Editing Diurnal motion

{{Short description|Apparent motion of celestial objects around Earth}}
{{More citations needed|date=May 2021}}
[[File:Iotw2246a - Beginning to End of a Total Lunar Eclipse.jpg|thumb|[[Star trail]]s captured during a total [[lunar eclipse]]]]

In [[astronomy]], '''diurnal motion''' ({{etymology|la|diurnus|daily}},  {{etymology|la|diēs|day}}) is the apparent [[motion (physics)|motion]] of [[astronomical object|celestial objects]] (e.g. the [[Sun]] and [[star]]s) around [[Earth]], or more precisely around the two [[celestial pole]]s, over the course of one day. It is caused by [[Earth's rotation]] around its [[axis of rotation|axis]], so almost every star appears to follow a [[Arc (geometry)#Circular arc|circular arc]] path, called the '''diurnal circle''',<ref>{{Cite web|title=Diurnal Motion {{!}} COSMOS|url=https://astronomy.swin.edu.au/cosmos/d/Diurnal+Motion|access-date=2021-05-27|website=astronomy.swin.edu.au}}</ref> often depicted in [[star trail]] photography.

The time for one complete rotation is 23 hours, 56 minutes, and 4.09 seconds – one [[sidereal day]]. The first experimental demonstration of this motion was conducted by [[Léon Foucault]]. Because Earth orbits the Sun once a year, the sidereal time at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after a year has passed, one additional sidereal "day" has elapsed compared to the number of solar days that have gone by.<!--accumulate to [[Leap year]] ? -->

==Relative direction==
[[File:Astronomy-Diurnal motion-43N-12S-en.gif|300px|thumb|Diurnal motion of the sky on the December solstice at 43°N ( Bilbao, Milwaukee, Sapporo, Vladivostok).
The stars represented are: α Cma, α Car, α Boo, α Lyr, α Aur, α Cmi, α Eri, α Ori, α Aql, α Sco, α PsA, α Cyg. α Uma, α Umi and α Ari]]
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

Thus, northern [[circumpolar star]]s move counter[[clockwise]] around [[Polaris]], the north [[pole star]].

At the [[North Pole]], the [[cardinal direction]]s 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 pole]]s 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 [[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.

==Variations according to latitude and time of year==
The diurnal motion is different in places of different latitudes and within each latitude according to the time of year, as can be seen in the following images.<br />

The variations according to latitude at a given time can be seen in the first three images.<br />

{{Multiple image
 | align             = center
 | direction         = horizontal
 | width             = 225
 | background color  = white

 | header            = Diurnal motion on the December [[solstice]] according to latitude
 | header_align      = center
 | header_background = white

 | footer            = -
 | footer_align      = center
 | footer_background = white

 | image1            = Astronomy-Diurnal motion-80N-12S-en.gif
 | width1            = 200
 | caption1          = at 80ºN

 | image2            = Astronomy-Diurnal motion-43N-12S-en.gif
 | width2            = 200
 | caption2          = at 43ºN 

 | image3            = Astronomy-Diurnal motion-10N-12S-en.gif
 | width3            = 200
 | caption3          = at 10ºN
}}

The following four images show diurnal motion at 43ºN on different times<br/>

{{Multiple images
 | align             = center
 | direction         = horizontal
 | width             = 225
 | background color  = white

 | header            = Diurnal motion at 43ºN at different times
 | header_align      = center
 | header_background = white

 | footer            = -
 | footer_align      = center
 | footer_background = white

 | image1            = Astronomy-Diurnal motion-43N-03E-en.gif
 | width1            = 200
 | caption1          = on March [[equinox]]

 | image2            = Astronomy-Diurnal motion-43N-06S-en.gif
 | width2            = 200
 | caption2          = on June [[solstice]]

 | image3            = Astronomy-Diurnal motion-43N-09E-en.gif
 | width3            = 200
 | caption3          = on September equinox
 | caption_align3    = center

 | image4            = Astronomy-Diurnal motion-43N-12S-en.gif
 | width4            = 200
 | caption4          = on December solstice
}}

== External links ==
* [https://www.youtube.com/watch?v=Gw-80tLQY8M Timelapse video of a 5 hour diurnal motion - Youtube]

==See also==
*[[Direction determination]]
*[[Position of the Sun]]

==References==
{{Reflist}}

{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{DEFAULTSORT:Diurnal Motion}}

[[Category:Astrometry]]
[[Category:Stellar astronomy]]