Are the Stars Actually Fixed? Wait a Few Thousand Years and Look Again
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Stars drift across the sky at rates too small to notice in a lifetime, and measuring that drift was the discovery that ended the idea of a fixed celestial sphere.
What is being measured
Every star moves through space, and part of that motion carries it across the line of sight while part carries it towards or away from us. The sideways component shows up as a slow change in the star's position against more distant background objects, measured as an angle per year. The values are tiny, and the largest known is about ten seconds of arc per year, which is roughly the width of the full moon in two centuries. Most stars move far less. The other component, along the line of sight, produces no positional change and is measured from the spectrum instead.
Why it is hard to detect
Several effects swamp the real motion and have to be removed:
- •The Earth orbits, which shifts nearby stars back and forth yearly
- •The Earth's axis wobbles over thousands of years
- •The atmosphere bends light by an amount depending on altitude
- •Instruments flex, settle and change with temperature
- •The reference stars used for comparison are themselves moving
- •The real signal is smaller than most of those effects
The discovery
Edmond Halley announced in 1718 that three bright stars were no longer where Greek astronomers had recorded them, having compared contemporary positions with the catalogue attributed to Hipparchus and preserved by Ptolemy. The differences were large enough not to be measurement error and could not be explained by the known wobble of the Earth's axis, which affects all stars together. The conclusion was that the stars themselves had moved. That ended a very old assumption that the stars were fixed points on a sphere, and it is one of the few discoveries that depended entirely on having records from two thousand years earlier.
Where the constellations are going
The measured drift is large enough to redraw the sky over long periods, which is a striking consequence to make concrete. The seven bright stars of the Plough are mostly moving together as a group, apart from the two at either end, which move differently, so the shape will visibly distort over tens of thousands of years and eventually cease to resemble anything recognisable. All constellations do the same. Conversely, running the motions backwards reconstructs the sky as it appeared in the distant past, which has occasionally been used to date ancient descriptions and alignments.
What it is used for now
The measurement is fundamental rather than merely curious and modern surveys have made it precise for enormous numbers of stars. Combining it with distance gives a star's actual speed across the sky in kilometres per second, and combining that with the line of sight speed gives its full motion through the galaxy. Stars sharing a common motion are identified as groups born together, which is how moving clusters and stellar streams are found. Large drift usually indicates a nearby star, which is how several close neighbours were first noticed. And the wobble of a star's path reveals unseen companions.
The takeaway
The sideways component of a star's motion shows up as a tiny angular drift per year, at most about ten seconds of arc, which must be separated from the Earth's own orbit, axis wobble and atmospheric bending. Halley detected it in 1718 by comparing his own era's positions with a Greek catalogue, which ended the idea of fixed stars. It now identifies groups born together and unseen companions.