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One of a chain of three prominent craters running along the western edge of Mare Nectaris, the Sea of Nectar, in the southeastern quadrant of the Moon's visible face. All three of these craters approach a diameter of 100km; Cyrillus is 98km from rim to rim, and is the central crater of the three. To the south lies Catharina, while to the north is the younger crater named Theophilus. Theophilus in fact slightly overlaps Cyrillus, with its southwestern wall interrupting Cyrillus' northeastern edge (showing that Theophilus was formed by a more recent impact event).
Cyrillus is an ancient crater with highly eroded walls, and along parts of its edges, the crater wall is worn to the point that it is barely distinguishable. The crater reaches a depth of some 3.6km, and contains a number of heights reaching up from the crater floor. Most notable among these are a group of three peaks in the northeastern part of the crater, named Cyrillus Alpha, Delta and Eta.
Enclosed entirely within the rim of Cyrillus, in the crater's southwestern quadrant, is a secondary crater designated Cyrillus A. Measuring seventeen kilometres along its longest axis, Cyrillus A has a slightly elongated shape, showing that the impact that created it must have hit the older crater Cyrillus at a angled trajectory.
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