Sidereal year
The
sidereal year is the time for the
Sun to return to the same position in respect to the
stars of the
celestial sphere. The sidereal year is the orbital period of
Earth. A sidereal year equals 365.25636042
mean solar days. The sidereal year is
20 minutes and
24 seconds longer than the
tropical year.
As the Sun and the stars cannot be seen at the same time, this requires a little explanation. If you look every
dawn at the eastern sky, the last stars you see appearing are not always the same. In a week or so you notice an upward shift. So in July in the Northern Hemisphere you cannot see
Orion in the dawn sky, but in August it begins to be visible. In a year, all the constellations rotate through the entire sky.
For those in the habit of looking at the sky before dawn, this motion is much more noticeable and much easier to measure than the north-south shift of the sunrise point in the horizon, which defines the
tropical year that the
Gregorian calendar is based on. This is why many cultures started their years on the first day a particular special star (
Sirius, for instance) could be seen at 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.
Up to the time of
Hipparchus, the years measured by the stars were thought to be exactly as long as the tropical years. In fact, sidereal years are very slightly longer than tropical years. The difference is caused by the
precession of the equinoxes. One sidereal year is roughly equal to 1 + 1/26000 or 1.000039 tropical years.
*
Anomalistic year*
Gaussian year*
Julian year (astronomy)*
Tropical year
