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Diurnal motion (Latin: diurnus, lit. 'daily', from dies, lit. "day") is an astronomical term referring to the apparent daily motion of stars around Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation around its axis, so almost every star appears to move along a circular arc path called the diurnal circle. The time for one complete rotation is 23 hours, 56 minutes, and 4.09 seconds (1 sidereal day). The first experimental demonstration of this motion was conducted by Léon Foucault.
The relative direction of diurnal motion in the Northern Celestial Hemisphere are as follows:
- Facing north, below Polaris: rightward, or eastward
- Facing north, above Polaris: leftward, or westward
- Facing south: rightward, or westward
At the North Pole, the cardinal directions do not apply to diurnal motion. Within the circumpolar circle, all the stars move simply rightward, or looking directly overhead, counterclockwise around the zenith, where Polaris is.
Southern Celestial Hemisphere observers are to replace north with south, left with right, and Polaris with Sigma Octantis, sometimes called the south pole star. The circumpolar stars move clockwise around Sigma Octantis. East and west are not interchanged.
As seen from the Equator, the two celestial poles are on the horizon due north and south, and the motion is counterclockwise (i.e. leftward) around Polaris and clockwise (i.e. rightward) around Sigma Octantis. All motion is westward, except for the two fixed points.
The daily arc path of an object on the celestial sphere, including the possible part below the horizon, has a length proportional to the cosine of the declination. Thus, the speed of the diurnal motion of a celestial object equals this cosine times 15° per hour, 15 arcminutes per minute, or 15 arcseconds per second.
- up to one Sun or Moon diameter (about 0.5° or 30') every 2 minutes
- up to one diameter of the planet Venus in inferior conjunction (about 1' or 60") about every 4 seconds
- 2,000 diameters of the largest stars per second
Star trail and time-lapse photography capture diurnal motion blur. The apparent motion of stars near the celestial pole seems slower than that of stars closer to the celestial equator. Conversely, following the diurnal motion with the camera to eliminate its arcing effect on a long exposure, can best be done with an equatorial mount, which requires adjusting the right ascension only; a telescope may have a sidereal motor drive to do that automatically.