# The New Student's Reference Work/Satellite

Satellite (săt′ ĕl-līt), a small planet or moon.  The term is generally employed in astronomy to denote one planet revolving about another; and hence satellites are sometimes called secondary planets, to distinguish them from the primary or larger planets.  One of the chief points of interest in connection with satellites is that a satellite enables one to determine the mass of the primary as soon as the radius of its orbit, r, and its period, T, are known.  For then, if we denote the mass of the primary by M and the mass of the secondary by m, we have

${\displaystyle {\mbox{M}}+m=\left({\frac {2\pi }{\mbox{T}}}\right)^{3}r^{2}}$

and, since m is practically always negligible in comparison with M, we have

${\displaystyle {\mbox{M}}=\left({\frac {2\pi }{\mbox{T}}}\right)^{3}r^{2}}$

For a list of the known satellites see Webster’s Unabridged Dictionary, article Solar System.  On the evolution of satellites see Time and Tide by Sir Robert Ball in the Romance of Science Series.