Astronomical day

{{Short description|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.{{cite web|url=http://www.thefreedictionary.com/Astronomical+day|title=Astronomical day|publisher=The Free Dictionary|accessdate=1 August 2013}}{{cite journal|last=Campbell|first=W.W.|title=The Beginning of the Astronomical Day|volume=30|issue=178|pages=358|journal=Publications of the Astronomical Society of the Pacific |bibcode=1918PASP...30..358C|year=1918|doi=10.1086/122784|doi-access=free}}{{cite arXiv|last=Finkleman|first=David|title=The Future of Time: UTC and the Leap Second|eprint=1106.3141|class=astro-ph.IM|year=2011}}

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.{{cite web|title=Logbooks of the Bologna astronomical Observatory for the year 1761.|url=http://transits.mhs.ox.ac.uk/contribute/record.php?recnum=8&compiled_name=MuseodellaSpecola%2CDepartmentofAstronomy%2CUniversit%3FdiBologna&size=orig&contributor_id=51|work=Transits of Venus|publisher=University of Oxford|accessdate=29 August 2013|archive-date=4 March 2016|archive-url=https://web.archive.org/web/20160304055855/http://transits.mhs.ox.ac.uk/contribute/record.php?recnum=8&compiled_name=MuseodellaSpecola%2CDepartmentofAstronomy%2CUniversit%3FdiBologna&size=orig&contributor_id=51|url-status=dead}}{{cite web|title=Transit of Venus Bibliography|url=http://www.staff.science.uu.nl/~gent0113/venus/venus_text1.htm|publisher=Institute for History and Foundations of Mathematics and the Natural Sciences|accessdate=29 August 2013}} An astronomical day can also refer to one rotation of Earth or any other planet depending on its spin speed.{{cn|date=June 2022}}