Radiant:
The point on the celestial sphere from which meteors of a given shower appear
to radiate. The effect is due to perspective.
Retrograde motion:
Westward or backward drift of a planet resulting from the forward motion of the
earth as it passes the planet.
Revolution:
The orbital motion of a body around its primary.
Rift:
An apparent division of the Milky Way caused by dark clouds of superposed dust.
Right ascension (RA):
Angle, measured eastward, from the vernal equinox to the footof a star's hour
circle.
Rotation:
Turning of an object on its axis.