Astronomical day

Length of day from Noon, not midnight

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.^{[citation needed]}

References

^ "Astronomical day". The Free Dictionary. Retrieved 1 August 2013.
^ ^a ^b 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. Archived from the original on 4 March 2016. 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

Past
Present
Future
Eternity

Measurement
and standards

Chronometry	UTC UT TAI Unit of time Orders of magnitude (time)
Measurement systems	Italian six-hour clock Thai six-hour clock 12-hour clock 24-hour clock Relative hour Daylight saving time Chinese Decimal Hexadecimal Hindu Metric Roman Sidereal Solar Time zone
Calendars	Main types Solar Lunar Lunisolar Gregorian Julian Hebrew Islamic Solar Hijri Chinese Hindu Panchang Maya List
Clocks	Main types astronomical astrarium atomic quantum hourglass marine sundial watch mechanical stopwatch water-based Cuckoo clock Digital clock Grandfather clock History Timeline

Philosophy of time

Religion
Mythology

Human experience
and use of time

Time in science

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 Instant Proper time Spacetime Theory of relativity Time domain Time translation symmetry Time reversal symmetry
Other fields	Chronological dating Chronobiology Circadian rhythms Clock reaction Glottochronology Time geography

Category
Commons

v t e 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 IERS Reference Meridian 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 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	Gregorian Hebrew Hindu Holocene Islamic (lunar Hijri) Julian Solar Hijri Astronomical Dominical letter Epact Equinox Intercalation Julian day 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	Instant 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