2017 Autumn Equinox Happens Friday, Sept. 22, in Northern Hemisphere

Summary: The 2017 autumn equinox, which is the Southern Hemisphere’s spring equinox, happens Friday, Sept. 22, in the Northern Hemisphere.

Earth at instant of 2017 southward equinox, Friday, Sept. 22, at 20:02 Coordinated Universal Time (4:02 p.m. Eastern Daylight Time): John Walker/Earth and Moon Viewer, Public Domain, via Fourmilab Switzerland

The 2017 autumn equinox, which the Southern Hemisphere observes as a spring equinox, happens Friday, Sept. 22, in the Northern Hemisphere at 20:02, according to Coordinated Universal Time (UTC), the world’s official time standard.

Earth’s time zones, which convert Coordinated Universal Time into local time, either place the 2017 September equinox on the same day as UTC’s date of Friday, Sept. 22, or on the following day, Saturday, Sept. 23. Time zones that are ahead of UTC by four or more hours have Saturday dates for the 2017 September equinox.

Victoria, capital of the Republic of the Seychelles, lies in the Seychelles Time Zone (SCT). Conversion to local time places the 115-island archipelago in the Indian Ocean four hours ahead of Coordinated Universal Time. The identifier of UTC + 4 expresses the Seychelles Time Zone's four hour offset. The September equinox takes place in Victoria on Saturday, Sept. 23, at 12:02 a.m. SCT. Also, lying on the fourth parallel south in the Southern Hemisphere, Victoria observes the September equinox as a spring equinox because the Northern Hemisphere and the Southern Hemisphere experience seasons oppositely.

According to Coordinated Universal Time, the equinoctial date variations range from Sept. 21 to Sept. 23 during the 21st century’s 100-year span from Jan. 1, 2001, through Dec. 31, 2100. Sept. 21 occurs most infrequently as the equinoctial date. Sept. 21’s two equinoctial claims both occur in the century’s last decade. The first Sept. 21 occurrence happens in 2092 at 23:41 UTC (7:41 p.m. Eastern Daylight Time). The second occurrence happens in 2096 at 22:55 UTC (6:55 p.m. EDT).

Sept. 22 claims most frequent status, with 76 occurrences in the 21st century. The century opens and closes with Sept. 22 equinoxes, according to Coordinated Universal Time. Also, the date monopolizes the century’s second half for more than three straight decades. From 2060 through 2091, the astronomical event falls exclusively on Sept. 22.

Sept. 23 claims the second most occurrences in the century. According to Coordinated Universal Time, the astronomical event makes 22 appearances on that date.

The September equinox is also known as the southward equinox. The directional designation references the sun’s apparent southward crossing of the celestial equator, the imaginary great circle projection of Earth’s equator onto the imaginary Earth-centered celestial sphere.

As the second of the year’s two equinoxes, the southward equinox reverses the year’s first equinox. The March equinox is known as the northward equinox because of the sun’s apparent northward crossing of the celestial equator.

Also both equinoxes open opposite astronomical seasons in the Northern Hemisphere and Southern Hemisphere. The Northern Hemisphere knows the March equinox as a spring equinox and the September equinox as an autumn, or fall, equinox. Contrastingly, the Southern Hemisphere embarks upon astronomical autumn with the March equinox and astronomical spring with the September equinox.

The year’s two equinoxes join with the year’s two solstices in driving Earth’s seasons. The two equinoxes are associated with autumn and spring while the two solstices are responsible for summer and winter.

The tilt of Earth’s rotational axis with respect to the plane of its orbit around the sun occasions the astronomical seasons opened by equinoxes and solstices. For solstices, the tilt away from or toward the sun at the polar points of the axis favors different hemispheres. For example, the tilt of the south polar axial point toward the sun during the December solstice initiates the Southern Hemisphere’s summer because greater portions of the Southern Hemisphere and lesser portions of the Northern Hemisphere are exposed to the sun’s incoming rays.

Equinoxes represent the perpendicularity of Earth’s axis to Earth's orbital plane around the sun. Consequently, the sun’s rays equally illuminate both the Northern Hemisphere and the Southern Hemisphere. Equinoxes (Latin: aequus, “equal” + nox, “night”) imply the global phenomenon of equal hours of daylight and of darkness experienced worldwide. Actual dates of 12 hours of day and night vary according to latitude.

The takeaway for the 2017 autumn equinox that happens, according to Coordinated Universal Time, Friday, Sept. 22, in the Northern Hemisphere is that the Southern Hemisphere knows the September equinox as a spring equinox.

Contour plot of daylight hours as a function of date and latitude shows 12 hours of daylight roughly experienced during the year’s two equinoxes: Cmglee, CC BY SA 3.0 Unported, 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 2017 southward equinox, Friday, Sept. 22, at 20:02 Coordinated Universal Time (4:02 p.m. Eastern Daylight Time): John Walker/Earth and Moon Viewer, Public Domain, via Fourmilab Switzerland @ http://www.fourmilab.ch/cgi-bin/Earth

Contour plot of daylight hours as a function of date and latitude shows 12 hours of daylight roughly experienced during the year’s two equinoxes: Cmglee, CC BY SA 3.0 Unported, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Hours_of_daylight_vs_latitude_vs_day_of_year_cmglee.svg

For further information:

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Available @ https://www.timeanddate.com/calendar/autumnal-equinox.html

Byrd, Deborah. “All You Need to Know: September Equinox.” EarthSky > Astronomy Essentials. Sept. 22, 2016.
Available @ http://earthsky.org/astronomy-essentials/everything-you-need-to-know-september-equinox

“Equinox: Equal Day and Night, Almost.” Time And Date > Sun & Moon.
Available @ https://www.timeanddate.com/astronomy/equinox-not-equal.html

“Equinoxes.” U.S. Naval Observatory Astronomical Applications Department > FAQ (Frequently Asked Questions) > Phenomena of the Sun and Moon. Last modified June 14, 2011.
Available @ http://aa.usno.navy.mil/faq/docs/equinoxes.php

Espenak, Fred. “Solstices and Equinoxes: 2001 to 2100 Greenwich Mean Time.” AstroPixels > Ephemeris.
Available @ http://www.astropixels.com/ephemeris/soleq2001.html

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Available @ https://www.timeanddate.com/worldclock/fixedtime.html?iso=20170922T2002&p1=1440

Marriner, Derdriu. “First Point of Aries for Spring Equinox Actually Happens in Pisces.” Earth and Space News. Wednesday, March 8, 2017.
Available @ https://earth-and-space-news.blogspot.com/2017/03/first-point-of-aries-for-spring-equinox.html

“The September Equinox.” Time And Date > Sun & Moon.
Available @ https://www.timeanddate.com/calendar/september-equinox.html