Any motion toward or away from an unmoving observer is called radial velocity, and the motion of any object is defined by both speed and direction. To define direction of the object, however, the frame of reference of the observer must be known. In normal, three-dimensional space, the observer has a frame of reference that is fixed, with any number of objects moving toward or away from his or her location.
Planets in mostly circular orbits possess little radial velocity with respect to their suns, but for fixed observers, outside the solar system, such a planet changes its motion toward and away from them throughout its orbit. The planet is seen to possess two maximum radial velocities: one positive, as the planet moves away from the observer to the far side of its sun and one negative, as the planet moves out from behind its sun toward the observer. When astronomers use telescopes to observe systems of orbiting bodies, the data is detected as electromagnetic energy. The energy waves received by the telescopes are different, depending on whether the orbiting object is moving toward or away from the scope.
The fact that energy waves from objects moving toward the observer are compressed and appear to possess a higher frequency than waves from objects moving away from the observer is called the Doppler shift, proposed by Christian Doppler in 1842. For example, as planets orbit distant stars, they tug them away from their centers of gravity, causing them to move toward or away from the observer. The slight movement of the star toward or away causes its spectrum, the rainbow colors of its light, to shift towards the blue as it moves closer and towards the red as it moves farther away. Using this radial velocity method, the timing of the shift from red to blue and back again, gives astronomers information about the mass and orbital cycle of planets orbiting distant stars.
This method can also be used in astronomy to gage the constant velocities of stars orbiting distant galaxies when they are viewed edge on. Light or radio waves received from stars moving toward the telescope shift to higher frequencies, whereas light or radio waves from stars moving away from the telescope shift toward lower frequency wavelengths. The amount of shift indicates both the relative velocity of the stars with respect to the observer and the angular velocity of the stars in orbit about the galaxy.
Weather forecasting has been helped greatly by radial velocity maps as measured by Doppler weather radar. Just as the radial velocity recorded for a rotating galaxy shows rotation by red and blue shifting of light waves, the change in frequency of radio waves indicates the rotational movement in storms such as cyclones, hurricanes, and tornados. Weather forecasters can put out tornado warnings early when they see the Doppler shift in severe weather systems.
The Doppler shift, or radial velocity method, can be used on any body or systems of bodies that are in orbit, or vibrating around a common center. Both celestial objects and weather patterns display a red shift or a blue shift, depending on whether objects are approaching or receding from the observer in the radial direction. The upper limit of radial velocity was described by Albert Einstein as the speed of light in a vacuum, and his special theory of relativity applies to this direct line-of-sight, radial motion.