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| September equinox | |
|---|---|
| Name | September equinox |
| Date | ≈ 22–23 September |
| Significance | Earth's subsolar point crosses equator; day and night approximately equal |
September equinox is the moment each year when the Sun's apparent geocentric longitude crosses the celestial equator moving southward, producing a near-equal division of daylight and darkness across Earth. This event is tied to the tilt and orbit of Earth around the Sun and figures prominently in calendars, navigation, agriculture, and religious observances from Mesopotamia to Mesoamerica.
The equinox occurs when the apparent center of the Sun lies on the celestial equator, a consequence of Earth's axial tilt relative to its orbital plane, the ecliptic, and the geometry of the Earth–Sun system. The phenomenon is described in the context of celestial mechanics, involving perturbations examined by researchers at institutions such as European Space Agency, Jet Propulsion Laboratory, and Harvard-Smithsonian Center for Astrophysics. Classical models by Claudius Ptolemy and refinements by Sir Isaac Newton and Pierre-Simon Laplace situate the equinox with respect to precession and nutation, effects catalogued by International Astronomical Union standards and handled in ephemerides produced by United States Naval Observatory, Institut de mécanique céleste et de calcul des éphémérides, and Jet Propulsion Laboratory Development Ephemeris.
The equinox typically falls on 22 or 23 September in the Gregorian calendar; historical records show occasional occurrences on 21 or 24 in adjoining years due to leap-year patterns established by Pope Gregory XIII and earlier by Julius Caesar with the Julian calendar. Timing is computed in Universal Time and coordinated by organizations including the International Earth Rotation and Reference Systems Service, National Aeronautics and Space Administration, and observatories such as Royal Greenwich Observatory and Paris Observatory. Astronomers reference the equinox instant using the Barycentric Coordinate Time and Terrestrial Time scales that appear in work by Karl Schwarzschild and modern chronologists affiliated with Royal Astronomical Society.
Around the equinox, observers note nearly equal day and night durations at most latitudes, a condition discussed by Hipparchus and modeled in studies by Johannes Kepler and Galileo Galilei. Phenomena include the Sun rising due east and setting due west as seen from mid-latitudes, relevant to historical alignments at sites like Stonehenge, Chichen Itza, Machu Picchu, Newgrange, and Ggantija Temples. Atmospheric refraction, studied by Isaac Newton and Anders Celsius, modifies apparent sunrise and sunset times, a factor in calculations by U.S. Naval Observatory and in navigational practice within fleets such as Royal Navy. Sky events tied to the equinox have been recorded in chronicles involving Christopher Columbus and expeditions of James Cook.
Human cultures have linked the equinox to harvest and renewal cycles across regions including Ancient Egypt, Babylon, Aztec Empire, Inca Empire, Imperial China, Heian period Japan, and Viking Age Scandinavia. Seasonal festivals and rites appear in texts from Homer, inscriptions from Assyria, and court chronicles of Emperor Qin Shi Huang. Calendrical reforms by Pope Gregory XIII, the Ottoman Empire, and states like United Kingdom and Spain influenced how civil observance tracked the astronomical event. Scholars at University of Oxford, University of Cambridge, and Sorbonne University have analyzed links between equinox timing and activities recorded in documents from Pliny the Elder, Ibn al-Haytham, and Al-Biruni.
Various names and celebrations mark the occasion: the Autumnal equinox festivals of China known as Mid-Autumn Festival and rituals tied to Q'ingming; the Día de la Independencia timings in some Latin American calendars; harvest festivals like Sukkot in Judaism and Mid-Autumn Festival in Vietnam and Korea (Chuseok); Japanese observance Higan and the public holiday Shūbun no Hi; Iranian Mehregān and Zoroastrian rites; and indigenous ceremonies among Haida and Navajo Nation. Religious calendars—Hebrew calendar, Islamic calendar, Chinese calendar, Mayan calendar, and Hindu calendar—interact with the equinox in ways examined by historians at Yale University, Princeton University, and University of California, Berkeley.
Secular changes such as axial precession, a ~26,000-year cycle described by Hipparchus and modeled by Pierre-Simon Laplace, shift the equinox relative to the fixed stars and constellations, a phenomenon central to debates involving Nicolaus Copernicus and later refined in works by Johannes Kepler and Edmond Halley. Milankovitch cycles, named after Milutin Milanković, link orbital eccentricity and obliquity variations to long-term climate patterns recorded in ice cores studied by teams from National Oceanic and Atmospheric Administration and British Antarctic Survey. Secular acceleration in Earth's rotation, leap-second adjustments by International Earth Rotation and Reference Systems Service, and tidal interactions with Moon introduce small timing shifts managed by International Astronomical Union conventions and computational centers such as Fugro and USNO.
Category:Astronomical events