Field guide · Reading the sky
ConstellationPrecession
The Earth wobbles like a slowing top, so the pole of the sky drifts in a great circle every 26,000 years. Polaris has not always been the North Star.
- Period
- About 25,800 years
- Pole star now
- Polaris
- Pole star 2700 BC
- Thuban
- Pole star AD 14,000
- Vega
The Earth’s axis is tipped, and the pull of the Sun and Moon on its equatorial bulge makes it wobble slowly, tracing a circle on the sky about once every 25,800 years.
Hipparchus noticed around 130 BC, comparing his star positions with ones measured a century and a half earlier. It means the pole star changes: when the pyramids were built it was Thuban in Draco; in 12,000 years it will be Vega.
It also slides the equinoxes backwards through the zodiac, which is why the dates in horoscopes no longer match the constellation the Sun is actually in. The map precesses every catalogue position to the date on the clock, so you can scrub back and watch it happen.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.