At night, the stars seem to revolve uniformly around a fixed point—the celestial north pole—in 23 hours and 56 minutes. Because of the four-minute advance of diurnal rotation, the following day at the same hour the visible stars appear to have shifted slightly westward.

The accumulation of the daily shifts entails, in a full year, a further complete uniform rotation of the stars around the celestial pole.

Beginning in the 15th C., the annual and diurnal apparent rotations of the stars were used to determine the hour of night thanks to an instrument known as a nocturnal, or nocturnlabe, built in different versions.

One side of the instrument records the months and days of the year or an equivalent zodiacal calendar. Holding the handle of the nocturnal perpendicular to the ground, the user aims at the celestial north pole, identified for simplicity's sake with the Pole Star in the constellation of Ursa Minor, Little Dipper, through the hole of the central rivet.

Next, the user places another disk, bearing an hour quadrant, in such a way that the related index intercepts the tack for the current day on the underlying calendar. The hour quadrant is thus oriented with respect to the stars.

Lastly, the user rotates a second index, placing it so that, when aimed at the Big Square, in Ursa Major, Big Dipper, it grazes the first two stars.

At this point, just like a watch hand, the index intercepts the hour of night on the quadrant.