Autumnal Equinox 2015 in Northern Hemisphere on Wednesday, Sept. 23

Summary: Wednesday, Sept. 23, is the happening date for autumnal equinox 2015 in the Northern Hemisphere.

view of instant of autumnal equinox 2015 from 150122759 kilometers above 0 degrees north latitude 40 degrees west longitude: John Walker, Public Domain, via Fourmilab Switzerland

In the Northern Hemisphere, summer officially ends and autumn officially begins on the autumnal equinox, which occurs annually, usually on Sept. 22 or 23, and rarely on the 24th.

The rare falling of the autumnal  equinox on Sept. 24 is occasioned by Earth’s orbit of 365.25 days around the sun within the context of the Gregorian calendar, the modern Western calendar introduced in 1582 by Gregory XIII, Pope of the Catholic Church from May 13, 1572, until his death on April 10, 1585.

The extra quarter of a day for Earth’s orbit translates into a later occurrence of the autumnal equinox by six hours each year that ultimately leads to the placement of the autumnal equinox on Sept. 24. Last happening in 1931, the late date occurs again in around four centuries, in 2303, as part of the 400-year cycle of repetitive patterning to keep equinoxes at nearly the same date every year.

The autumnal equinox, also known as September equinox, marks the placement of the noon sun directly overhead, known as its zenith, as viewed from the Earth’s equator.

With the sun rising due east and setting due west on September’s equinox, the date celebrates approximately equal lengths of day and night worldwide, reflecting the word’s meaning of “equal night” (Latin: aequus, “equal” + nox, “night”).

Disparity between daytime and nighttime hours emerges in October, with the sun’s rising south of due east and setting south of due west producing shorter days and longer nights in the Northern Hemisphere.

With seasons reversed between the Northern and Southern Hemispheres, September’s equinox is known as the spring equinox south of the equator. In October, the Southern Hemisphere is welcoming spring’s longer days and shorter nights.

The autumnal equinox in the Northern Hemisphere happens in 2015 at 8:21 Coordinated Universal Time (UTC), which equates to a range from early morning to late evening in U.S. time zones.

The equinox starts at 1:21 a.m. Pacific Daylight Time (PDT).

Onset occurs at 2:21 a.m. Mountain Daylight Time (MDT).

The equinox begins at 3:21 a.m. Central Daylight Time (CDT).

Onset occurs at 4:21 a.m. Eastern Daylight Time (EDT).

The equinox starts at 5:21 a.m. Atlantic Daylight Time (ADT).

Onset occurs at 9:21 p.m. Samoa Standard Time (SST).

The equinox occurs at 6:21 a.m. Chamorro Standard Time (ChST).

As suggested by Bruce McClure, chief writer for EarthSky Tonight, celebration of autumnal equinox 2015 could include noting the placements of the rising and setting sun in the skies as revelatory of exact directions of due east and due west, respectively. Linked with familiar landmarks, the exact directional identification serves as a helpful tool for backyard astronomy specifically and for geographical orientation generally.

Equinox with noon sun in highest position in sky (= zenith), directly overhead: Tau’olunga, CC BY SA 2.5, 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:

view of autumnal equinox 2015 from 150122759 kilometers above 0 degrees north 40 degrees west: John Walker, Public Domain, via Fourmilab Switzerland @ https://www.fourmilab.ch/cgi-bin/Earth

equinox with sun in zenith at noon: Tau’olunga, CC BY SA 2.5, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Equinox-0.jpg

For further information:

Breyer, Melissa. “8 things you didn’t know about the autumnal equinox.” MNN.com > Earth Matters > Climate & Weather. Sept. 22, 2014.
Available @ http://www.mnn.com/earth-matters/climate-weather/stories/8-things-you-didnt-know-about-the-autumnal-equinox

McClure, Bruce. "Dates of sun's entry into each zodiac constellation." EarthSky > Science Wire > Space. Aug. 10, 2015.
Available @ http://earthsky.org/space/can-i-find-a-list-of-zodiacal-constellations-with-degrees

McClure, Bruce. “Sun over Earth’s equator at equinox.” EarthSky > Tonight > Sep 22 – Sep 22. Sept. 22, 2015.
Available @ http://earthsky.org/tonight/sun-over-earths-equator-at-equinox