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 365.2564 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.

If you are 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.