SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/astro-doppler-radial-only

A star moves sideways across our line of sight, not toward or away. What Doppler shift do we see?

None. Only motion toward or away shifts the lines.

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  1. A redshift
  2. A blueshift
  3. No Doppler shift ✔

Why: The Doppler effect measures only radial velocity, the part of motion along our line of sight. Purely sideways motion leaves the wavelengths unchanged.

Huh, didn't know that: Astronomers also use the terms redshift and blueshift for radio waves and X-rays.

The dad joke
The star moved sideways and the spectrograph didn't notice. Some stars just fly under the radar.
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References

Answer: No Doppler shift. The Doppler effect measures only radial velocity, the part of motion along our line of sight. Purely sideways motion leaves the wavelengths unchanged.

  1. OpenStax Astronomy 2e, 5.6 The Doppler Effect openstax.org/books/astronomy-2e/pages/5-6-the-doppler-effect
    the Doppler effect is produced only by a motion toward or away from the observer, a motion called radial velocity .
    Checked 2026-10-10.

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