Empirical Earth · The Sky — Sun, Moon & Stars

Proper motion — the ‘fixed’ stars are not fixed

The claim

“The constellations never change, so the stars are fixed lamps on a nearby dome.”

What is measured

The constellations are a freeze-frame, not a fixture. Barnard’s Star, found by the astronomer E. E. Barnard in 1916, alone shifts ~10.3 arcseconds a year, a full-Moon width every ~180 years, and the Big Dipper was a different shape 50,000 years ago and will be unrecognizable in 100,000.

What would show this is wrong

a modern star catalogue that matches the ancient Greeks’ positions with no drift, constellations that fail to deform in any planetarium run across millennia, or a Gaia data set showing zero proper motion.

Sources

  1. Barnard’s Star & proper motion. E. E. Barnard measured its motion at ~10.3 arcseconds/year in 1916 — the largest proper motion of any known star, ~90 km/s across the line of sight. Britannic link
  2. Halley’s 1718 discovery that stars move. Comparing his positions with the ancient catalogue of Hipparchus, Halley found Sirius, Arcturus and Aldebaran each displaced by more than half a degr link
  3. The changing constellations & Gaia’s survey. Run forward, the Big Dipper deforms within ~50,000 years and is gone by ~100,000; ESA’s Gaia has measured positions and proper motions for nearly link
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