All Topics  
Diurnal motion

 

 
   Email Print
   Bookmark   Link
 

 

Diurnal motion
 
 
  Topics Diurnal motion Topic Home

Diurnal motion is an astronomical
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 term referring to the apparent daily motion
Motion (physics)

In physics, motion means a constant change in the location of a body. Change in motion is the result of applied force. Motion is typically described in terms of velocity, acceleration, Displacement , and time....
 of star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. 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. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
, 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 axis of rotation, "infinitely 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).

Direction of the motion in the Northern hemisphere:

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. Circumpolar stars are therefore visible for the entire night on every night of the year ....
s move counterclockwise around the North Star
North Star

The North Star is the prominent pole star that lies closest in the sky to the celestial pole and which appears directly overhead to an observer at the Earth's North Pole; currently, this is Polaris....
.

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

In broad terms, the zenith is the direction pointing directly above a particular location . Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms....
.

For the southern hemisphere, interchange north/south and left/right, and replace North Star by southern celestial pole.






Discussion
Ask a question about 'Diurnal motion'
Start a new discussion about 'Diurnal motion'
Answer questions from other users
Full Discussion Forum



Encyclopedia


Diurnal motion is an astronomical
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 term referring to the apparent daily motion
Motion (physics)

In physics, motion means a constant change in the location of a body. Change in motion is the result of applied force. Motion is typically described in terms of velocity, acceleration, Displacement , and time....
 of star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. 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. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
, 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 axis of rotation, "infinitely 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).

Direction of the motion in the Northern hemisphere:
  • looking to the north, below the North Star
    North Star

    The North Star is the prominent pole star that lies closest in the sky to the celestial pole and which appears directly overhead to an observer at the Earth's North Pole; currently, this is Polaris....
    : 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. Circumpolar stars are therefore visible for the entire night on every night of the year ....
s move counterclockwise around the North Star
North Star

The North Star is the prominent pole star that lies closest in the sky to the celestial pole and which appears directly overhead to an observer at the Earth's North Pole; currently, this is Polaris....
.

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

In broad terms, the zenith is the direction pointing directly above a particular location . Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms....
.

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

The equator is the intersection of the Earth's surface with the Plane perpendicular to the Earth's rotation and containing the Earth's center of mass....
 both 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 imagination rotation sphere of "gigantic radius", concentric spheres and coaxial with the Earth....
, 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....
. 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 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....
):
  • 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 time-exposure photography
Photography

Photography is the process, activity and art of creating still or moving by recording radiation on a sensitive medium, such as a photographic film, or an ....
. 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 that has one rotational axis parallel to the Earth's axis of rotation. This type of mount is used with telescopes, satellite dishes, 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....
 only; a telescope
Telescope

A telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century....
 may have a motor to do that automatically (sidereal drive).

The first experimental demonstration of this motion was undertaken by Léon Foucault
Léon Foucault

Jean Bernard L?on Foucault was a France physics best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation....
.