HOME
The Info List - Sidereal Time


--- Advertisement ---



Sidereal time
Sidereal time
/saɪˈdɪəriəl/ is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars"[1] From a given observation point, a star found at one location in the sky will be found at the same location on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun. Just as the Sun
Sun
and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation
Earth's rotation
about its polar axis, solar time following the Sun
Sun
while sidereal time roughly follows the stars. More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox
March equinox
and both celestial poles, and is usually expressed in hours, minutes, and seconds.[2] Common time on a typical clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's annual revolution around the Sun
Sun
of slightly less than 1° per day (in fact to the nearest arcsecond, it takes 365.2422 days to revolve, therefore 360 degrees/365.2422 days = 0.9856° or 59′ 8″ per day, i.e., slightly less than 1 degree per day). A sidereal day is approximately 23 hours, 56 minutes, 4.0905 SI seconds. The March equinox
March equinox
itself precesses slowly westward relative to the fixed stars, completing one revolution in about 26,000 years, so the misnamed sidereal day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than Earth's period of rotation relative to the fixed stars.[3] The slightly longer "true" sidereal period is measured as the Earth Rotation Angle (ERA), formerly the stellar angle.[4] An increase of 360° in the ERA is a full rotation of the Earth. Because Earth orbits the Sun
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.

Contents

1 Comparison to solar time 2 Precession
Precession
effects 3 Modern definition

3.1 Earth Rotation Angle definition 3.2 Sidereal time
Sidereal time
definition 3.3 Relationship between solar time and sidereal time intervals

4 Sidereal days compared to solar days on other planets 5 See also 6 Notes 7 Citations 8 References 9 External links

Comparison to solar time[edit]

Sidereal time
Sidereal time
vs solar time. Above left: a distant star (the small orange star) and the Sun
Sun
are at culmination, on the local meridian m. Centre: only the distant star is at culmination (a mean sidereal day). Right: a few minutes later the Sun
Sun
is on the local meridian again. A solar day is complete.

Solar time
Solar time
is measured by the apparent diurnal motion of the Sun, and local noon in apparent solar time is the moment when the Sun
Sun
is exactly due south or north (depending on the observer's latitude and the season). A mean solar day (what we normally measure as a "day") is the average time between local solar noons ("average" since this varies slightly over the year). Earth makes one rotation around its axis in a sidereal day; during that time it moves a short distance (about 1°) along its orbit around the Sun. So after a sidereal day has passed, Earth still needs to rotate slightly more before the Sun
Sun
reaches local noon according to solar time. A mean solar day is, therefore, nearly 4 minutes longer than a sidereal day. The stars are so far away that Earth's movement along its orbit makes nearly no difference to their apparent direction (see, however, parallax), and so they return to their highest point in a sidereal day. Another way to see this difference is to notice that, relative to the stars, the Sun
Sun
appears to move around Earth once per year. Therefore, there is one fewer solar day per year than there are sidereal days. This makes a sidereal day approximately 365.24/366.24 times the length of the 24-hour solar day, giving approximately 23 h 56 min 4.1 s (86,164.1 s). Precession
Precession
effects[edit] Earth's rotation
Earth's rotation
is not a simple rotation around an axis that would always remain parallel to itself. Earth's rotational axis itself rotates about a second axis, orthogonal to Earth's 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 Earth in a manner more complicated than a simple constant rotation. For this reason, to simplify the description of Earth's orientation in astronomy and geodesy, it was conventional to chart the positions of the stars in the sky according to right ascension and declination, which are based on a frame that follows Earth's precession, and to keep track of Earth's rotation, through sidereal time, relative to this frame as well.[a] In this reference frame, Earth's rotation
Earth's 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 Earth's seasons, represents one orbit of Earth around the Sun. The precise definition of a sidereal day is the time taken for one rotation of Earth in this precessing reference frame. Modern definition[edit] In the past, time was measured by observing stars with instruments such as photographic zenith tubes and Danjon astrolabes, and the passage of stars across defined lines would be timed with the observatory clock. Then, using the right ascension of the stars from a star catalog, the time when the star should have passed through the meridian of the observatory was computed, and a correction to the time kept by the observatory clock was computed. Sidereal time
Sidereal time
was defined such that the March equinox
March equinox
would transit the meridian of the observatory at 0 hours local sidereal time.[6] Beginning in the 1970s the radio astronomy methods Very Long Baseline Interferometry (VBLI) and pulsar timing overtook optical instruments for the most precise astrometry. This lead to the determination of UT1 (mean solar time at 0° longitude) using VBLI, a new measure of the rotation of the Earth named Earth Rotation Angle, and new definitions of sidereal time. These changes were put into practice on 1 January 2003.[7] Earth Rotation Angle definition[edit] The Earth Rotation Angle (ERA) measures the rotation of the Earth from an origin on the celestial equator, the Celestial Intermediate Origin, that has no instantaneous motion along the equator; it was originally referred to as the non-rotating origin. ERA replaces Greenwich Apparent Sidereal Time
Time
(GAST). The origin on the celestial equator for GAST, called the true equinox, does move, due to the movement of the equator and the ecliptic. The lack of motion of the origin of ERA is considered a significant advantage.[8]. ERA, measured in radians, is defined as[3]

