Autumnal Equinox

Sources

• U.S. Naval Observatory, Astronomical Applications Department, Earth's Seasons — Equinoxes, Solstices, Perihelion, and Aphelion (2026, Universal Time) — https://aa.usno.navy.mil/calculated/seasons?dst=false&submit=Get+Data&tz=0&tz_label=false&tz_sign=-1&year=2026 → every 2026 figure in the Timeline and the FAQ: March equinox March 20 at 14:46 UT, June solstice June 21 at 08:24 UT, aphelion July 6 at 17:30 UT, autumnal equinox September 23 at 00:05 UT, December solstice December 21 at 20:50 UT.
• U.S. Naval Observatory, Astronomical Applications Department, Earth's Seasons (2027, Universal Time) — https://aa.usno.navy.mil/calculated/seasons?dst=false&submit=Get+Data&tz=0&tz_label=false&tz_sign=-1&year=2027 → the 2027 autumnal equinox at September 23, 06:02 UT.
• NASA Science, Embracing the Equinox — https://science.nasa.gov/solar-system/skywatching/night-sky-network/embracing-the-equinox/ → the equinox as the moment the center of the Sun crosses the plane of the equator; the September crossing as southward, continuing to the December solstice; that the length of day and night on the equinox is not precisely equal and that the date of the even split depends on latitude; the Sun as a sphere rather than a point light source whose edge is refracted by the atmosphere, adding minutes of light; the Sun directly overhead at noon at the equator; both poles lit simultaneously only at the equinoxes; the Latin derivation of "equinox" as equal night; the 2026 equinox in U.S. local time as September 22 at 5:05 p.m. PDT; the hemispheres reversing at the September equinox.
• NOAA National Weather Service, Tucson, The Astronomical Seasons — https://www.weather.gov/twc/Astronomical_Seasons → Earth's 23.5-degree axial tilt as the cause of the seasons; the equinoxes as the two times the axis is tilted neither toward nor away from the Sun; refraction described as bending the light so the Sun appears above the horizon when its actual position is below it; the day-length figures of about 12 hours 6.5 minutes at the equator, 12 hours 8 minutes at 30 degrees and 12 hours 16 minutes at 60 degrees; the Sun directly overhead at noon at the equator; the Latin aequus and nox.
• NOAA National Weather Service, glossary entry for equinox — https://forecast.weather.gov/glossary.php?word=equinox → the September equinox as the start of astronomical autumn in the Northern Hemisphere, with the Sun approaching the Southern Hemisphere; the date given as approximately September 22; and the statement that daylight is "everywhere 12 hours," quoted in the FAQ as the claim the other two sources correct.

Source conflict, resolved and disclosed: the National Weather Service glossary says daylight on the equinox is "everywhere 12 hours." NASA says the split is not precisely equal, and the National Weather Service's own Tucson page gives measured day lengths above twelve hours and explains the refraction that causes it. This page follows NASA and the Tucson page, because they give the physical mechanism and the figures rather than a one-line approximation, and it names the glossary's version rather than quietly dropping it.

Not claimed, for want of a source: the term "equilux"; the common claim that equal day and night falls a few days after the September equinox specifically (NASA says only that it may fall earlier or later, depending on latitude); the range of calendar dates the September equinox can occupy and why it drifts; and that the Sun rises due east and sets due west at the equinox.

Rejected as sources: nationaltoday, nationaldaycalendar, checkiday, timeanddate, and content-farm or AI-generated listicles. Encyclopaedia Britannica's article on the autumnal equinox was reachable but its body text did not retrieve, so nothing here is cited to it.