The Night Sky

Orienting yourself without equipment.

The night sky can be navigated with no equipment once a few anchors are known. Constellations are not physical groupings — the stars in one may be wildly different distances away — but they are a durable and shared coordinate system.

Starting points

In the northern hemisphere, the Big Dipper's two outer bowl stars point at Polaris, which sits almost exactly above the north pole and therefore barely moves all night. Everything else appears to rotate around it. In the southern hemisphere the Southern Cross and the Pointers serve a similar function, though there is no bright pole star.

Magnitude runs backwards

Brightness is measured on a scale where lower is brighter, inherited from ancient Greek practice. Sirius is −1.5, Polaris about 2.0, and the naked-eye limit under dark skies is around 6. Each step of 1 is a factor of about 2.5 in brightness, so 5 magnitudes is exactly 100 times.

Why the sky changes

Nightly rotation carries stars westward. Orbital motion shifts them about four minutes earlier each night, which is why constellations are seasonal — Orion in winter, Scorpius in summer. The stars themselves have not moved; Earth's night side now faces a different direction.

Telling planets from stars

Planets shine steadily; stars twinkle, because a star is effectively a point source and atmospheric turbulence deflects that single beam, while a planet is a small disc whose many beams average out. Planets also stay near the ecliptic and shift position against the background over weeks.