With all this talk about an expanding universe, perhaps you're wondering how it
is that astronomers know things are moving up there, let alone how fast they're
moving. But astronomers routinely measure the speeds of stellar objects. How
they do it, it turns out, is grounded in everyday experience. Ever noticed how
the pitch of an ambulance's siren or a train's whistle changes as it speeds by?
If so, then you're already familiar with the concept used in gauging
interstellar speeds.
Determining something's radial velocity—how fast it is moving towards or
away from you—is quite easy. By taking note of something called the Doppler
effect and making close observations of the light from stars, we're able to
tell not only if those stars are moving to or away from us but how fast they're
moving.
You can find out how it's done and also determine stellar velocities yourself
with the activity below.
Moving Targets