The sidereal year is the time taken for the Sun to return to the same position with respect to the stars of the celestial sphere. It is the orbital period of Earth, equal to 365.25636042 mean solar days (31,558,149.540 seconds), that is 366.25636042 earth rotations or sidereal days. The sidereal year is 20 minutes and 24 seconds longer than the tropical year.
The word "sidereal" means "relating to the stars". It derives from the Latin sidus, meaning "star".
Apparent motion of the Sun against the stars
As the Earth orbits the Sun, the apparent position of the Sun against the stars gradually moves along the ecliptic, passing through the twelve traditional constellations of the zodiac, and returning to its starting point after one sidereal year. This motion is difficult to observe directly because the stars cannot be seen when the Sun is in the sky. However, if one looks regularly at the sky before dawn, the annual motion is very noticeable: the last stars seen to rise are not always the same, and within a week or two an upward shift can be noted. As an example, in July in the Northern Hemisphere, Orion cannot be seen in the dawn sky, but in August it becomes visible.
This effect is easier to measure than the north/south movement of the position of sunrise,citation needed which defines the seasonal cycle and the tropical year on which the Gregorian calendar is based. For this reason many cultures started their year on the first day a particular special star (Sirius, for instance) could be seen in the east at dawn. In Hesiod's Works and Days, the times of the year for sowing, harvest, and so on are given by reference to the first visibility of stars. Such a calendar is effectively using the sidereal year.
Up to the time of Hipparchus,[1] the years measured by the stars (sidereal years) were thought to be exactly as long as the years measured by the seasons (tropical years). In fact, sidereal years are very slightly longer than tropical years: one sidereal year is roughly equal to 1 + 1/26000 or 1.000039 tropical years. The difference is caused by the precession of the equinoxes, and means that over long periods of time a calendar based on the sidereal year will drift wildly out of sync with the seasons.
Notes
- ^ Although Hipparchus is usually credited with the discovery of the precession of the equinoxes, there is speculation that various ancient cultures, long before his time, were aware of it.
See also
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