater in September 1993.

Ocean of Storms (Oceanus Procellarum): Bigger than the average lunar mare, this has a diameter of 1,611 miles. The area is easily spotted with the naked eye because of its sheer size. The manned mission Apollo 12 landed here, along with a number of other unmanned Soviet and U.S. missions.

Contemplating Craters

If you are lucky enough to have exceptional viewing conditions, then some of the Moon’s more prominent craters may be visible too. Most of the craters visible to the naked eye have two things in common. First, they have bright rays radiating out from them. This is ejecta material that was thrown out during their formation. These radial lines make them appear bigger, brighter, and hence more visible than just an impact crater.

Second, they sit in maria. Impact craters are usually fairly light in color as they are relatively new and have not been as weathered by the solar wind as other areas. A bright impact crater sitting in a dark mare is more easily spotted with the naked eye because of the contrast between the colors of the two.

Some of the more visible craters are:

Aristarchus: This is a prominent crater that sits in the Ocean of Storms. It is one of the brightest formations on the Moon’s surface. The crater is larger than the Grand Canyon in size, having a diameter of 25 miles across and a depth of 2.2 miles. Riccioli named it after the Greek astronomer Aristarchus of Samos.

Copernicus: This crater is about 62 miles in diameter, and has an extensive system of rays. This crater looks its best when viewed close to the terminator (the division between the illuminated and dark hemispheres of the Moon) when the prominent rim is lit up against the contrasting shadow of the crater floor.

Kepler: This bright crater, like its near neighbor Copernicus, has a system of rays. Unlike most features, which are best seen at or near the terminator, the rays are most visible when the Moon is full. Its diameter is approximately 20 miles.

Tycho: This crater sits near the south pole region of the near side of the Moon. It is pleasingly circular in shape and is surrounded by bright ejecta radiating out from it. It has a fresh look about it and is thought to be relatively young as its rim and ejecta area seem not to have been pummeled by lunar impacts. It has a diameter of 52 miles.

Earthshine: Ghost Moon

As well as spotting features on the lunar surface, an interesting thing to look out for when observing the Moon with the naked eye is a phenomenon called earthshine. It appears as a pale glow that lights up the unlit portion of the Moon, like a ghostly version of a full moon.

Earthshine is most clearly visible a few days before and after a new moon, when the crescent moon is close to the horizon at sunset or sunrise. In the past, people looked at this spectacle and wondered what was happening. They called the phenomena “ashen glow,” or said that “the old moon is in the new moon’s arms.”

This astronomical marvel was eventually explained by the polymath Leonardo da Vinci, who, as well as all his other achievements, had a keen interest in astronomy. He realized that as well as light leaving the Sun, hitting the Moon, and being reflected back to the viewer on Earth, some of the sunlight could also hit Earth, be reflected up to the Moon (some of it hitting the night side of the Moon), and then get re-reflected back to an observer on Earth. The Earth light, hitting the night side of the Moon, is what causes the earthshine.

Excerpt from the new book The Book of the Moon: A Guide to Our Closest Neighbor (Abrams Image) by Dr. Maggie Aderin-Pocock © 2019 Dr. Maggie Aderin-Pocock.