_Celestial Bodies 137 
which take into account not only the planets’ motion about 
the sun, but also their mutual attractions and perturbations 
according to the Law of Universal Gravitation and Newton’s 
Laws of Motion. Positions of celestial bodies are listed by 
two quantities: right ascension and declination on the celes- 
tial sphere, just as geographical positions are given by longi- 
tude and latitude on the earth. Right ascension is measured 
from a standard meridian in the sky, the vernal equinoctial 
colure, as longitude on the earth is measured from the Green- 
wich meridian. Declination is measured north and south of 
the celestial equator, as latitude on earth is measured north 
and south of the equator. 
THE ZODIAC 
Furthermore the sun, moon, and planets all follow, more or 
less closely, a definite path in the sky called the ecliptic. 
Along the ecliptic are a series of twelve constellations, 
groups or configurations of stars, called the Signs of the 
Zodiac. A rough way of locating the sun, moon or the planets 
is to designate the particular Sign of the Zodiac in which 
the body is at the time. The apparent positions of the celes- 
tial bodies are essential to celestial navigation. By means of a 
comparison between the right ascension and declination of 
celestial bodies as given in an almanac, and the observed 
height above the horizon (altitude) and bearing (azimuth) 
of the same bodies, the navigator is able to determine with 
sufficient accuracy the position of his ship at sea (i.e., its 
latitude and longitude). 
THE STARS 
Besides the sun, moon and planets, the other visible astro- 
nomical objects are several thousand points of light of vari- 
