SECT, xii.] SIDEREAL TIME. 93 



SECTION XII. 



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 depending upon 

 the Position of the Solar Perigee Inequality of the Lengths of the Seasons 

 in the two Hemispheres Application of Astronomy 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, dis- 

 tance, magnitude, and velocity. The mean sidereal day 

 measured by the time elapsed between two consecutive 

 transits of any star at the same meridian, and the mean 

 sidereal year which is the time included between two conse- 

 cutive 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 

 meridian of any place, is a variable quantity, from the effects 

 of precession and nutation. Clocks showing apparent sidereal 

 time are employed for observation, and are so regulated that 

 they indicate O b O m s at the instant 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 meridian, 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 



