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In astronomy, diurnal motion (from Latin diurnus 'daily', from Latin diēs 'day') is the apparent motion of celestial objects (e.g. the Sun and stars) around Earth, or more precisely around the two celestial poles, over the course of one day. It is caused by Earth's rotation around its axis, so almost every star appears to follow a circular arc path, called the diurnal circle, 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.
Relative direction
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 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.
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.
Apparent speed
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.
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, 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.
External links
- Timelapse video of a 5 hour diurnal motion - Youtube
See also
- Direction determination
- Position of the Sun
References
- "Diurnal Motion | COSMOS". astronomy.swin.edu.au. Retrieved 2021-05-27.
This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Diurnal motion news newspapers books scholar JSTOR May 2021 Learn how and when to remove this message In astronomy diurnal motion from Latin diurnus daily from Latin dies day is the apparent motion of celestial objects e g the Sun and stars around Earth or more precisely around the two celestial poles over the course of one day It is caused by Earth s rotation around its axis so almost every star appears to follow a circular arc path called the diurnal circle often depicted in star trail photography Star trails captured during a total lunar eclipse 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 Leon 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 Relative directionThe 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 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 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 Apparent speedThe 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 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 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 External linksTimelapse video of a 5 hour diurnal motion YoutubeSee alsoDirection determination Position of the SunReferences Diurnal Motion COSMOS astronomy swin edu au Retrieved 2021 05 27 Portals AstronomyStarsSpaceflightOuter spaceSolar System