θ (

t

U

) = 2 π ( 0.779

057

273

2640 + 1.002

737

811

911

354

48

t

U

)

displaystyle theta (t_ U )=2pi (0.779,057,273,2640+1.002,737,811,911,354,48t_ U )

where tU is the Julian UT1
UT1
date − 2451545.0. The ERA may be converted to other units; for example, the Astronomical Almanac for the Year
Year
2017 tabulated it in degrees, minutes, and seconds.[9] As an example, the Astronomical Almanac for the Year
Year
2017 gave the ERA at 0 h 1 January 2017 UT1
UT1
as 100° 17′ 12.4365″.[10] Sidereal time
Sidereal time
definition[edit] Although ERA is intended to replace sidereal time, there is a need to maintain definitions for sidereal time during the transition, and when working with older data and documents. Similarly to mean solar time, every location on Earth has its own local sidereal time (LST), depending on the longitude of the point. Since it is not feasible to publish tables for every longitude, astronomical tables make use of Greenwich sidereal time (GST), which is sidereal time on the IERS Reference Meridian, less precisely called the Greenwich, or prime meridian. There are two varieties, mean sidereal time if the mean equator and equinox of date are used, or apparent sidereal time if the apparent equator and equinox of date are used. The former ignores the effect of nutation while the latter includes nutation. When the choice of location is combined with the choice of including nutation or not, the acronyms GMST, LMST, GAST, and LAST result. The following relationships hold:[11] local mean sidereal time = GMST + east longitude local apparent sidereal time = GAST + east longitude The new definitions of Greenwich mean and apparent sidereal time (since 2003, see above) are

G M S T

(

t

U

, t ) = θ (

t

U

) −

E

P R E C

( t )

displaystyle mathrm GMST (t_ U ,t)=theta (t_ U )-E_ mathrm PREC (t)

G A S T

(

t

U

, t ) = θ (

t

U

) −

E

0

( t )

displaystyle mathrm GAST (t_ U ,t)=theta (t_ U )-E_ 0 (t)

where θ is the Earth Rotation Angle, EPREC is the accumulated precession, and E0 is equation of the origins, which represents accumulated precession and nutation.[12] The calculation of precession and nutation was described in Chapter 6 of Urban & Seidelmann. As an example, the Astronomical Almanac for the Year
Year
2017 gave the ERA at 0 h 1 January 2017 UT1
UT1
as 100° 17′ 12.4365″. The GAST was 6 h 43 m 20.7109 s. For GMST the hour and minute were the same but the second was 21.1060.[10] Relationship between solar time and sidereal time intervals[edit] If a certain interval I is measured in both mean solar time (UT1) and sidereal time, the numerical value will be greater in sidereal time than in UT1, because sidereal days are shorter than UT1
UT1
days. The ratio is

I

m e a n

s i d e r e a l

I

U T 1

=

r ′

= 1.002

737

379

093

507

95 + 5.9006 ×

10

− 11

t − 5.9 ×

10

− 15

t

2

displaystyle frac I_ mathrm mean,sidereal I_ mathrm UT1
UT1
=r'=1.002,737,379,093,507,95+5.9006times 10^ -11 t-5.9times 10^ -15 t^ 2

where t represents the number of Julian centuries elapsed since noon 1 January 2000 Terrestrial Time.[13] Sidereal days compared to solar days on other planets[edit] Of the eight solar planets, all but Venus
Venus
and Uranus
Uranus
have prograde rotation—that is, they rotate more than once per year in the same direction as they orbit the Sun, so the Sun
Sun
rises in the east.[14] Venus
Venus
and Uranus, however, have retrograde rotation. For prograde rotation, the formula relating the lengths of the sidereal and solar days is

number of sidereal days per orbital period = 1 + number of solar days per orbital period

or equivalently

length of solar day = length of sidereal day/1 − length of sidereal day/orbital period.

On the other hand, the formula in the case of retrograde rotation is

number of sidereal days per orbital period = −1 + number of solar days per orbital period

or equivalently

length of solar day = length of sidereal day/1 + length of sidereal day/orbital period.

All the solar planets more distant from the Sun
Sun
than Earth are similar to Earth in that, since they experience many rotations per revolution around the Sun, there is only a small difference between the length of the sidereal day and that of the solar day—the ratio of the former to the latter never being less than Earth's ratio of 0.997. But the situation is quite different for Mercury and Venus. Mercury's sidereal day is about two-thirds of its orbital period, so by the prograde formula its solar day lasts for two revolutions around the Sun— three times as long as its sidereal day. Venus
Venus
rotates retrograde with a sidereal day lasting about 243.0 Earth days, or about 1.08 times its orbital period of 224.7 Earth days; hence by the retrograde formula its solar day is about 116.8 Earth days, and it has about 1.9 solar days per orbital period. By convention, rotation periods of planets are given in sidereal terms unless otherwise specified. See also[edit]

Anti-sidereal time Earth's rotation International Celestial Reference Frame Nocturnal (instrument) Sidereal month Sidereal year Synodic day Transit instrument

