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

CHAPTER III.
OF PARgALLAX, REFRACTION, AND TWILIGHT.
80. PARALLAX is the apparent change of place which bodies
undergo by being viewedfrom dfferent points. This change is
called diurnal parallax, when one point is at the surface of the
~__ Fig. 6.
earth, and the other at the center. Thus (Fig. 6), an observer
on the surface at A would see the moon F at p on the sky, while
from the center C, it would appear at P; or, if the moon were
at E, H would be its apparent place as seen from the center,
and A from the surface. The angle AEC, or AFOC, is the diuerncal pcarallax; and Hh or Pp is the parallactic arc. It appears,
therefore, that the diurnal parallax of a body is the angle at
that body, subtended by the earth's radius.
81. Since then a heavenly body is liable to be referred to
different points on the celestial vault, when seen from different
parts of the earth, and thus some confusion occasioned in the
determination of points on the celestial sphere, astronomers
have agreed to consider the true place of a celestial object to
be that where it would appear if seen from the center of the