Billy 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...

 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 at the southern fringes of the Oceanus Procellarum

Oceanus Procellarum

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...

, in the western hemisphere of the Moon

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...

. It lies to the southeast of the similar-sized crater Hansteen

Hansteen (crater)

Hansteen is a lunar crater that lies near the southwest edge of the Oceanus Procellarum. To the southeast is the flooded crater Billy. The rim of Hansteen is somewhat polygonal in form, especially along the eastern side. There are a few terraces along the northwestern inner wall. The inner floor...

, and west-southwest of the flooded Letronne

Letronne (crater)

Letronne is the lava-flooded remnant of a lunar crater. The northern part of the rim is completely missing, and opens into the Oceanus Procellarum, forming a bay along the southwestern shore...

.

The interior floor of Billy has been flooded by basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

, leaving a dark surface due to the low albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

. The portion of the rim remaining above the surface is narrow and low, with a thin inner wall. Only a few tiny craterlets mark the interior.

To the north of the crater is a triangular mountainous formation named Mons Hansteen, from the nearby crater. Southeast of Billy is a rille

Rille

Rille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...

, designated Rima Billy, that runs 70 kilometers to the south.

Satellite craters

By convention these features are identified on lunar maps by placing the letter on the side of the crater mid-point that is closest to Billy.

Billy	Latitude	Longitude	Diameter
A	14.3° S	46.3° W	7 km
B	12.2° S	47.6° W	25 km
C	16.1° S	49.0° W	6 km
D	14.9° S	48.3° W	11 km
E	15.0° S	49.6° W	2 km
H	15.6° S	49.6° W	3 km
K	12.9° S	48.7° W	4 km