Astronomical day

An astronomical day refers to a length of day of exactly or nearly 24 hours beginning at noon instead of at midnight. The exact length has been variously defined as either that of a solar day or of a sidereal day.[1][2][3]

Astronomical days were historically used by astronomers (in contrast most commonly to solar days), but since the Industrial Revolution this usage has generally fallen out of favor, in order to avoid confusion with more conventional timekeeping.[4][2][5] An astronomical day can also refer to one rotation of Earth or any other planet depending on its spin speed.

References

"Astronomical day". The Free Dictionary. Retrieved 1 August 2013.
Campbell, W.W. (1918). "The Beginning of the Astronomical Day". Publications of the Astronomical Society of the Pacific. 30 (178): 358. Bibcode:1918PASP...30..358C. doi:10.1086/122784.
Finkleman, David (2011). "The Future of Time: UTC and the Leap Second". arXiv:1106.3141 [astro-ph.IM].
"Logbooks of the Bologna astronomical Observatory for the year 1761". Transits of Venus. University of Oxford. Retrieved 29 August 2013.
"Transit of Venus Bibliography". Institute for History and Foundations of Mathematics and the Natural Sciences. Retrieved 29 August 2013.

Time

Key concepts

Measurement
and standards

Chronometry	UTC UT TAI Unit of time Planck time New Earth Time Second Minute Hour Day Week Month Season Year Decade Century Millennium Tropical year Sidereal year Samvatsara
Measurement systems	6-hour clock (Italian) 6-hour clock (Thai) 12-hour clock 24-hour clock Daylight saving time Decimal Hexadecimal Metric Sidereal Solar Time zone
Calendars	Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year

Clocks

Chronology
History

Religion
Mythology

Philosophy of time

Human experience
and use of time

Time in

Geology	Geological time age chron eon epoch era period Geochronology Geological history of Earth
Physics	Absolute space and time Arrow of time Chronon Coordinate time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time dilation gravitational Time domain Time translation symmetry Time reversal symmetry
Other subject areas	Chronological dating Chronobiology Circadian rhythms Dating methodologies in archaeology Time geography

Related topics

Category
Commons

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 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 zone 180th meridian
Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time translation symmetry T-symmetry
Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema
Calendar	Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Holocene 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 Decimal time Metric time System time Time metrology Time value of money Timekeeper

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