Astronomy Answers: From the Astronomical Dictionary

# Astronomy AnswersFrom the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the doppler effect

after C.J. Doppler (1803-1853)

The doppler effect is the phenomenon that the frequency of a wave depends on the speed of the source of the waves relative to the receiver. When the source moves towards the receiver (or the receiver towards the source; only the relative speed matters) then the frequency is higher; if the source and the receiver move apart, then the frequency is lower. The relative change in the frequency is approximately equal to the ratio of the speed difference and the speed of propagation of the waves, as long as the speed difference is much smaller than the propagation speed.

For sound, the doppler effect changes the pitch: if a car, motor cycle, or train passes by you fast then the pitch of the sounds they produce goes down. The speed of sound is about 340 m/s (1200 km/h) at sea level on Earth, so the pitch of the sounds coming from a passing car change by one standard note (about 6 percent of frequency change) for about each 9 m/s (32 km/h) of its speed relative to you.

A speed measured through its doppler effect is called a doppler speed. The shift of characteristics in the frequency spectrum of waves (such as spectral lines) is called doppler shift.

Astronomers can use the doppler effect to fairly simply determine the speeds of stars and galaxies relative to us, and also of material in the photosphere of the Sun.