Why is the ninth month called September?
|sidereal time |
Time based on the rotation of the Earth with respect to the background of fixed stars. Astronomers generally use sidereal time rather than solar time because it is better suited to observations beyond the solar system. ◇ A sidereal day is the time required for one complete rotation of the Earth on its axis with respect to a fixed star. It is an unvarying unit equal to 23 hours, 56 minutes, 4.09 seconds of solar time. ◇ A sidereal month is the average period of revolution of the Moon around the Earth with respect to a fixed star, equal to 27 days, 7 hours, 43 minutes of solar time. ◇ A sidereal year is the time required for one complete revolution of the Earth around the Sun with respect to a fixed star, equal to 365 days, 6 hours, 9 minutes, 9.54 seconds of solar time. Compare solar time.
time as measured by the apparent motion about the Earth of the distant, so-called fixed, stars, as distinguished from solar time, which corresponds to the apparent motion of the Sun. The primary unit of sidereal time is the sidereal day, which is subdivided into 24 sidereal hours, 1,440 sidereal minutes, and 86,400 sidereal seconds. Astronomers rely on sidereal clocks because any given star will transit the same meridian at the same sidereal time throughout the year. The sidereal day is almost 4 minutes shorter than the mean solar day of 24 of the hours shown by ordinary timepieces.