Thursday, May 21, 2009

Sideral Time

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars. They differ by the precession of the vernal equinox in right ascension relative to the stars.

Earth's sidereal day also differs from its rotation period relative to the background stars by the amount of precession in right ascension during one day. Sideral time means to measure time relative to the position of the stars.

Sidereal time is defined as the hour angle of the vernal equinox. When the meridian of the vernal equinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal Time is the hour angle of the vernal equinox at the prime meridian at Greenwich, England, local values differ according to longitude. When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours.

Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observed at a given time. Objects are located in the night sky using right ascension and declination relative to the celestial equator, and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or at which time it is best placed for observation.

Solar time is measured by the apparent diurnal 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 or north depending on the observer's latitude and the season). The average time taken for the sun to return to its highest point is 24 hours.

During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the Earth moves a short distance (approximately 1°) along its orbit around the sun. After a sidereal day, the Earth still needs to rotate a small extra angular distance before the sun reaches its highest point. A solar day is, nearly 4 minutes longer than a sidereal day.

The stars, are so far away that the Earth's movement along its orbit makes a generally negligible difference to their apparent direction and so they return to their highest point in a sidereal day. A sidereal day is almost 4 minutes shorter than a mean solar day. Another way to see this difference is to notice that, relative to the stars, the Sun appears to move around the Earth once per year. Which means, there is one less solar day per year than there are sidereal days.