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Moving Targets
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Illustration: car with trumpeter, and microphones positioned around displaying waveforms

Determining Speed
If we ask a trumpeter in our car to play a middle C, we can tell if the car is moving to or away from us and how fast it's going by measuring the frequency. If the car is approaching, the note will sound higher than a C. How much higher tells us how fast the car's moving. If it's moving away, the note will sound lower than a C. Again, how much lower tells us how fast.

Illustration: white lightbulb moving toward telescope, with light waveforms and blue light visible in telescope viewpiece

Illustration: white lightbulb moving toward telescope, with light waveforms and blue light visible in telescope viewpiece

Light, too, is a wave, and so is influenced by the Doppler effect. However, instead of pitch we perceive differences in light's frequency as color. The higher the frequency of light, the closer its color is to the blue end of the spectrum. Cars' headlights don't appear blue on approach because they move too slowly to "catch up" enough with the light they produce. However, not everything moves as slowly as a car. Also, we have instruments far more sensitive than our eyes.

A key element in determining our trumpeter's speed was knowing that, if the car wasn't moving, we would have heard a middle C. Astronomers, unfortunately, don't have the luxury of asking stellar objects to pull over. What if instead of a single note our trumpeter was playing a familiar tune? Looking at the notes in the context of the song's pattern makes placing them quite simple. It turns out that the heavens are full of familiar tunes.

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