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Information about Sidereal Day

For the novel Sidereal Time see Christopher Meredith.


Sidereal time is a measure of the position of the Earth in its rotation around its axis. A sidereal day is, simply put, the time needed for Earth to complete one rotation around its axis, relative to the stars.

Sidereal time

Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. They differ by the precession of the vernal equinox relative to the stars.

Sidereal time is defined as the hour angle of the vernal equinox. When the meridian of the vernal equinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal time is the hour angle of the vernal equinox at the prime meridian at Greenwich, England; local values differ according to longitude. When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours. Greenwich sidereal time and UT1 differ from each other by a constant rate (GST=1.00273790935×UT1).[1] Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time. Objects are located in the night sky using right ascension and declination relative to the celestial equator (analogous to longitude and latitude on Earth), and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or culmination, at which time it is best placed for observation, as atmospheric extinction is minimised.

Sideral time and solar time



Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south in the northern hemisphere and due north in the southern hemisphere). The average time taken for the sun to return to its highest point is 24 hours.

During the time needed by the Earth to complete a rotation around its axis (a sidereal day), the earth moves a short distance (around 1°) along its orbit around the sun. Therefore, after a sidereal day, the Earth still needs to rotate a small extra angular distance before the sun reaches its highest point. A solar day is, therefore, around 4 minutes longer than a sidereal day.

The stars, however, are so far away that the earth's movement along its orbit makes a generally negligible difference to their apparent direction (see, however parallax), and so they return to their highest point in a sidereal day. A sidereal day is around 4 minutes shorter than a mean solar day.

Another way to see this difference is to notice that, relative to the stars, the Sun appears to move around the Earth once per year. Therefore, there is one less solar day per year than there are sidereal days. This makes a sidereal day a factor of approximately 365.25/366.25 shorter than the 24 hour solar day, giving approximately 23 hours, 56 minutes, 4.1 seconds (86,164.1 seconds).

Precession effects

The Earth rotation is not simply a simple rotation around an axis that would always remain parallel to itself. The Earth's rotation axis itself rotates about a second axis, orthogonal to the Earth orbit, taking about 25,800 years to perform a complete rotation. This phenomenon is called the precession of the equinoxes. Because of this precession, the stars appear to move around the Earth in a manner more complicated than a simple constant rotation.

For this reason, to simplify the description of Earth orientation in astronomy and geodesy, it is conventional to describe Earth rotation relative to a frame which is itself precessing slowly. In this reference frame, Earth rotation is close to constant, but the stars appear to rotate slowly with a period of about 25,800 years. It is also in this reference frame that the tropical year, the year related to the Earth's seasons, represents one orbit of the Earth around the sun. The precise definition of a sidereal day is the time taken for one rotation of the Earth in this precessing reference frame.

Exact duration and its variation

A mean sidereal day is about 23h 56m 4.1s in length. However, due to variations in the rotation rate of the Earth the rate of an ideal sidereal clock deviates from any simple multiple of a civil clock. In practice, the difference is kept track of by the difference UTC–UT1, which is measured by radio telescopes and kept on file and available to the public at the IERS and at the United States Naval Observatory.

Given a tropical year of 365.242190402 days from Simon et al.[2] this gives a sidereal day of 86,400×365.242190402/366.242190402, or 86,164.09053 seconds.

According to Aoki et al.,[3] an accurate value for the sidereal day at the beginning of 2000 is (1/1.002737909350795) times a mean solar day of 86,400 seconds, which gives 86,164.090530833 seconds. For times within a century of 1984, the ratio only alters in its 11th decimal place. This web based sidereal time calculator uses a truncated ratio of 1/1.00273790935.

