solar time............

Solar Time
May 6, 2010

SOLAR TIME Solar Time, or sun time, is determined in two ways. Apparent Solar Time is the system of days and hours which goes strictly by the sun itself and is thus continually changing in value from day to day. It is the time between two successive transits of the sun over the same meridian. Mean Solar Time is the system of days and hours mathematically computed in order to give the average value to every hour and day. It is 24 hours. The difference in value between apparent and mean solar time is known as equation of time.

SIDEREAL TIME OR STAR TIME Whereas the sun moves sometimes slow and sometimes fast, with a total range of half-an-hour from one extreme to the other, the stars provide a perfect time-piece. But they do not operate according to the conventional systems of days and hours that our calendars follow. A star takes 23 hours 56 minutes of mean solar time and 4.09 seconds to complete one rotation of the earth, covering 360°. This interval is called a sidereal day, which is thus about 4 minutes shorter than
the mean solar day of 24 hours.