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Equinox

This article is about an astronomical event. For the celestial coordinates, see Equinox (celestial coordinates). For other uses, see Equinox (disambiguation).

UT date and time of
equinoxes and solstices on Earth[1][2]
event equinox solstice equinox solstice
month March[3] June[4] September[5] December[6]
year day time day time day time day time
2019 20 21:58 21 15:54 23 07:50 22 04:19
2020 20 03:50 20 21:43 22 13:31 21 10:03
2021 20 09:37 21 03:32 22 19:21 21 15:59
2022 20 15:33 21 09:14 23 01:04 21 21:48
2023 20 21:25 21 14:58 23 06:50 22 03:28
2024 20 03:07 20 20:51 22 12:44 21 09:20
2025 20 09:02 21 02:42 22 18:20 21 15:03
2026 20 14:46 21 08:25 23 00:06 21 20:50
2027 20 20:25 21 14:11 23 06:02 22 02:43
2028 20 02:17 20 20:02 22 11:45 21 08:20
2029 20 08:01 21 01:48 22 17:37 21 14:14

A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around 20 March and 23 September.[a]

More precisely, an equinox is traditionally defined as the time when the plane of Earth's equator passes through the geometric center of the Sun's disk.[7][8] Equivalently, this is the moment when Earth's rotation axis is directly perpendicular to the Sun-Earth line, tilting neither toward nor away from the Sun. In modern times[when?], since the Moon (and to a lesser extent the planets) causes Earth's orbit to vary slightly from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the apparent geocentric longitude of the Sun is 0° and 180°.[9]

The word is derived from the Latin aequinoctium, from aequus (equal) and nox (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. Contrary to popular belief,[10][11] they are not exactly equal because of the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality, equatorial cultures noted the day when the Sun rises due east and sets due west, and indeed this happens on the day closest to the astronomically defined event. As a consequence, according to a properly constructed and aligned sundial, the daytime duration is 12 hours.

In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox. In the Southern Hemisphere, the reverse is true. During the year, equinoxes alternate with solstices. Leap years and other factors cause the dates of both events to vary slightly.[12]

Hemisphere-neutral names are northward equinox for the March equinox, indicating that at that moment the solar declination is crossing the celestial equator in a northward direction, and southward equinox for the September equinox, indicating that at that moment the solar declination is crossing the celestial equator in a southward direction.

Daytime is increasing at the fastest at the vernal equinox and decreasing at the fastest at the autumnal equinox.

  1. ^ Astronomical Applications Department of USNO. "Earth's Seasons - Equinoxes, Solstices, Perihelion, and Aphelion". Retrieved 1 August 2022.
  2. ^ "Solstices and Equinoxes: 2001 to 2100". AstroPixels.com. 20 February 2018. Retrieved 21 December 2018.
  3. ^ Équinoxe de printemps entre 1583 et 2999
  4. ^ Solstice d’été de 1583 à 2999
  5. ^ Équinoxe d’automne de 1583 à 2999
  6. ^ Solstice d’hiver
  7. ^ "Equinoxes". Astronomical Information Center. United States Naval Observatory. 14 June 2019. Archived from the original on 21 August 2019. Retrieved 9 July 2019. On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon. The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level region
  8. ^ "ESRL Global Monitoring Division - Global Radiation Group". NOAA. www.esrl.noaa.gov. U.S. Department of Commerce. Retrieved 9 July 2019.
  9. ^ Astronomical Almanac. United States Naval Observatory. 2008. Glossary.
  10. ^ Grieser, Justin (22 September 2014). "Autumn arrives: The fall equinox explained in six images". The Washington Post. Archived from the original on 8 June 2021. Retrieved 29 June 2024.
  11. ^ Plait, Phil (22 September 2023). "The Equinox Is Not What You Think It Is". Scientific American. Retrieved 29 June 2024.
  12. ^ Cite error: The named reference YallopEtAl was invoked but never defined (see the help page).


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