Midnight Sun and Polar Night

node · earth · Both polar regions, poleward of approximately 66.5 degrees latitude

The consequence of an axial tilt of about 23.4 degrees: poleward of the polar circles at roughly 66.5 degrees latitude the sun fails to set for at least one full day in summer and fails to rise for at least one full day in winter, and at the pole itself each condition lasts approximately six months. The two are one phenomenon seen from opposite ends of the year, and they are the metronome of polar biology - the spring return of light triggers the ice-algal bloom that starts the whole food chain, and the autumn loss of it drives migration, hibernation, fat storage or, in the emperor penguin's case, breeding straight into the dark. Polar night is not uniformly black: refraction and long twilight mean usable light persists for weeks beyond the geometric sunset, which is why the polar circles are a poor guide to when biology actually shuts down.