Seasons: Vernal (March) Equinox, Summer Solstice, Autumnal Equinox, Winter Solstice

U.S. Naval Observatory

US Naval Observatory
Astronomical Information Center (Sun, Moon, Time, Seasons, etc.)

 

TimeAndDate.com
/seasons.html

On the equinoxes the Sun shines directly on the equator and the length of day and night is nearly equal – but not quite.
Solstices are opposite on either side of the equator, so the Summer Solstice in the Northern Hemisphere is the Winter Solstice in the Southern Hemisphere and vice versa.

Year March Equinox June Solstice September Equinox December Solstice
2016 Mar 20 12:30 AM EDT Jun 20 6:34 PM EDT Sep 22 10:21 AM EDT Dec 21 5:44 AM EST
2017 Mar 20 6:29 AM EDT Jun 21 12:24 AM EDT Sep 22 4:02 PM EDT Dec 21 11:28 AM EST
2018 Mar 20 12:15 PM EDT Jun 21 6:07 AM EDT Sep 22 9:54 PM EDT Dec 21 5:23 PM EST
2019 Mar 20 5:59 PM EDT Jun 21 11:54 AM EDT Sep 23 3:50 AM EDT Dec 21 11:19 PM EST
2020 Mar 19 11:50 PM EDT Jun 20 5:44 PM EDT Sep 22 9:31 AM EDT Dec 21 5:02 AM EST
2021 Mar 20 5:37 AM EDT Jun 20 11:32 PM EDT Sep 22 3:21 PM EDT Dec 21 10:59 AM EST

/spring-equinox.html
The Vernal (Spring) Equinox in the Northern Hemisphere is the Autumnal (Fall) Equinox in the Southern Hemisphere and vice versa.
/summer-solstice.html
Solstices are opposite on either side of the equator, so the Summer Solstice in the Northern Hemisphere is the Winter Solstice in the Southern Hemisphere and vice versa.
/autumnal-equinox.html
Equinoxes are opposite on either side of the equator, so the autumnal (fall) equinox in the Northern Hemisphere is the spring (vernal) equinox in the Southern Hemisphere and vice versa.
/december-solstice.html
It is the winter solstice in the Northern Hemisphere, where it is the shortest day of the year. In the Southern Hemisphere, it is the summer solstice and the longest day of the year.

Definitions:

Dictionary.com

/equinox
[ee-kwuh-noks, ek-wuh-] noun
1. the time when the sun crosses the plane of the earth’s equator, making night and day of approximately equal length all over the earth andoccurring about March 21 (vernal equinox or spring equinox) and September 22 (autumnal equinox)
2. either of the equinoctial points.
1350-1400; Middle English < Medieval Latin equinoxium, for Latin aequinoctium the time of equal days and nights ( aequi- equi- + noct- (stem of nox) night + -ium -ium )

/solstice
[sol-stis, sohl-] noun
1. Astronomy. either of the two times a year when the sun is at its greatest distance from the celestial equator: about June 21, when the sun reaches its northernmost point on the celestial sphere, or about December 22, when it reaches its southernmost point.
Compare summer solstice, winter solstice.
either of the two points in the ecliptic farthest from the equator.
2. a furthest or culminating point; a turning point.
1200-50; < Middle English < Old French < Latin s?lstitium, equivalent to s?l sun + -stit-, combining form of stat-, variant stem of sistere to make stand (see stand ) + -ium -ium; see -ice )

Merriam-Webster.com

equinox
noun equi·nox
1: either of the two points on the celestial sphere where the celestial equator intersects the ecliptic
2: either of the two times each year (as about March 21 and September 23) when the sun crosses the equator and day and night are everywhere on earth of approximately equal length

solstice
noun sol·stice
1: either of the two points on the ecliptic at which its distance from the celestial equator is greatest and which is reached by the sun each year about June 22 and December 22
2: the time of the sun’s passing a solstice which occurs about June 22 to begin summer in the northern hemisphere and about December 22 to begin winter in the northern hemisphere

TheFreeDictionary.com

equinox
e·qui·nox
n.
1. Either of two points on the celestial sphere at which the ecliptic intersects the celestial equator.
2. Either of the two times during a year when the sun crosses the celestial equator and when the length of day and night are approximately equal; the vernal equinox or the autumnal equinox.
[Middle English, from Old French equinoxe, from Medieval Latin aequinoxium, from Latin aequinoctium : aequi-, equi- + nox, noct-, night; see nekw-t- in Indo-European roots.]

solstice
sol·stice
n.
Either of two times of the year when the sun is at its greatest angular distance from the celestial equator. The summer solstice in the Northern Hemisphere occurs about June 21, when the sun is at the zenith on the Tropic of Cancer; the winter solstice occurs about December 21, when the sun is at zenith on the Tropic of Capricorn. The summer solstice is the longest day of the year; the winter solstice is the shortest.
[Middle English, from Old French, from Latin s?lstitium : s?l, sun; see s?wel- in Indo-European roots + -stitium, a stoppage; see st?- in Indo-European roots.]

Wikipedia.com

Illumination of Earth by the Sun at the March equinox

Illumination of Earth by the Sun at the March equinox

Equinox
An equinox is an astronomical event in which the plane of Earth’s equator passes through the center of the Sun,[2] which occurs twice each year, around 20 March and 23 September. On an equinox, day and night are of approximately equal duration all over the planet.

Solstice
A solstice is an astronomical event that occurs twice each year (in June and December) as the Sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere. The seasons of the year are directly connected to both the solstices and the equinoxes.

Earth's seasons as seen from the north. Far right: December solstice.

Earth’s seasons as seen from the north. Far right: December solstice.

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