March Equinox Astronomically Opens Northern Spring and Southern Autumn

Summary: The annual March equinox astronomically opens northern spring and southern autumn, as Earth’s Northern and Southern Hemispheres have opposite seasons.

Earth at instant of 2019 March equinox, Wednesday, March 20, 2019, at 21:58 Coordinated Universal Time: John Walker/Earth and Moon Viewer, Public Doman, via Fourmilab Switzerland

The annual March equinox astronomically opens northern spring and southern autumn, as Earth’s Northern and Southern Hemispheres experience opposite seasons.

The March equinox is the year’s first of two equinoxes. The second equinox occurs in September. September’s equinox astronomically opens northern autumn and southern spring.

The instant of the 2019 March equinox occurs Wednesday, March 20, at 21:58 Coordinated Universal Time. The instant of the 2019 September equinox occurs Monday, Sept. 23, at 01:54 UTC.

The astronomical term of equinox derives from Latin (aequus, “equal” + nox, “night”). The U.S. Naval Observatory in Washington, D.C., defines equinox as the instant of the sun’s passage through “Either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator.”

The ecliptic designates the sun’s apparent path around the celestial sphere, the imaginary sphere projected outward from Earth by astronomers and celestial navigators. The celestial equator references astronomy’s projection of Earth’s equator outward onto the imaginary celestial sphere. The ecliptic tips the celestial equator at an angle of 23.5 degrees.

The two points of intersection between the celestial equator and the ecliptic are known as nodes. The U.S. Naval Observatory defines the vernal (Latin: vernalis, “of or pertaining to spring”) equinox as the sun’s location at “the ascending node of the ecliptic on the celestial equator.” The sun’s apparent ecliptic longitude is 0 degrees. The autumnal equinox finds the sun at “the descending node of the ecliptic on the celestial sphere.” The sun’s apparent ecliptic longitude is 180 degrees.

The subpolar point concerns the point where solar rays form a perpendicular angle, 90 degrees, with Earth’s surface. The subsolar point appears to cross the equator, moving northward, at the March equinox. At the September equinox, the subsolar point appears to cross the equator, moving southward.

Earth’s axial inclination of 23.5 degrees to Earth’s orbit around the sun accounts for the positioning of Earth’s Northern and Southern Hemispheres with respect to the sun’s rays. At the March and September equinoxes, Earth’s axis is perpendicular to the sun’s rays.

The equinoxes annually mark the only two instances of perpendicularity by both the Northern and Southern Hemispheres to the sun’s rays. After the March equinox, the axial tilt toward the sun favors the Northern Hemisphere. After the September equinox, the axial tilt away from the sun favors the Southern Hemisphere.

The U.S. Naval Observatory’s Astronomical Applications Department website notes that equal length of day with night does not actually occur at the time of the year’s two equinoxes. The website explains: “On the dates of the equinoxes, the day is about 7 minutes longer than the night at latitudes up to about 25 degrees, increasing to 10 minutes or more at latitude 50 degrees.”

The dates of 12-hour long day and night occur before or after the equinoxes, according to latitude. In the Southern Hemisphere, the dates of equal day and night follow the March equinox and precede the September equinox. In the Northern Hemisphere, the dates of equal day and night precede the March equinox and follow the September equinox.

At 5 degrees north latitude, the date of equal day and night arrives about Feb. 25. At 40 degrees north latitude, the date is about March 17.

The U.S. Naval Observatory notes: “Daytime continues to be longer than nighttime until after the September equinox.” After the September equinox, the date of equal day and night happens about Oct. 15 at 5 degrees north latitude and about Sept. 26 at 40 degrees north latitude.

The sun’s geometry and atmospheric refraction account for the actual dates for the occurrence of equal day and night. As a disk, not a point, the sun’s geometric center is below the horizon at sunrise and at sunset. Sunrise references the instant of visibility of the solar disk’s leading edge on the horizon. Sunset indicates the instant of disappearance of the solar disk’s trailing edge below the horizon. Additionally, atmospheric refraction makes the solar disk’s upper edge appear higher above the horizon than its true position.

The takeaway for the annual March equinox that astronomically marks northern spring and southern autumn is that the date of equal day and night does not actually coincide with the March equinox but instead occurs before the equinox in the Northern Hemisphere and after the equinox in the Southern Hemisphere.

The sun's geometry (as a disk, not a point) and Earth’s atmospheric refraction account for discrepancy between sun’s apparent versus actual horizonal positions, which determine the actual dates of equal day and night before or after the year's two equinoxes: TWCarlson, CC BY SA 3.0, via Wikimedia Commons

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Earth at instant of 2019 March equinox, Wednesday, March 20, 2019, at 21:58 Coordinated Universal Time: John Walker/Earth and Moon Viewer, Public Domain, via Fourmilab Switzerland @ https://www.fourmilab.ch/cgi-bin/Earth

The sun's geometry (as a disk, not a point) and Earth’s atmospheric refraction account for discrepancy between sun’s apparent versus actual horizonal positions, which determine the actual dates of equal day and night before or after the year's two equinoxes: TWCarlson, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Sunrise-Sunset_angle.svg#mw-jump-to-license

For further information:

Case, Jack. “The Sun’s Declination, the Equinoxes and the Solstices.” Astro Navigation Demystified > Sun/Earth.
Available @ https://astronavigationdemystified.com/the-suns-declination-the-equinoxes-and-the-solstices/

Marriner, Derdriu. “2017 Spring Equinox Happens Monday, March 20, in Northern Hemisphere.” Earth and Space News. Wednesday, March 15, 2017.
Available @ https://earth-and-space-news.blogspot.com/2017/03/2017-spring-equinox-happens-monday.html

Marriner, Derdriu. “2018 Northern Hemisphere Spring Equinox Happens Tuesday, March 20.” Earth and Space News. Wednesday, March 14, 2018.
Available @ https://earth-and-space-news.blogspot.com/2018/03/2018-northern-hemisphere-spring-equinox.html

Marriner, Derdriu. “American Samoa Has Autumn Equinox While United States Has Spring Equinox.” Earth and Space News. Wednesday, March 1, 2017.
Available @ https://earth-and-space-news.blogspot.com/2017/03/american-samoa-has-autumn-equinox-while.html

Marriner, Derdriu. “American Samoa Has Summer Solstice As Rest of U.S. Has Winter Solstice.” Earth and Space News. Wednesday, Dec. 12, 2018.
Available @ https://earth-and-space-news.blogspot.com/2018/12/american-samoa-has-summer-solstice-as.html

U.S.N.O. Astronomical Applications Department. “Earth’s Seasons and Apsides.” U.S. Naval Observatory Astronomical Applications Department > Data Services > Equinoxes, Solstices, Perihelion, and Aphelion.
Available @ http://aa.usno.navy.mil/data/docs/EarthSeasons.php

U.S.N.O. Astronomical Applications Department. “Equinox.” U.S. Naval Observatory Astronomical Applications Department > Information Center > Terms Used on This Website > The Astronomical Almanac Online.
Available @ http://asa.usno.navy.mil/SecM/Glossary.html#_E

U.S.N.O. Astronomical Applications Department. “Equinoxes.” U.S. Naval Observatory Astronomical Applications Department > Information Center > Phenomena of the Sun and Moon > Length of Day and Night at the Equinoxes.
Available @ http://aa.usno.navy.mil/faq/docs/equinoxes.php