2018 Autumn Equinox Happens Sunday, Sept. 23, in Northern Hemisphere

Summary: The 2018 autumn equinox happens Sunday, Sept. 23, in the Northern Hemisphere while the Southern Hemisphere experiences the event as a spring equinox.

sunlit Earth, autumn equinox, Sept. 20, 2011; imaged by Spinning Enhanced Visible and Infrared Imager (SEVIRI) on EUMETSAT’s (European Organisation for the Exploitation of Meteorological Satellites) Meteorsat-9: via NASA Earth Observatory

The 2018 autumn equinox happens Sunday, Sept. 23, at 01:54 Coordinated Universal Time (2:54 a.m. British Summer Time, Sunday, Sept. 23; 9:54 p.m. Eastern Daylight Time, Saturday, Sept. 22) in the Northern Hemisphere while the event occurs as a spring equinox in the Southern Hemisphere.

The September equinox opens astronomical autumn in the Northern Hemisphere and signals the end of astronomical summer. In the Southern Hemisphere, the September equinox marks the start of astronomical spring and the end of astronomical winter.

Earth’s time zones convert Coordinated Universal Time, Earth’s primary time standard, into local time. Time zone differences account for the 2018 September equinox’s occurrence either Saturday, Sept. 22, or Sunday, Sept. 23.

Time zones that lag two or more hours behind Coordinated Universal Time (UTC) claim the 2018 September equinox’s Saturday occurrence. Time zones that are behind UTC have a negative UTC time offset. The negative UTC offsets apply to locations to the west of the prime meridian, defined as zero degrees longitude. The Saturday, Sept. 22, date for the 2018 September equinox happens in time zones with UTC offsets of UTC-2 through UTC-12.

Time zones that are ahead of Coordinated Universal Time agree with the Sunday, Sept. 23, date for the September 2018 equinox. These time zones have a positive UTC time offset. The easternmost time zone’s offset is UTC+14.

Time zones that equate to Coordinated Universal Time have a UTC time offset of UTC+0. Greenwich Mean Time (GMT) is the time zone that equates to Coordinated Universal Time. Places in the Northern Hemisphere observing Greenwich Mean Time year-round include Iceland and the West African countries of Burkina Faso, Cote d’Ivoire, Guinea, Mali, Mauritania, Senegal and Togo. In the Southern Hemisphere, the British overseas territory of Saint Helena, Ascension and Tristan da Cunha observe Greenwich Mean Time year-round.

The September equinox registers the instant of the sun’s crossing of the celestial equator from north to south. The celestial equator is an imaginary line projected outward from Earth’s equator into space.

The September equinox occurs as the second of two annual equinoxes. The first equinoctial event is known as the March equinox. In the Northern Hemisphere, the March equinox marks the end of astronomical winter and the start of astronomical spring. In the Southern Hemisphere, the March equinox signals the end of astronomical summer and the beginning of astronomical autumn.

The year’s two equinoxes join with the year’s two solstices in defining annual astronomical seasons. The June solstice opens the Northern Hemisphere’s astronomical summer and the Southern Hemisphere’s astronomical winter. The December solstice ushers in the Northern Hemisphere’s astronomical winter and the Southern Hemisphere’s astronomical summer.

Earth’s tilted axis and orbit around the sun occasion the four astronomical seasonal events that are known as equinoxes and solstices. Earth rotates around an imaginary line, known as an axis, that passes through Earth from the North Pole to the South Pole. Earth’s axis currently tilts at an angle of 23.4 degrees to the imaginary perpendicular line of the ecliptic, the imaginary plane of Earth’s orbit around the sun. During the twice yearly equinoxes, Earth’s axis tilts neither away from nor toward the sun.

The perpendicularity of Earth’s axis to the sun causes approximately equal reception of the sun’s rays by the Northern Hemisphere and the Southern Hemisphere. This twice-yearly event’s name, equinox (Latin: aequus, “equal” + nox, “night”), references an approximate equality in the length of day and night.

EarthSky Tonight’s lead writer, Bruce McClure, observes that, in reality, daylight’s minutes exceeds nighttime’s minute tally at mid-temperate latitudes on the equinoctial dates. He explains the extra minutes as due to the solar disk and to atmospheric refraction.

Unlike stars, the sun is not pointlike. Sunrise calculations consider the first appearance of the top of the sun at the eastern horizon. Yet, the rest of the sun has yet to emerge. Sunset calculations concern the final touching of the western horizon by the sun’s trailing limb.

Atmospheric refraction references the effect of air density, as a function of altitude, on the velocity of light. The Earth’s atmosphere displaces the sun’s image at sunrise and also at sunset. McClure notes the raising of the solar image by about 0.5 degrees from its true geometrical position.

“This advances the sunrise yet retards the sunset, adding several minutes of daylight at each end of the day,” explains McClure.

The takeaway for 2018 autumn equinox that happens Sunday, Sept. 23, at 01:54 Coordinated Universal Time in Northern Hemisphere is that the September event, which roughly equates daytime with nighttime in length, announces astronomical autumn in the Northern Hemisphere and astronomical spring in the Southern Hemisphere.

Atmospheric refraction adds extra moments to the actual sunset time: Francisco Javier Blanco González, 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:

sunlit Earth, autumn equinox, Sept. 20, 2011; imaged by Spinning Enhanced Visible and Infrared Imager (SEVIRI) on EUMETSAT’s (European Organisation for the Exploitation of Meteorological Satellites) Meteorsat-9: via NASA Earth Observatory @ https://earthobservatory.nasa.gov/images/52248

Atmospheric refraction adds extra moments to the actual sunset time: Francisco Javier Blanco González, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Refracci%C3%B3n.png

For further information:

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

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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

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Available @ https://earth-and-space-news.blogspot.com/2016/09/2016-northern-autumnal-equinox-happens.html

Marriner, Derdriu. “2017 Autumn Equinox Happens Friday, Sept. 22, in Northern Hemisphere.” Earth and Space News. Wednesday, Sept. 20, 2017.
Available @ https://earth-and-space-news.blogspot.com/2017/09/2017-autumn-equinox-happens-friday-sept.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. “Autumnal Equinox 2015 in Northern Hemisphere on Wednesday, Sept. 23.” Earth and Space News. Tuesday, Sept. 22, 2015.
Available @ https://earth-and-space-news.blogspot.com/2015/09/autumnal-equinox-2015-in-northern.html

McClure, Bruce. “Day and Night Exactly Equal at Equinoxes?” EarthSky > Astronomy Essentials > Earth. Sept. 18, 2018.
Available @ http://earthsky.org/?p=196819

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