Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for one complete rotation is 23 hours, 56 minutes and 4.09 seconds (1 sidereal day). The first experimental demonstration of this motion was undertaken by Léon Foucault.
Direction of the motion in the Northern hemisphere:
- Looking to the north, below the North Star: left-right, west-east
- Looking to the north, above the North Star: right-left, east-west
- Looking to the south: left-right, east-west
Thus northern circumpolar stars move counterclockwise around the North Star.
At the North Pole, north, east and west are not applicable, the motion is simply left-right, or looking vertically upward, counterclockwise around the zenith.
For the southern hemisphere, interchange north/south and left/right, and replace North Star by southern celestial pole. The circumpolar stars move clockwise around it. East/west are not interchanged.
At the equator the two celestial poles are at the horizon and motion is counterclockwise (i.e. to the left) around the North Star and clockwise (i.e. to the right) around the southern celestial pole. All motion is from east to west, except for the two stationary points.
The daily 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 is this cosine times 15 °/hr = 15'/min = 15"/s, i.e. (compare angular diameter):
- Up to a Sun or Moon diameter every two minutes
- ca. four seconds for the largest planet
- 2000 diameters of the largest stars per second
Diurnal motion can be seen in stop motion photography. Circumpolar stars close to the celestial pole move only slowly. Conversely, following the diurnal motion with the camera, to eliminate it on the photograph, can best be done with an equatorial mount, which requires adjusting the right ascension only; a telescope may have a motor to do that automatically (sidereal drive).
Famous quotes containing the words diurnal and/or motion:
“No motion has she now, no force;
She neither hears nor sees;
Rolled round in earth’s diurnal course,
With rocks, and stones, and trees.”
—William Wordsworth (1770–1850)
“It is the fixed that horrifies us, the fixed that assails us with the tremendous force of mindlessness. The fixed is a Mason jar, and we can’t beat it open. ...The fixed is a world without fire--dead flint, dead tinder, and nowhere a spark. It is motion without direction, force without power, the aimless procession of caterpillars round the rim of a vase, and I hate it because at any moment I myself might step to that charmed and glistening thread.”
—Annie Dillard (b. 1945)