Diurnal motion
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Diurnal motion (from
The time for one complete rotation is 23 hours, 56 minutes, and 4.09 seconds – one
Relative direction
The relative direction of diurnal motion in the
- Facing north, below Polaris: rightward, or eastward
- Facing north, above Polaris: leftward, or westward
- Facing south: rightward, or westward
Thus, northern circumpolar stars move counterclockwise around Polaris, the north pole star.
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.
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.
Apparent speed
The daily
Per a certain period of time, a given angular distance travelled by an object along or near the celestial equator may be compared to the angular diameter of one of the following objects:
- up to one Sun or Moon diameter (about 0.5° or 30') every 2 minutes
- up to one diameter of the planet 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.
See also
References
- ^ "Diurnal Motion | COSMOS". astronomy.swin.edu.au. Retrieved 2021-05-27.