That component of an objects velocity directed away from or toward Earth.
radiation pressure
The force exerted on the surface of a body by its absorption of light. Small particles floating in the solar system can be blown outward by the pressure of the sun-light.
resolving power
The ability of a telescope to reveal fine detail. Depends on the diameter of the telescope.
retrograde motion
The apparent backward (westward) motion of planets as seen against the background of stars.
revolution
Orbital motion about a point located outside the orbiting body.
RNA (ribonucleic acid)
Long carbon-chain molecules that use the information stored in DNA to manufacture complex molecules necessary to the organism.
rotation
Motion aound an axis passing through the rotating body.
rotation curve
A graph of orbital velocity versus orbital radius of stars in the galaxy.
rotation curve method
A method of determing a galaxy's mass by observing the orbital velocity and orbital radius of stars in the galaxy.