Because this is the period of rotation in a precessing reference frame, it is not directly related to the mean rotation rate of the Earth in an inertial frame, which is given by ω=2π/T where T is the slightly longer stellar day given by Aoki et al.[4] as 86,164.09890369732 seconds. This can be calculated by noting that ω is the magnitude of the vector sum of the rotations leading to the sidereal day and the precession of that rotation vector. In fact, the period of the Earth's rotation varies on hourly to interannual timescales by around a millisecond [5], together with a secular increase in length of day of about 2.3 milliseconds per century which mostly results from slowing of the Earth's rotation by tidal friction. [6]

See also

References

1. ^ P. Kenneth Seidelmann, ed., Explanatory supplement to the Astronomical Almanac, (Mill Valley, Cal.: University Science Books, 1992) pp. 52 and 698.
2. ^ Simon, J. L., P. Bretagnon, J. Chapront, M. Chapront-Touzé, G Francou and J. Laskar: Numerical expressions for precession formulas and mean elements for the moon and the planets. Astronomy and Astrophysics 282(2), 663-683, 1994.
3. ^ Aoki, S., B. Guinot, G. H. Kaplan, H. Kinoshita, D. D. McCarthy and P. K. Seidelmann: The new definition of Universal Time. Astronomy and Astrophysics 105(2), 359-361, 1982.
4. ^ Aoki, S., B. Guinot, G. H. Kaplan, H. Kinoshita, D. D. McCarthy and P. K. Seidelmann: The new definition of Universal Time. Astronomy and Astrophysics 105(2), 359-361, 1982.
5. ^ Hide, R., and J. O. Dickey: Earth's variable rotation. Science 253, 629-637, 1991.
6. ^ Stephenson, F.R.: Historical eclipses and Earth's rotation. Cambridge University Press, 1997, 557pp.

External links

Christopher Meredith
Born: 1955
Tredegar
Occupation: Teacher
Nationality: Welsh
Genres: Poems & novels
Debut works: Shifts

Christopher Meredith is a poet and novelist from Tredegar, Wales.
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EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
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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
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equinox can have two meanings:
  • The moment when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment - see below.

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Precession refers to a change in the direction of the axis of a rotating object. In physics, there are two types of precession, torque-free and torque-induced, the latter being discussed here in more detail.
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In astronomy, the hour angle is one of the coordinates used in the equatorial coordinate system for describing the position of a point on the celestial sphere. The hour angle of a point is the angle between the half plane determined by the Earth axis and the zenith (half of the
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meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the horizon, and through the nadir, and is perpendicular to the local horizon.
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Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London — it is the meridian at which longitude is defined to be 0 degrees.
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equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0. Longitude is the east-west geographic coordinate measurement most commonly utilized in cartography and global navigation.
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Right ascension (abbrev. RA; symbol α) 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.
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In astronomy, declination (abbrev. dec or δ) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle.
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equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0. Latitude, usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator.
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Extinction is a term used in astronomy to describe the absorption and scattering of light emitted by astronomical objects by matter (dust and gas) between the emitting object and the observer.
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Solar times are measures of the apparent position of the Sun on the celestial sphere. They are not actually the physical time, but rather hour angles, that is, angles expressed in time units.
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Northern Hemisphere or northern hemisphere[1] is the half of a planet that is north of the equator—the word hemisphere literally means 'half ball'. It is also that half of the celestial sphere north of the celestial equator.
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Southern Hemisphere or southern hemisphere[1] is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'. It is also that half of the celestial sphere south of the celestial equator.
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Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, caused by the motion of an
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Solar times are measures of the apparent position of the Sun on the celestial sphere. They are not actually the physical time, but rather hour angles, that is, angles expressed in time units.
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The precession of Earth's axis of rotation with respect to inertial space is also called the precession of the equinoxes. Like a wobbling top, the direction of the Earth's axis is changing; while today, the North Pole points roughly to Polaris, over time it will change.
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Geodesy (IPA North American English /dʒiˈɑdɪsi/; British, Australian English etc. /dʒɪˈɒdəsi/), also called geodetics
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A tropical year (also known as a solar year) is the length of time the Sun, as seen from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere) relative to the equinoxes and solstices.
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For the novel Sidereal Time see Christopher Meredith.


Sidereal time is a measure of the position of the Earth in its rotation around its axis.
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Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation and other discrepancies.
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This article needs copy editing for grammar, style, cohesion, tone and/or spelling.
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United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States. Located in Northwest Washington, D.C., it is one of the very few observatories located in an urban area; when it was initially constructed, it was far from the light
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Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation and other discrepancies.
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International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earth's geoid.
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Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. It is the proper time experienced by a clock located on the geoid. In astronomy it is used as the time coordinate for apparent ephemerides for an Earthbound viewer.
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time.

One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence, and time itself is something that can be measured.
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sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one sidereal day).
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