Sidereal Year - Apparent Motion of The Sun Against The Stars

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 (except in high-latitude regions), 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 effectively uses the sidereal year.

Up to the time of Hipparchus, years measured by the stars (sidereal years) were thought to be the same as years measured by the seasons (tropical years). In fact, sidereal years are slightly longer than tropical years: One sidereal year is roughly equal to 1 + 1/26000 or 1.0000385 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 out of sync with the seasons at the rate of about one day every 72 years.