2016 Northern Autumnal Equinox Happens Thursday, Sept. 22

Summary: The 2016 northern autumnal equinox marking the start of autumn in the Northern Hemisphere takes place Thursday, Sept. 22, at 14:21 UTC.

view of instant of 2016 northern autumnal equinox from 150122759 kilometers above 0 degrees north latitude 40 degrees west longitude: John Walker, Public Domain, via Fourmilab Switzerland

The 2016 northern autumnal equinox, which marks the start of autumn in the Northern Hemisphere, happens Thursday, Sept. 22, at 14:21 Coordinated Universal Time.

The 2016 northern autumnal equinox is one of two equinoxes that mark the sun’s twice yearly crossing of the celestial equator. The solar crossing of the imaginary circle in the sky above Earth’s equator takes place annually in March and again in September.

In March, the sun crosses the celestial equator from south to north. In September, the crossing is directed from north to south.

Two celestial coordinate systems are usually referenced to specify the sun’s position at the time of the March and September equinoxes. The ecliptic coordinate system identifies the spring equinox as occurring with the solar position at 0 degrees longitude. The solar position at geocentric ecliptic longitude 180 degrees. The equatorial coordinate system pinpoints the March equinox at right ascension 0 hours and September’s equinox at right ascension 12 hours.

The year’s two equinoxes mark opposite seasons northward and southward of the equator. The March equinox is known in the Northern Hemisphere as the northern vernal (Latin: vernalis, “of or pertaining to spring”) equinox because it announces the start of spring. The Southern Hemisphere knows the March equinox as the southern autumnal equinox because it ushers in autumn. The September equinox that is an autumnal placeholder in the Northern Hemisphere marks the beginning of spring in the Southern Hemisphere.

The vernal point serves as the origin of the ecliptic longitude in the ecliptic coordinate system and of the right ascension in the equatorial coordinate system. The vernal point identifies the instant of the vernal equinox with the sun’s northward crossing of the celestial equator.

The ecliptic coordinate system bases the orbits and positions of solar system objects either upon the center of the Earth for geocentric (Ancient Greek γῆ, gê, “earth” + κεντρικός , kentrikós, “central”) ecliptic longitude or upon the center of the sun for heliocentric (Ancient Greek ἥλιος, hḗlios, “sun”) ecliptic coordinates. The ecliptic longitude for the autumnal and spring equinoxes is geocentric.

The equatorial coordinate system relates the positions of celestial objects to the Earth’s center. Right ascension (RA) measures the angular distance along the celestial equator from the vernal point eastward to the celestial object’s hour circle. Hour circles are perpendicular to the celestial equator and are analogous to a globe’s lines of longitude. Right ascension refers to the view of a celestial object’s rising from Earth’s equator. The intersection of the celestial equator with the horizon at Earth’s equator forms a right angle.

As with geocentric ecliptic coordinates, the equatorial coordinate system imagines a plane projected from Earth’s equator onto an imaginary celestial sphere that is concentric with Earth. The plane’s primary direction is toward the vernal point. A right-handed convention presents north and east positions with positive coordinates.

The takeaway for the 2016 northern autumnal equinox is that the Northern Hemisphere has now shed summer’s greenery for autumn’s golds. The balance of daylight and nighttime is shifting. By Monday, Nov. 7, about one and one-half months after the autumnal equinox, clock hands in a multitude of locations in the Northern Hemisphere will have fallen backward as a balance to their spring forward in March.

equinox with sun in zenith at noon: 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 2016 from 150122759 kilometers above 0 degrees north latitude 40 degrees west longitude: 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:

“Autumnal Equinox - Fall Equinox.” Time And Date > Sun & Moon > Autumnal Equinox.
Available @ http://www.timeanddate.com/calendar/autumnal-equinox.html

Byrd, Deborah. “Everything you need to know: September equinox.” EarthSky > Astronomy Essentials. Sept. 21, 2016.
Available @ http://earthsky.org/?p=26181

“Daylight Saving Time Around the World 2016.” Time And Date > Time Zones.
Available @ http://www.timeanddate.com/time/dst/2016.html

Fisher, Kurt. “Equinox Line.” EPOD USRA (Universities Space Research Association’s Earth Science Picture of the Day) > Blog > October 2006. Oct. 21, 2006.
Available @ http://epod.usra.edu/blog/2006/10/equinox-line.html

McClure, Bruce. “Sun over Earth’s equator at equinox.” EarthSky Tonight. Sept. 21, 2016.
Available @ http://earthsky.org/tonight/sun-over-earths-equator-at-equinox

Owens, Steve. “Equinox, Equilux, and Twilight Times.” Dark Sky Diary > General Astronomy, Time and Date. March 20, 2010.
Available @ https://darkskydiary.wordpress.com/2010/03/20/equinox-equilux-and-twilight-times/

Richmond, Michael W. “Some basic astrometry.” Rochester Institute of Technology SPIFF > Classes > Physics 445 > Lectures.
Available @ http://spiff.rit.edu/classes/phys445/lectures/astrom/astrom.html

“What is the September Equinox?” Time And Date > Sun & Moon > September Equinox.
Available @ http://www.timeanddate.com/calendar/september-equinox.html