What defines a sidereal day?

A sidereal day is defined as the time it takes for the Earth to complete one full rotation around its axis relative to a particular star, typically the distant background stars. This duration is approximately 23 hours, 56 minutes, and 4 seconds. Since it is based on the Earth's rotation relative to stars rather than the sun, it accounts for the slight difference that occurs between the time it takes for the Earth to rotate once on its axis and the time it takes to orbit the sun, known as a solar day.

The alternatives presented involve different astronomical measurements. A complete solar day relates to the sun and takes about 24 hours, marking the time from one solar noon to the next. The average time for the Earth to orbit the sun pertains to a year, not a day. The movement of the moon is associated with lunar phases and cycles, rather than defining a day. These distinctions clarify why the definition of a sidereal day focuses solely on Earth’s rotation in relation to the stars.