Diurnal motion
"If the stars should appear one night in a thousand years,
how would men believe and adore, and preserve for many generations
the remembrance of the city of God which had been shown.
But every night come out these envoys of beauty,
and light the universe with their admonishing smile."
R. W. Emerson, Nature
What is diurnal motion?
How long does it last before repeating?
Is circumpolar motion one form of diurnal motion?
"He whose roof is heaven, who has no other cover, over whom the stars continually rise and set in one and the same course, makes the beginnings of his affairs and his knowledge of time depend upon them." Al-Biruni
Diurnal motion is the daily motion by which the stars appear to revolve around the north pole roughly once every 24 hours. It is as though the stars, one might imagine, were bright spots fixed on the inside surface of a giant celestial sphere which rotates around us.
Stars that are close to the north pole never set beneath the horizon, and their circular paths are referred to as circumpolar. Stars lying farther away from the pole dip below the horizon, which cuts off the circular pattern of their motion. These stars appear to rise in the east, ascend to the meridian, and then set in the west, roughly once per day.
Daily Motion: the alternation of day and night.
The Sun, Moon, planets, and stars all move westward across the sky each day (that is, from east to west). They repeat this general westward motion about a day later. All rise roughly in the east, ascend in the eastern sky until they reach their maximum height (when they "transit the meridian"), then descend in the western sky until they set roughly in the west. This is called their daily or "diurnal" motion.
"Diurnal" derives from the Latin noun "dies" which means "day" (as in "diary"), and from "diurnalis," which means "daily." The latter, pronouncing the "i" as a "j," is the etymological source for "journal" as well.
Diurnal Motion pages:
Related Terms
Diurnal Motion as seen from the Northern Hemisphere
Circumpolar Stars
Sphaera Recta
Application: Orientation and Navigation by fixed stars.
Focus Questions
Diurnal Motion as seen from the Southern Hemisphere.