This diagram shows the position of the Earth, in relation to the Sun, at the time of the Autumnal Equinox, as well as the other equinox and solstices of the year.
(Image Source: ©1999, Eric G. Canali, former Floor Operations Manager of the original Buhl Planetarium and Institute of Popular Science (a.k.a. Buhl Science Center - Pittsburgh's science and technology museum from 1939 to 1991) and Founder of the South Hills Backyard Astronomers amateur astronomy club; permission granted for only non-profit use with credit to author.)
By Glenn A. Walsh
Reporting for SpaceWatchtower
Equinox, the beginning of the season of Autumn or Fall in the
Northern Hemisphere of Earth, begins Tuesday morning. In Earth's
Southern Hemisphere, this equinox marks the astronomical beginning of
the season of Spring.
The September Equinox occurs Tuesday Morning, 2020 September 22 at 9:31 a.m. Eastern Daylight Saving Time (EDT) / 13:31 Coordinated Universal Time (UTC).
On the day of the Equinox, the Sun appears directly overhead at local Noon on the Equator. At the moment of Equinox, the Northern and Southern Hemispheres of Earth are illuminated equally. And, the time of Equinox is the only time when the Earth Terminator (dividing line on Earth between daylight and darkness) is perpendicular to the Equator.
This, and the reason for seasons on Earth in the first place, is due to the fact that Earth rotates on its axis, which is tilted at an approximate 23.44-degree angle from the plane of the Earth's orbit around the Sun. As the Earth revolves around the Sun, this axial tilt causes one hemisphere of the planet to receive more direct solar radiation during that hemisphere's season of Summer and much less direct solar radiation a half-year later during that hemisphere's season of Winter. As mentioned, during an Equinox (about half-way between Summer and Winter, and about half-way between Winter and Summer) both planetary hemispheres receive an equal amount of solar radiation.
Although "Equinox" in Latin means equal-night, the day of the Equinox does not actually have an equal amount of daylight and nightfall, as it appears on the Earth's surface. If the Sun was just a pin-point of light in our sky, as all other stars appear, day and night would be equal.
But, because the Sun is a disk, part of the Sun has risen above the horizon before the center of the Sun (which would be the pin-point of light); so there are extra moments of light on the Equinox. Likewise, part of the Sun is still visible, after the center of the Sun has set.
Additionally, the refraction of sunlight by our atmosphere causes sunlight to appear above the horizon, before sunrise and after sunset.
September 25 will mark the Equilux ("equal-light"), the actual day with equal hours and minutes of the Sun above the horizon, and equal hours and minutes of the Sun below the horizon. The Equilux occurs twice each year, approximately 3-to-4 days before the Vernal Equinox, when Spring begins, and 3-to-4 days after the Autumnal Equinox.
An urban legend that has been making
the rounds for decades has it that eggs can be stood on their ends
only during an Equinox, whether the Vernal Equinox in the Spring or
the Autumnal Equinox in the Fall. This is completely false. Depending
greatly on the size and shape of the particular egg, eggs can be
stood on their ends any day of the year! Astronomy has nothing to do
with whether an egg can stand on its end. If an egg can stand on its
end on the Equinox (and, due to the shape and size of some eggs, this
is not even possible), it can stand the same way any other day of the
In the last few years, with the help of the Internet and Social Media, another urban legend has become prevalent. Now it is claimed that brooms can stand, on their own, on their bristles, only on an Equinox day. This is also false. Again, as with eggs, if a broom can stand on its bristles by itself (this usually only works with newer brooms, with more even bristles) on an Equinox, it can do so any day of the year!
September 22 is also designated as the annual Falls Prevention Awareness Day for this year.
Internet Links to Additional Information ---
Autumnal Equinox: Link >>> http://scienceworld.wolfram.com/astronomy/AutumnalEquinox.html
Earth's Seasons: Link >>> http://en.wikipedia.org/wiki/Season
Tilt of a planet's axis: Link >>> http://en.wikipedia.org/wiki/Axial_tilt
Urban legend of eggs and brooms standing on their own, only on an Equinox:
Link >>> http://www.snopes.com/science/equinox.asp
Falls Prevention Awareness Day: Link >>> http://www.ncoa.org/improve-health/center-for-healthy-aging/falls-prevention/falls-prevention-awareness.html
Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
Tuesday, 2020 September 22.
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Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
< http://www.planetarium.cc >
* Adler Planetarium, Chicago:
< http://adlerplanetarium.tripod.com >
* Astronomer, Educator, Optician John A. Brashear:
< http://johnbrashear.tripod.com >
* Andrew Carnegie & Carnegie Libraries:
< http://www.andrewcarnegie.cc >