78 SOLAR TIME. SECT. XII. 



time are compared ; the oscillations of the isochronous 

 pendulum measure its smaller portions. By these in- 

 variable 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 h 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 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 

 sphere. But as the sun moves with greater rapidity at 

 the winter than at the summer solstice, the astronomi- 

 cal day is more nearly equal to the sidereal day in sum- 

 mer than in winter. The obliquity of the ecliptic also 

 affects its duration ; for near the equinoxes the arc of 

 the equator is less than the corresponding arc of the 

 ecliptic, and in the solstices it is greater (N. 145). The 

 astronomical day is therefore diminished in the first 

 case, and increased in the second. If the sun moved 

 uniformly in the equator at the rate of 59' 8"- 33 every 

 day, the solar days would be all equal. The time there- 

 fore which is reckoned by the arrival of an imaginary 

 sun at the meridian, or of one which is supposed to 

 move uniformly in the equator, is denominated mean 

 solar time, such as is given by clocks and watches in 

 common life. When it is reckoned by the arrival of the 

 real sun at the meridian it is apparent time, such as is 

 given by dials. The difference between the time shown 

 by a clock and a dial is the equation of time given in 



