It is customary to call them all alike ``stars.'' But
since the planets more or less rapidly change their places in the sky,
in consequence of their revolution about the sun, while the stars
proper seem to remain always in the same relative positions, the
latter are spoken of as ``fixed stars.'' In the beginnings of
astronomy it was not known that the ``fixed stars'' had any motion
independent of their apparent annual revolution with the whole sky
about the earth as a seeming center. Now, however, we know that the
term ``fixed stars'' is paradoxical, for there is not a single really
fixed object in the whole celestial sphere. The apparent fixity in the
positions of the stars is due to their immense distance, combined with
the shortness of the time during which we are able to observe them. It
is like viewing the plume of smoke issuing from a steamer, hull down,
at sea: if one does not continue to watch it for a long time it
appears to be motionless, although in reality it may be traveling at
great speed across the line of sight. Even the planets seem fixed in
position if one watches them for a single night only, and the more
distant ones do not sensibly change their places, except after many
nights of observation. Neptune, for instance, moves but little more
than two degrees in the course of an entire year, and in a month its
change of place is only about one-third of the diameter of the full
moon.
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