Cyrillus 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...

 located on the northwest edge of Mare Nectaris

Mare Nectaris

The Sea of Nectar is a small lunar mare or sea located between the Sea of Tranquillity and the Sea of Fecundity . Montes Pyrenaeus borders the mare to the west and the large crater near the south center of the mare is known as Rosse...

. Intruding into the northeast rim is the equally large, and younger crater Theophilus

Theophilus (crater)

Theophilus is a prominent lunar impact crater that lies between Sinus Asperitatis in the north and Mare Nectaris to the southeast. It partially intrudes into the comparably sized crater Cyrillus to the southwest. To the east is the smaller crater Mädler and further to the south-southeast is Beaumont...

. To the south is another prominent crater named Catharina

Catharina (crater)

Catharina is an ancient lunar impact crater located in the southern highlands. It lies in a rugged stretch of land between the Rupes Altai scarp to the west and Mare Nectaris in the east. To the west-northwest is the crater Tacitus, and the lava-flooded Beaumont lies to the east along the shore of...

. Together these three craters form a prominent trio in the southeast quadrant 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...

. To the northwest is Ibn-Rushd

Ibn-Rushd (crater)

Ibn-Rushd is a lunar impact crater located to the northwest of the larger crater Cyrillus. To the northwest is the crater Kant and to the north is Mons Penck, a mountain promontory. The crater is somewhat eroded with age, and the southern rim is overlain by a pair of smaller craters named Cyrillus...

. Cyrillus is named after Saint Cyril of Alexandria

Cyril of Alexandria

Cyril of Alexandria was the Patriarch of Alexandria from 412 to 444. He came to power when the city was at its height of influence and power within the Roman Empire. Cyril wrote extensively and was a leading protagonist in the Christological controversies of the later 4th and 5th centuries...

, a 4th-century Coptic Pope and theologian.

The floor of Cyrillus contains a reduced central hill and the considerable crater Cyrillus A. The walls of the broken formation of Cyrillus remain intact up until the point of junction with Theophilus. Slightly northeast of its center, three rounded mountains rise to heights of 1,000 meters above Theophilus' floor: Cyrillus Alpha, Delta, and Eta.

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 Cyrillus.

Cyrillus	Latitude	Longitude	Diameter
A	13.8° S	23.1° E	17 km
C	12.3° S	21.5° E	12 km
E	15.8° S	25.3° E	11 km
F	15.3° S	25.5° E	44 km
G	15.6° S	26.6° E	8 km

The following craters have been renamed by the IAU

International Astronomical Union

The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

.

• Cyrillus B — See Ibn-Rushd crater
  Ibn-Rushd (crater)
  Ibn-Rushd is a lunar impact crater located to the northwest of the larger crater Cyrillus. To the northwest is the crater Kant and to the north is Mons Penck, a mountain promontory. The crater is somewhat eroded with age, and the southern rim is overlain by a pair of smaller craters named Cyrillus...
  
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