Sidereal, << sy DIHR ee uhl, >> time measures the rotation of the earth in relation to the stars. By contrast, mean solar time (average solar time), which we use to tell time in our daily lives, measures the earth’s rotation in relation to the sun. Astronomers use sidereal time because the stars are in the same place in the sky at the same sidereal time each night. The stars are not in the same place at the same mean solar time each night.
A sidereal day consists of 24 sidereal hours. It is the time the earth takes to make one rotation on its axis with respect to an imaginary line that extends from earth’s center to a point in the sky called the vernal equinox (see Equinox ). To measure a solar day, astronomers determine the time required for the earth to make one rotation with respect to an imaginary line from the earth’s center to the sun (see Time (Measuring time) ).
A mean solar day is longer than a sidereal day. To understand why this is so, consider a point on the earth’s surface that is on the imaginary line to the vernal equinox and the imaginary line to the sun. During the time this point takes to make one rotation with respect to the line to the vernal equinox (one sidereal day), the earth moves about one degree along its orbit. As a result, the point has not yet reached the line to the sun. The point needs nearly 4 additional minutes to rotate to this line. A mean solar day equals 24 hours 3 minutes 56.555 seconds of sidereal time.
