Sidereal Clock

Sidereal Clock is a specially-regulated clock for measuring sidereal time. Accuracy is the great essential of observatory clocks, and so carefully are they now made that their variation seldom exceeds a second per day. Jewelling the holes has greatly diminished the errors due to friction, but careful compensation of the pendulum [PENDULUM], so that its length is the same whatever be the temperature, has been the chief cause of the accuracy now attainable. Time measured in an observatory begins, not at noon as in an ordinary clock, but at the moment when the vernal point of intersection of ecliptic and equator - the first point of Aries - crosses the meridian, and from this point the hours go on till 24 o'clock. The time of the clock is constantly checked by means of the transits of certain stars. A number of such reference stars have had their right ascensions very accurately determined. Every day some of these are observed, and give the error of the clock. To find the right ascension of any celestial object, the time of its transit is noted, and its angular right ascension is got by converting the time into angles at the rate of 15° for each hour.