Sidereal time - Definition and Overview

Sidereal time is time measured by the motion of the stars rather than the sun.

Solar time is measured by the motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south in the northern hemisphere and due north in the southern hemisphere). The time taken for the sun to return to its highest point is exactly 24 hours, or a solar day.

However, the stars move in a slightly different way. During the course of one day, the earth has moved a short distance along its orbit around the sun, and so must rotate a small extra angular distance before the sun reaches its highest point. The stars, however, are so distant that the earth's movement along its orbit makes no difference to their apparent direction, and so they return to their highest point in slightly less than 24 hours. A sidereal day is about 23h56m in length.

Sidereal time is defined so that at local midnight on the Vernal equinox, local sidereal time is 00:00. Sidereal clocks are used at astronomical observatories, because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time. Objects are located in the night sky using Right ascension and declination (analogous to longitude and latitude), and when the sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, at which time it is best placed for observation as atmospheric extinction is minimised.

See also

• Terrestrial Time
• Coordinated Universal Time
• Universal Time
• International Atomic Time
• sidereal day
• sidereal year
• solar time
• time

External links

• For more details, see the article on sidereal time (http://docs.kde.org/en/3.1/kdeedu/kstars/ai-sidereal.html) from Jason Harris' Astroinfo, which comes along with KStars, a Desktop Planetarium for Linux/KDE.
• See also KStars website (http://edu.kde.org/kstars/index.phtml).