How the Sky Turns · 7 of 8

Why the Sky Moves

Why isn’t your sky tonight the same as someone else’s?

Rotation, latitude and precession — the three reasons the sky you see tonight is not the sky someone else sees.

You’ll be able to say why the sky turns, why your latitude tilts it, and why the pole drifts over millennia.

11 steps · about 8 min

Before this: Finding the North Star (6 min), The Southern Cross (6 min)

Where and which night

Working out when this is best placed from where you are…

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Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

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← GUIDES

Eight hours: every star draws an arc but one, which barely moves.

It was Polaris. The grid laid over the sky shows why: every line meets beside it, because Earth's axis aims there. One turn takes 23 h 56 min. Nothing up there is moving — you are, and from the ground you cannot tell which turns.

Now bolt the camera to the stars and run a day. The sky stops and the Earth turns, carrying the pin — you — through the night. The same day seen two ways, and both are true. On the ground you are bolted to the Earth, which is why the sky seems to move.

Twelve hours. A star is circumpolar if its declination, distance north of the sky's equator, tops 90° minus your latitude.

Take it to the limit. At the North Pole the celestial pole is straight overhead: every star circles at a fixed height, nothing rises or sets, and every way is south.

Now walk south, a stop at a time. The pole sinks a degree for every degree you go, and the cap of never-setting stars shrinks. At the equator every star rises and sets.

The walk from outside. The pin is you; the disc through it, your horizon. The axis stands still; the disc swings at each stop, until at the equator it lies in your plane.

On to Buenos Aires: the disc swings beyond level, and the south celestial pole stands clear of your horizon. One ball, one axis, one plane; latitude is their angle.

At midnight, walk to Buenos Aires's latitude: the sky wheels round the south celestial pole, which no bright star marks, and the Cross never sets. See The Southern Cross.

Last, the axis itself drifts, round the ring once every 25,772 years: . The measurement is how far the pole has moved since Thuban held the job, when the pyramids were laid out. Why it drifts is The Wandering Equinox.

The pole's ring, from The Wandering Equinox

Tonight, go out and find the one star that doesn't move.

Earth spins: the sky turns
Your latitude tilts the wheel
The axis drifts round the ring

Why the Sky Moves

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