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

30 TTHE EARTH.
by the semi-diameter of the earth as seen perpendicularly from
the body. Or if the horizontal parallax is given, the parallax
at any altitude may be found, for
P'- =P x sin zenith distance.
Hence, in the zenith the parallax is nothing, and is at its
maximnunm in the horizon.
S4. It is evident from the figure, that the effect of parallax
upon the place of a celestial body is to depress it. Thus, in
consequence of parallax, E is depressed by the are HA; F by
the are Pp; G by the are Rr; while 0 sustains no change.
Hence, in all calculations respecting the altitude of the sun,
moon, or planets, the amount of parallax is to be added; the
stars, as we shall see hereafter, have no sensible parallax. As
the depression which arises from parallax is in the direction of
a vertical circle, a body, when on the meridian, has only a
parallax in declination; but in other situations, there is at the
same time a parallax in declination and right ascension; for
its direction being oblique to the equinoctial, it can be resolved
into two parts, one of which (the declination) is perpendicular,
and the other (the right ascension) is parallel to the equinoctial.
85. The mode of determining the horizontal paralx, is as
follows:
Let 0, O' (Fig. 7), be two places on the earth, situated under the same meridian, at a Fig. i.
great distance from each other; one place, for example, at Y_ M
the Cape of Good Hope, and \
the other in the north of Europe. The latitude of each
place being known, the are of
the meridian 00' is known,
and the angle OCO' also is
known. Let the celestial /
body M (the moon for example), be observed simultane-
ously at 0 and 0', and its
zenith distance at each place