Notes[edit]

^ The conventional reference frame, for purposes of star catalogues, was replaced in 1998 with the International Celestial Reference Frame, which is fixed with respect to extra-galactic radio sources. Because of the great distances, these sources have no appreciable proper motion.[5]

Citations[edit]

^ NIST n.d. A more precise definition is given later in the lead. ^ Urban & Seidelmann 2013, "Glossary" s.v. hour angle, hour circle, sidereal time. ^ a b Urban & Seidelmann 2013, p. 78. ^ IERS 2013. ^ Urban & Seidelmann 2013, p. 105. ^ ES1 1961, Ch 3, "Systems of Time
Time
Measurement". ^ Urban & Seidelmann 2013, pp. 78–81, 112. ^ Urban & Seidelmann 2013, p. 6. ^ Astronomical Almanac 2016, pp. B21–B24. ^ a b Astronomical Almanac 2016, p. B21. ^ Urban & Seidelmann 2013, p. 80. ^ Urban & Seidelmann 2013, pp. 78–79. ^ Urban & Seidelmann 2013, p. 81. ^ Bakich 2000.

References[edit]

Astronomical Almanac for the Year
Year
2017. Washington and Taunton: US Government Printing Office and The UK Hydrographic Office. 2016. ISBN 978-0-7077-41666.  Bakich, Michael E. (2000). The Cambridge Planetary Handbook. Cambridge University Press. ISBN 0-521-63280-3.  "Earth Rotation Angle". International Earth Rotation and Reference System Service. 2013. Retrieved 20 March 2018.  Explanatory Supplement to the Ephemeris. London: Her Majesty's Stationery Office. 1961.  " Time
Time
and Frequency from A to Z, S to So". National Institute of Standards and Technology.  Urban, Sean E.; Seidelmann, P. Kenneth, eds. (2013). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, CA: University Science Books. ISBN 1-891389-85-8. 

External links[edit]

Look up sidereal time in Wiktionary, the free dictionary.

Web based Sidereal time
Sidereal time
calculator

v t e

Time

Key concepts

Past

history deep time

Present Future Futures studies Far future in religion Far future in science fiction and popular culture Timeline
Timeline
of the far future Eternity Eternity
Eternity
of the world

Measurement and standards

Chronometry

UTC UT TAI Unit of time Planck time Second Minute Hour Day Week Month Season Year Decade Century Millennium Tropical year Sidereal year Samvatsara

Measurement systems

Time
Time
zone Six-hour clock 12-hour clock 24-hour clock Daylight saving time Solar time Sidereal time Metric time Decimal time Hexadecimal time

Calendars

Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year

Clocks

Horology History
History
of timekeeping devices Main types

astrarium atomic

quantum

marine sundial sundial markup schema watch water-based

Chronology History

Astronomical chronology Big History Calendar
Calendar
era Chronicle Deep time Periodization Regnal year Timeline

Religion Mythology

Dreamtime Kāla Kalachakra Prophecy Time
Time
and fate deities Wheel of time Immortality

Philosophy of time

A-series and B-series B-theory of time Causality Duration Endurantism Eternal return Eternalism Event Multiple time dimensions Perdurantism Presentism Static interpretation of time Temporal finitism Temporal parts The Unreality of Time

Human experience and use of time

Accounting period Chronemics Fiscal year Generation time Mental chronometry Music Procrastination Punctuality Temporal database Term Time
Time
discipline Time
Time
management Time
Time
perception

Specious present

Time-tracking software Time-use research Time-based currency
Time-based currency
(time banking) Time
Time
value of money Time
Time
clock Timesheet Yesterday – Today – Tomorrow

Time
Time
in

Geology

Geological time

age chron eon epoch era period

Geochronology Geological history of Earth

Physics

Absolute time and space Arrow of time Chronon Coordinate time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time
Time
dilation

gravitational

Time
Time
domain Time
Time
translation symmetry Time
Time
reversal symmetry

other subject areas

Chronological dating Chronobiology Circadian rhythms Dating methodologies in archaeology Time
Time
geography

Related topics

Carpe diem Clock
Clock
position Space System time Tempus fugit Time
Time
capsule Time
Time
complexity Time
Time
signature Time
Time
travel

Time
Time
portal Category

v t e

Time
Time
measurement and standards

Chronometry Orders of magnitude Metrology

International standards

Coordinated Universal Time

offset

UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 6-hour clock 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second Solar time Terrestrial Time Time
Time
zone 180th meridian

Obsolete standards

Ephemeris time Greenwich Mean Time Prime meridian

Time
Time
in physics

Absolute time and space Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time
Time
dilation Gravitational time dilation Time
Time
domain Time
Time
translation symmetry T-symmetry

Horology

Clock Astrarium Atomic clock Complication History
History
of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History
History
of sundials Sundial
Sundial
markup schema

Calendar

Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology

Chronological dating Geologic time scale International Commission on Stratigraphy

Astronomical chronology

Galactic year Nuclear timescale Precession Sidereal time

Other units of time

Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium

Related topics

Chronology Duration

music

Mental chronometry Metric time System time Time
Time
value o

.