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

TIME. 39
ages of the world, show that they all perform their diurnal
revolutions in the same time, and that their motion during any
part of the revolution is perfectly uniform.
101. Solar time is reckoned by the apparent revolution of
the sun, from the meridian round to the same meridian again.
Were the sun stationary in the heavens, like a fixed star, the
time of its apparent revolution would be equal to the revolution of the earth on its axis, and the solar and the sidereal days
would be equal. But since the sun passes from west to east,
through 360~ in 365k- days, it moves eastward nearly 1~ a day
(59' 8".3). While, therefore, the earth is turning round on its
axis, the sun is moving in the same direction, so that when we
have come round under the same celestial meridian from which
we started, we do not find the sun there, but he has moved
eastward nearly a degree, and the earth must perform so much
more than one complete revolution, in order to come under
the sun again. Now since a place on the earth gains 360~ in
24 hours, how long will it take to gain 1~?
360: 24:: 1: =4 nearly.
Hence the solar day is about 4 minutes longer than the sidereal; and if we were to reckon the sidereal day 24 hours, we
should reckon the solar day 24h. 4:m. To suit the purposes of
society at large, however, it is found most convenient to reckon
the solar day 24 hours, and to throw the fraction into the sidereal day. Then,
24h. 4m.: 24: 24: 23h. 56m. (23h. 56m 4".09)=the length of
a sidereal day.
102. The solar days, however, do not always differ from
the sidereal by precisely the same fraction, since the increments
of right ascension (Art. 37), which measure the difference between a sidereal and a solar day, are not equal to each other.
Apparent time is time reckoned by the revolutions of the sun
from the meridian to the meridian again. These intervals
being unequal, of course the apparent solar days are unequal
to each other.
103. lean time is time reckoned by the average length of