Celestial Bodies 141 
ages. Before the introduction of the marine chronometer, ob- 
servations of the moon (so-called “lunars”) were necessary 
for a determination of longitude. In addition to the observa- 
tions, precise tables of the moon’s position were also re- 
quired. Many of the best scientific minds—Newton, Laplace, 
Delaunay—spent their time trying to unravel the mysteries 
of the moon’s motion. Modern lunar theories call for exten- 
sive use of the most advanced dynamical theories. They re- 
quire long series computations which involve hundreds of 
separate periodic terms depending on such different things 
as the distribution of density of materials in the earth, the 
earth's oblateness, all of the planets in the solar system, the 
retardation of the tides, the ellipticity of the earth’s orbit. 
Separating these various effects and determining their magni- 
tude empirically and theoretically is a painstaking and difh- 
cult task. Only recently has the theory of the moon’s motion 
been satisfactorily completed by the late Professor Ernest 
Brown of Yale who spent the major portion of his life on this 
task. He would awaken about two o'clock in the morning, 
do his calculations propped up in bed until breakfast time 
when he would get up and get ready to teach his classes. 
Lunar sights are no longer necessary however for determina- 
tion of time at sea in a world which enjoys the benefits of 
radio time signals and accurate chronometers. 
THE PLANETS 
The planets, of which the Earth is one, all revolve around 
the sun. They are the principal members of the solar system. 
Some are accompanied in their travels by smaller satellites. 
The Earth has one satellite, the Moon. Other planets have a 
number of satellites, some of these moons being the merest 
