Theaetetus is a lunar

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 impact crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 that is located to the southeast of the crater Cassini

Cassini (lunar crater)

Cassini is a lunar impact crater that is located in the Palus Nebularum, at the eastern end of Mare Imbrium. To the northeast is the Promontorium Agassiz, the southern tip of the Montes Alpes mountain range. South by south-east of Cassini is the crater Theaetetus. To the northwest is the lone peak...

 near the eastern edge of Mare Imbrium

Mare Imbrium

Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon and one of the larger craters in the Solar System. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...

. It lies just to the west of the Montes Caucasus

Montes Caucasus

Montes Caucasus is a rugged range of mountains in the northeastern part of the Moon. It begins at a gap of level surface that joins the Mare Imbrium to the west with the Mare Serenitatis to the east, and extends in an irregular band to the north-northeast to the western side of the prominent crater...

 range, which forms the eastern shore of the mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

. To the southwest is the prominent crater Aristillus

Aristillus (crater)

Aristillus is a prominent lunar impact crater that lies in the eastern Mare Imbrium. Directly to the south is the smaller crater Autolycus, while to the southwest is the large Archimedes. The area of mare to the southwest is named the Sinus Lunicus...

.

The rim of Theaetetus is distinctly polygon

Polygon

In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...

al in shape, with a slight rounding at the vertices. There is a low outer rampart and a slight central rise on the crater floor, which is offset to the northeast of the crater mid-point. The interior is otherwise relatively featureless.

This crater has been noted in the past as a site of possible transient lunar phenomena

Transient lunar phenomenon

A transient lunar phenomenon , or lunar transient phenomenon , is a short-lived light, color, or change in appearance on the lunar surface....

. In 1902, a white cloud was observed briefly in the vicinity of the crater. Other observers, including Patrick Moore

Patrick Moore

Sir Patrick Alfred Caldwell-Moore, CBE, FRS, FRAS is a British amateur astronomer who has attained prominent status in astronomy as a writer, researcher, radio commentator and television presenter of the subject, and who is credited as having done more than any other person to raise the profile of...

 and W. H. Pickering

William Henry Pickering

William Henry Pickering was an American astronomer, brother of Edward Charles Pickering. He was elected a Fellow of the American Academy of Arts and Sciences in 1883.-Work:...

, have also noted unusual appearances in this area.