Editing Diurnal motion (section)

==Apparent speed==
The daily [[arc (geometry)|arc]] path of an [[astronomical object|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 [[minute and second of arc|arcminutes]] per minute, or 15 arcseconds per second.

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 [[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 photography|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.