Which Stars Never Set? It Depends Entirely on Where You Are Standing

The sky appears to turn once a day about an axis passing through the celestial poles, and every star traces a circle around that axis. Whether a given star sets depends on whether its circle dips below the horizon. Stars close enough to the visible pole trace small circles that stay entirely above it, so they are visible every clear night of the year and never rise or set at all. Stars further away trace larger circles that cross the horizon, so they rise and set like the Sun.

A single comparison settles every case:

• •The visible pole sits above the horizon by your own latitude
• •A star never sets if it lies within that angle of the pole
• •So at sixty degrees north, anything within sixty degrees never sets
• •At the equator the angle is zero, so nothing qualifies
• •At the pole the angle is ninety, so nothing ever sets
• •The same rule applies in reverse to stars that never rise

The never-setting portion of the sky was a practical instrument long before it was a curiosity. Because the pole sits at an altitude equal to the observer's latitude, measuring the height of the pole star above the horizon gives latitude directly, which is the simplest position fix available at sea and was in use for centuries. Sailors also used the circling stars as a clock, since a known star's position around the pole indicates the hour of the night, and instruments were made specifically to read it. In the southern hemisphere there is no convenient bright pole star, which made the equivalent measurement considerably harder.

The rule has a mirror image that is less often stated and equally useful. Just as stars within your latitude of the visible pole never set, stars within that same angle of the opposite pole never rise, so an observer at fifty degrees north can never see anything within fifty degrees of the southern celestial pole from any location at that latitude, on any night of any year. That is why the southern constellations were entirely unknown to European astronomy until voyages south of the equator, and why several of them carry names given only in the seventeenth and eighteenth centuries.

The distinction between stars that die and stars that do not was noticed everywhere and carried meaning. Egyptian texts refer to the never-setting stars as the imperishable ones and associate them with eternal life, and shafts in the Great Pyramid are aligned to that region of the sky. The turning of the northern sky supplied the figure of a fixed point around which everything moves, which appears in Chinese, Greek and Norse material in different forms. And because the pole moves slowly over thousands of years, different stars have held the position, so the identity of the pole star is itself a dateable detail in old descriptions.

Stars within an angle of the pole equal to your latitude trace circles that never touch the horizon, so they are up every clear night. Nothing qualifies at the equator and everything does at the pole. Measuring the pole star's height above the horizon gives latitude directly, which was the simplest position fix at sea for centuries.