Diurnal motion
Encyclopedia
Diurnal motion is an astronomical
term referring to the apparent daily motion
of star
s around the Earth
, or more precisely around the two celestial pole
s. 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:
Thus northern circumpolar star
s 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
):
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).
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
term referring to the apparent daily motion
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...
of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s around the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, or more precisely around the two celestial pole
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...
s. 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
Léon Foucault
Jean Bernard Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation...
.
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 star
Circumpolar star
A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets , due to its proximity to one of the celestial poles...
s 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
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
.
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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
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
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
, including the possible part below the horizon, has a length proportional to the cosine of the declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
. 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
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
):
- 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
Stop motion
Stop motion is an animation technique to make a physically manipulated object appear to move on its own. The object is moved in small increments between individually photographed frames, creating the illusion of movement when the series of frames is played as a continuous sequence...
. 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
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
, which requires adjusting the right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
only; a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
may have a motor to do that automatically (sidereal drive).