Equinox Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! ===Celestial coordinate systems=== {{See also|Equinox (celestial coordinates)}} {{more citations needed section|date=March 2019}} [[File:Celestial sphere with ecliptic.svg|270px|thumb|Celestial sphere]] The [[March equinox]] occurs about when the Sun appears to cross the [[celestial equator]] northward. In the Northern Hemisphere, the term ''vernal point'' is used for the time of this occurrence and for the precise direction in space where the Sun exists at that time. This point is the origin of some [[celestial coordinate system]]s, which are usually rooted to an astronomical [[Epoch (astronomy)|epoch]] since it gradually varies ([[Axial precession|precesses]]) over time: * in the [[ecliptic coordinate system]], the vernal point is the origin of the [[ecliptic longitude]]; * in the [[equatorial coordinate system]], the vernal point is the origin of the [[right ascension]]. [[File:Equinox diagram.svg|270px|thumb|Diagram of the difference between the Sun's [[celestial longitude]] being zero and its [[declination]] being zero. Its [[celestial latitude]] never exceeds 1.2 [[Minute and second of arc|arcseconds]], but is exaggerated in this diagram.]] The modern definition of equinox is the instant when the Sun's apparent geocentric ecliptic longitude is 0Β° ([[March equinox|northward equinox]]) or 180Β° ([[September equinox|southward equinox]]).<ref>{{cite book|title=Astronomical Almanac 2008|date=2006|publisher=[[United States Naval Observatory]]}} Glossary Chapter.</ref><ref>{{cite book|last=Meeus|first=Jean|title=Mathematical Astronomy Morsels|year=1997}}</ref><ref>{{cite book|last=Meeus|first=Jean|title=Astronomical Algorithms|year=1998|edition=Second}}</ref> Note that at that moment, its latitude will not be exactly zero, since Earth is not exactly in the plane of the ecliptic. Its declination will also not be exactly zero, so the scientific definition is slightly different from the traditional one. The ''mean'' ecliptic is defined by the [[barycenter]] of Earth and the Moon combined, to minimize the fact that the orbital inclination of the Moon causes the Earth to wander slightly above and below the ecliptic.{{refn|"The IAU Working Group on Precession and the Ecliptic...have recommended that the ecliptic be more precisely defined as the plane perpendicular to the mean orbital angular momentum vector of the Earth-Moon barycenter passing through the Sun in the BCRS." [''Internal citations omitted''.]<ref>{{cite book |last1=Hilton |first1=James L. |author-link1=James L. Hilton |last2=McCarthy |first2=Dennis D. |author-link2=Dennis McCarthy (scientist) |editor1-last=Urban |editor1-first=S.E. |editor2-last=Seidelmann |editor2-first=P.K. |chapter=Precession, Nutation, Polar Motion, and Earth Rotation |year=2013 |title=Explanatory supplement to the astronomical almanac |edition=3rd |location=Mill Valley, CA |publisher=University Science Books |isbn=978-1-891389-85-6 |pages=205β206}}</ref>}} See the adjacent diagram. Because of the [[precession (astronomy)|precession of the Earth's axis]], the position of the vernal point on the [[celestial sphere]] changes over time, and the equatorial and the ecliptic coordinate systems change accordingly. Thus when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time can either be a conventional time (like [[J2000]]), or an arbitrary point in time, as for the [[equinox (celestial coordinates)|equinox of date]].<ref>{{cite book |title=Astronomy on the Personal Computer |year=1994 |url=https://archive.org/details/astronomyonperso00mont |url-access=limited |first1=Oliver |last1=Montenbruck |first2=Thomas |last2=Pfleger |publisher=Springer-Verlag |page=[https://archive.org/details/astronomyonperso00mont/page/n29 17] |isbn=0-387-57700-9}}</ref> The upper [[culmination]] of the vernal point is considered the start of the [[sidereal time|sidereal day]] for the observer. The [[hour angle]] of the vernal point is, by definition, the observer's [[sidereal time]]. Using the current official [[International Astronomical Union|IAU]] constellation boundaries β and taking into account the variable precession speed and the rotation of the celestial equator β the equinoxes shift through the constellations as follows<ref>{{cite book |first=J. |last=Meeus |title=Mathematical Astronomical Morsels |year=1997 |isbn=0-943396-51-4}}</ref> (expressed in [[astronomical year numbering]] when the {{nowrap|year 0 {{=}} 1 BC,}} {{nowrap|β1 {{=}} 2 BC,}} etc.): * The March equinox passed from [[Taurus (constellation)|Taurus]] into [[Aries (constellation)|Aries]] in {{nowrap|year β1865,}} passed into [[Pisces (constellation)|Pisces]] in {{nowrap|year β67,}} will pass into [[Aquarius (constellation)|Aquarius]] in {{nowrap|year 2597,}} and then into [[Capricornus]] in {{nowrap|year 4312.}} In 1489 it came within 10 [[arcminutes]] of [[Cetus]] without crossing the boundary. * The September equinox passed from Libra into [[Virgo (constellation)|Virgo]] in {{nowrap|year β729,}} will pass into [[Leo (constellation)|Leo]] in {{nowrap|year 2439.}} Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. 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