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

DIURNAL REVOLUTION. 21
lengths of the days and nights. All these points will be readily understood by inspecting figure 5.
5 7. Most of the phenomena of the diurnal revolution can be
explained, either on the supposition that the celestial sphere
actually all turns around the earth once in 24 hours, or that
this motion of the heavens is merely apparent, arising from
the revolution of the earth on its axis in the opposite direction,-a motion of which we are insensible, as we sometimes
lose the consciousness of our own motion in a ship or a steamboat, and observe all external objects to be receding from us
with a commdn motion. Proofs entirely conclusive and satisfactory, establish the fact, that it is the earth and not the
celestial sphere that turns; but these proofs are drawn from
various sources, and the student is not prepared to appreciate
their value, or even to understand some of them, until he has
made considerable proficiency in the study of astronomy, and
become familiar with a great variety of astronomical phenomena. To such a period of our course of instruction we therefore postpone the discussion of the hypothesis of the earth's
rotation on its axis.
58. While we retain the same place on the earth, the diurnal revolution -occasions no change in our horizon, but our
horizon goes round as well as ourselves. Let us first take our
station on the equator at sunrise; our horizon now passes
through both the poles, and through the sun, which we are to
conceive of as at a great distance from the earth, and therefore as cut, not by the terrestrial but by the celestial horizon.
As the earth turns, the horizon dips more and more below the
sun, at the rate of 15 degrees for every hour; and, as in the
case of the polar star (Art. 50), the sun appears to rise at the
same rate. In six hours, therefore, it is depressed 90 degrees
below the sun, which brings us directly under the sun, which,
for our present purpose, we may consider as having all the
while maintained the same fixed position in space. The earth
continues to turn, and in six hours more, it completely reverses
the -position of our horizon, so that the western part of the
horizon which at sunrise was diametrically opposite to the sun
now cuts the sun, and soon afterwards it rises above the level