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.
Related web pages
- Previous page: Bowl of
Night
- 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.
- Next page: The Celestial
Sphere