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

DOCTRINE OF THE SPHERE. 15
4AsCen~t.ywrn is the angular distance from the vernal equinox, measured on the equator. If a star is situated on the equator, then
its right ascension is the number of degrees of the equator between the star and the vernal equinox. But if the star is north
or south of the equator, then its right ascension is the are of
the equator intercepted between the vernal equinox and that
secondary to the equator which passes through the star.
Declinaction is the distance of a body from the equator, measured oil a secondary to the latter. Therefore, right ascension
and declination correspond to terrestrial longitude and latitude;
right ascension being reckoned frorm the equinoctial colure, in
the same manner as longitude is reckoned from the meridian
of Greenwich. On the other hand, celestial longitude and latitude are referred, not to the equator, but to the ecliptic.
Celestial Zonyitucde, is the distance of a body from the vernal
equinox reckoned on the ecliptic. Celestial Latituzce, is the
distance from the ecliptic measured on a secondary to the latter. Or, more briefly, Longitude is distance on the ecliptic:
Latitude, distance Jfiom the ecliptic. The North Polar DIislance of a star, is the complement of its declination.
Fig. 4.
38. Parallels of Latiztcle are Z
small circles parallel to the equator.
They constantly diminish in size as E
we go from the equator to the pole,
the radius being always equal to the c /
cosine of the latitude. In fig. 4, let1
HO be the horizon, EQ the equator,
PP the axis of the earth, ZN the Q
prime vertical, and ZL a parallel of P
latitude of any place Z. Then ZE is l,the latitude (Art. 31), and ZP the complement of the latitude;
but Zn, the radius of the parallel of latitude ZL, is the sine of
ZP, and therefore the cosine of the latitude.
39. The Troplics are the parallels of latitude that pass
through the solstices. The northern tropic is called the tropic
of Cancer; the southern, the tropic of Capricorn.
40. The Polar Circles are the parallels of latitude that pass