 | This articleneeds additional citations forverification. Please helpimprove this article byadding 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) |
Inastronomy,diurnal motion (from Latin diurnus 'daily', from Latin diēs 'day') is the apparentmotion ofcelestial objects (e.g. theSun andstars) aroundEarth, or more precisely around the twocelestial poles, over the course of one day. It is caused byEarth's rotation around itsaxis, so almost every star appears to follow acircular arc path, called thediurnal circle,[1] often depicted instar trail photography.
The time for one complete rotation is 23 hours, 56 minutes, and 4.09 seconds – onesidereal day. The first experimental demonstration of this motion was conducted byLéon 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.
The relative direction of diurnal motion in theNorthern Celestial Hemisphere are as follows:
Thus, northerncircumpolar stars move counterclockwise aroundPolaris, the northpole star.
At theNorth Pole, thecardinal directions do not apply to diurnal motion. Within the circumpolar circle, all the stars move simply rightward, or looking directly overhead, counterclockwise around thezenith, where Polaris is.
Southern Celestial Hemisphere observers are to replace north with south, left with right, and Polaris withSigma Octantis, sometimes called the south pole star. The circumpolar stars move clockwise around Sigma Octantis. East and west are not interchanged.
As seen from theEquator, the twocelestial 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 dailyarc path of anobject on thecelestial sphere, including the possible part below the horizon, has a length proportional to thecosine of thedeclination. Thus, the speed of the diurnal motion of a celestial object equals this cosine times 15° per hour, 15arcminutes per minute, or 15 arcseconds per second.
Per a certain period of time, a givenangular distance travelled by an object along or near thecelestial equator may be compared to theangular diameter of one of the following objects:
Star trail andtime-lapse photography capture diurnalmotion 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 along exposure, can best be done with anequatorial mount, which requires adjusting theright ascension only; atelescope may have a sidereal motor drive to do that automatically.
