"Doppler shift" refers to the stretching of waves (known as
"redshifting") from a source moving away from the observer, and
the compression of waves (called "blueshifting") from a source
approaching the observer. Redshifted waves have a longer
wavelength (or lower frequency) than they otherwise would, and
blueshifted waves have a shorter wavelength (or higher
frequency). This works with radio waves, light, and all other
forms of electromagnetic radiation. It's also noticeable in
sound waves-it causes a siren coming toward a listener to have a
higher pitch than a siren moving away. Just as police equipped
with radar guns use Doppler shift to determine the speed of a
car, scientists can use the phenomenon to determine the speed at
which a spacecraft or celestial object is moving toward or away
from Earth.