Radial velocity is the motion of an object towards or away from a stationary observer, defined by both velocity and direction. Planets in circular orbits have little radial velocity relative to their suns, but to stationary observers outside the solar system, the planet changes its motion towards and away from them during its orbit. The Doppler shift, proposed by Christian Doppler in 1842, is the fact that energy waves of objects moving towards the observer appear to possess a higher frequency than the waves of objects moving away from the observer. This method can be used to measure the constant velocities of stars orbiting distant galaxies and to aid weather forecasting. The upper limit of radial velocity is the speed of light in a vacuum, and Albert Einstein’s special theory of relativity applies to this direct line-of-sight, radial motion.
Any motion towards or away from a stationary observer is called radial velocity, and the motion of any object is defined by both velocity and direction. To define the direction of the object, however, it is necessary to know the frame of reference of the observer. In normal, three-dimensional space, the observer has a fixed frame of reference, with any number of objects moving towards or away from his position.
Planets in mostly circular orbits possess little radial velocity relative to their suns, but to stationary observers outside the solar system, such a planet changes its motion towards and away from them during its orbit. The planet is seen to have two maximum radial velocities: a positive one, as the planet moves away from the observer towards the opposite side of its sun, and a negative one, as the planet moves away from behind its sun towards the observer. When astronomers use telescopes to observe systems of orbiting bodies, the data is detected as electromagnetic energy. The energy waves received by telescopes are different, depending on whether the orbiting object approaches or moves away from the telescope.
The fact that the energy waves of objects moving towards the observer are compressed and appear to possess a higher frequency than the waves of objects moving away from the observer is called a Doppler shift, proposed by Christian Doppler in 1842. For example, as planets orbit distant stars, they push them away from their centers of gravity, causing them to move towards or away from the observer. The slight movement of the star towards or away causes its spectrum, the rainbow colors of its light, to shift towards blue as it approaches and towards red as it recedes. Using this radial velocity method, the timing of the transition from red to blue and back again provides astronomers with information about the mass and orbital cycle of planets orbiting distant stars.
This method can also be used in astronomy to measure the constant velocities of stars orbiting distant galaxies when they are seen edge-on. Light or radio waves received from stars moving towards the telescope move towards higher frequencies, while light or radio waves from stars moving away from the telescope move towards lower frequency wavelengths. The amount of displacement indicates both the relative speed of the stars with respect to the observer and the angular speed of the stars orbiting the galaxy.
Weather forecasting was greatly aided by radial velocity maps measured by Doppler weather radar. Just as the radial velocity recorded for a rotating galaxy shows rotation by the red and blue shift of light waves, the change in frequency of radio waves indicates rotational motion in storms such as cyclones, hurricanes, and tornadoes. Forecasters can issue early tornado warnings when they see Doppler shift in severe weather systems.
The Doppler shift, or radial velocity method, can be used on any body or system of bodies that are orbiting or vibrating around a common center. Both celestial objects and weather patterns show a redshift or a blueshift, depending on whether the objects are moving towards or away 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.
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