Toward and away

15 chapters, each an animated explainer with its physics and sources. Every drawing says whether it is an illustration, a model from stored values, or real data, and by how much it is exaggerated. Only the transit chapter is something SetuHQ itself does.

Chapter 3 of 15

Toward and away

Light from a star moving toward us arrives slightly bluer; moving away, slightly redder. Only the motion along our line of sight shifts the light. Motion across the sky (proper motion) shifts nothing.

Illustration
An observer at the bottom. A star moves toward and away from the observer along the line of sight, then across it. Below, a strip of five absorption lines slides blueward when the star approaches and redward when it recedes, and stays put when it moves across. The shift is exaggerated a million-fold and labelled.

How small the shift is

Δλ/λ = v_r/c, with v_r positive for motion away (redshift). For a wobble of tens of m/s the shift is a ten-millionth of the wavelength, so spectrographs measure it by cross-correlating thousands of lines (Fischer et al. 2016). The star's colour does not visibly change.

QuantityValueFrom
51 Peg b's K55.77 m/sexoplanet.eu (The Extrasolar Planets Encyclopaedia (exoplanet.eu), fetched 2026-10-18)
Δλ/λ for that K1.9e-07v/c [calc]
Shift at 500 nm95 fm[calc]
Planets in this chapter51 Peg b ›
Sources

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