An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

40 THE EARTt.
all the solar days throughout the year. This is the period
which constitutes the civil day of 24 hours, beginning when
the sun is on the lower meridian, namely, at 12 o'clock at
night, and counted by 12 hours fromn the lower to the upper
culmination, and from the upper to the lower. The astrononzical day is the apparent solar day counted through the whole
24 hours, instead of by periods of 12 hours each, and begins at
noon. Thus 10th day and 14th hour of astronomical time,
would be 11th day and 2d hour of civil time.
104. Clocks are usually regulated so as to indicate mean
solar time; yet as this is an artificial period, not marked off,
like the sidereal day, by any natural event, it is necessary to
know how much is to b9 added to or subtracted from the ai
parent solar time, in order to give the corresponding mean
time. The interval by which apparent time differs from mean
time, is called the equation of time. If a clock were constructed (as it may be) so as to keep exactly with the sun, going
faster or slower according as the increments of right ascension
were greater or smaller, and another clock were regulated to
mean time, then the difference of the two clocks at any period,
would be the equation of time for that moment. If the apparent clock were faster than the mean, then the equation of
time must be subtracted; but if the apparent clock were slower
than the mean, then the equation of time must be added, to
give the mean time. The two clocks would differ most about
the 3d of November, when the apparent time is 161m greater
than the mean (16m 17T). But, since apparent time is sometimes greater and sometimes less than mean time, the two
must obviously be sometimes equal to each other. This is in
fact the case four times a year, namely, April 15th, June 15th,
September 1st, and December 22d. These epochs, however,
do not remain constant; for, on account of the change in the
position of the perihelion, or the point where the earth is nearest the sun (which shifts its place from west to east about 12"
a year), the period when the sun's motions are most rapid, as
well as that when they are slowest, will occur at different parts
of the year. The change is indeed exceedingly small in a single year; but in the progress of ages, the time of year when the
sun's motion in its orbit is most accelerated, will not be, as