SECT. XII. SIDEREAL TIME. 83 



SECTION XIT. 



Mean and Apparent Sidereal Time Mean and Apparent Solar Time 

 Equation of Time English and French Subdivisions of Time Leap 

 Year Christian Era Equinoctial Time Remarkable Eras de- 

 pending upon the Position of the Solar Perigee Inequality of the 

 Lengths of the Seasons in the two Hemispheres Application of Astro- 

 nomy to Chronology English and French Standards of Weights and 

 Measures. 



ASTRONOMY has been of immediate and essential use in affording 

 invariable standards for measuring duration, distance, magnitude, 

 and velocity. The mean sidereal day measured by the time 

 elapsed between two consecutive transits of any star at the same 

 meridian (N. 148), and the mean sidereal year which is the time 

 included between two consecutive returns of the sun to the same 

 star, are immutable units with which all great periods of time 

 are compared; the oscillations of the isochronous pendulum 

 measure its smaller portions. By these invariable standards 

 alone we can judge of the slow changes that other elements of 

 the system may have undergone. Apparent sidereal time, which 

 is measured by the transit of the equinoctial point at the meri- 

 dian of any place, is a variable quantity, from the effects of pre- 

 cession and nutation. Clocks showing apparent sidereal time are 

 employed for observation, and are so regulated that they indicate 

 Qh Qm Q 8 a j. ^ ne ms t an f; the equinoctial point passes the meridian 

 of the observatory. And as time is a measure of angular motion, 

 the clock gives the distances of the heavenly bodies from the 

 equinox by observing the instant at which each passes the meri- 

 dian, and converting the interval into arcs at the rate of 15 to 

 an hour. 



The returns of the sun to the meridian and to the same equinox 

 or solstice have been universally adopted as the measure of our 

 civil days and years. The solar or astronomical day is the time 

 that elapses between two consecutive noons or midnights. It is 

 consequently longer than the sidereal day, on account of the 

 proper motion of the sun during a revolution of the celestial 



