SESK: Stuff Everyone Should Know · sesk.info/astro-doppler-formula
A spectral line shifts by 1/1000 of its wavelength. How fast is the source moving along our line of sight?
About 300 km/s, one thousandth of light speed.
- About 3 km/s
- About 300 km/s ✔
- About 30,000 km/s
Why: For speeds well below light, the fractional shift equals v divided by c. Light moves at about 300,000 km/s, so a 1/1000 shift means about 300 km/s.
Huh, didn't know that: Absorption lines are used because their exact wavelengths are known and easy to measure.
I asked the star how fast it was going. It said, just do the math, it's relative.
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References
Answer: About 300 km/s. For speeds well below light, the fractional shift equals v divided by c. Light moves at about 300,000 km/s, so a 1/1000 shift means about 300 km/s.
- OpenStax Astronomy 2e, 5.6 The Doppler Effect openstax.org/books/astronomy-2e/pages/5-6-the-doppler-effect
Solving this equation for the velocity, we find v = c × Δλ/λ.
Checked 2026-10-10.
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