Day

A day is one of the, if not the, most fundamental astronomical unit of time. There exist many different definitions of a day.

Solar Day

The most common definition of a day, the solar day is the time it takes for the Sun to return to the same position in the sky. This is the time between two successive noons. Alternatively, it can also be defined as the time it takes for the Earth to rotate once on its axis.

The length of a solar day is taken to be 24 hours.

Sidereal Day

Astronomers, both amateur and professional, often use the sidereal day as a standard unit of time. It is the time it takes for the Earth to rotate once on its axis, relative to the stars. Alternatively, it can also be defined as the time it takes for a star to return to the same position in the sky, for example, the time it takes for the star Vega to return to the same position in the sky.

The length of a sidereal day is roughly 23 hours, 56 minutes and 4 seconds.

A solar day is slightly longer than a sidereal day. This is because Earth not only rotates on its axis, but also revolves around the Sun. This means that the Earth has to rotate a little more than one full rotation to return to the same position relative to the Sun, since the Sun wouldn’t have returned to the same position relative to the observer at Earth yet. To complete a solar day, Earth must rotate an additional amount, equal to \(1/365\) of a full turn. The time required for this extra rotation is roughly \(1/365\) of a sidereal day, which is roughly 4 minutes.

Since most of the ordinary clocks are set to the solar day, stars rise 4 minutes earlier each day.

This is also the reason why the Sun appears to move eastward relative to the stars.