This diagram shows the position of the Earth, in relation to the Sun, at the time of the Vernal Equinox at the official beginning of the season of Spring in the Earth's Northern Hemisphere (Autumn in Earth's Southern Hemisphere), as well as the other equinox and solstices of the year.
(Graphic 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
Spring begins early Saturday morning at the moment of the Vernal Equinox in Earth's Northern Hemisphere. In the Southern Hemisphere of Earth, this marks the astronomical beginning of the season of Autumn.
Vernal Equinox on Earth
The Vernal Equinox occurs on Earth at precisely: 5:37 a.m. Eastern Daylight Saving Time (EDT) / 9:37 Coordinated Universal Time (UTC) on Saturday Morning, 2021 March 20.
As the diagram at the beginning of this
blog-post demonstrates, on the day of 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 a 23.439281-degree angle from the plane of the Earth's orbit around the Sun, which is part of the Ecliptic of our Solar System. 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.
March 16 marked 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 and 3-to-4 days after the Autumnal Equinox (Equilux is on September 25, while the Autumnal Equinox is ~ September 22 or 23).
An urban legend that has been making the rounds for decades, now exacerbated by the Internet and Social Media, 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 year.
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 and stiff bristles) on an Equinox, it can do so any day of the year!
In ancient times, the Vernal Equinox was considered the beginning of the new calendar year, as Spring brought new life after the cold Winter months. The calendar year was then defined as the time from one Vernal Equinox to the next. This is known as the Tropical Year: 365 days, 5 hours, 48 minutes, and 45 seconds.
This was when most of Western Civilization used the Julian Calendar, recommended by astronomer Sisogenes and approved by Roman leader Julius Caesar in 46 B.C. Due to the difference between the Julian Calendar and the calendar we use today, known as the Gregorian Calendar, the Vernal Equinox then occurred on March 25, later observed by Christians as the Feast of the Annunciation (observed nine full months before Christmas Day). As part of the Gregorian Calendar reform, in October of 1582, Roman Catholic Pope Gregory XIII chose the Feast of the Circumcision of Christ (January 1) as the beginning of the New Year in the Roman Catholic Church's Liturgical Year.
The Vernal Equinox continues to be considered the beginning of the New Year, or an important holy day, in several other places on Earth ---
* Beginning of New Year (using the Solar Calendar) - Nowruz: Afghanistan and Iran / Persia.
* Holy Day for adherents of the Zoroastrian Religion (the three Magi, who the Christian Bible reports visited the Christ Child after following the Star of Bethlehem / Christmas Star, were adherents of the Zoroastrian Religion).
* Holy Day for adherents of the Bahá'í Faith: Baha'i Naw-Ruz, one of nine holy days of the Bahá'í Faith.
NASA and the European Space Agency (ESA) observe Sun-Earth Day on or near the Vernal Equinox. This is a joint educational program started in 2000, to popularize the knowledge about the Sun, and the way it influences life on Earth, among students and the public. This is part of Solar Week, which is the calendar week that includes the Vernal Equinox.
The first week of Spring, beginning with the Vernal Equinox, has been declared by physicians as Medicine Cabinet Clean-Up Week. To avoid prescription drug abuse, particularly important at this time of the opioid crisis, physicians encourage everyone to get rid of unused and no-longer-needed medications and other drugs, which may have lingered in the household, as part of an annual Spring cleaning. Several states have prescription drug take-back locations, where these drugs can be dropped-off.
The week of the Vernal Equinox is the also the beginning of the National Cherry Blossom Festival held each year in Washington, DC, which begins on March 20. This festival commemorates the 1912 gift of 3,000 cherry trees from the Mayor of Tokyo to the City of Washington. The festival runs through April 11 this year and will be available both on-line and with events in Washington, according to the National Cherry Blossom Festival web-site: “with an innovative format that honors the tradition of the Festival, while prioritizing the health and safety of participants amidst the coronavirus pandemic.”
Internet Links to Additional Information ---
Vernal Equinox: Link >>> https://en.wikipedia.org/wiki/March_equinox
Season of Spring: Link >>> http://en.wikipedia.org/wiki/Spring_%28season%29
Equinox: Link >>> http://en.wikipedia.org/wiki/Equinox
Earth's Seasons: Link >>> http://en.wikipedia.org/wiki/Season
Tilt of a Planet's Axis: Link >>> http://en.wikipedia.org/wiki/Axial_tilt
Related Blog-Post ---
"Mon.: Winter Begins & Great Conjunction: Jupiter & Saturn; Tue: Ursid Meteors." Sun., 2020 Dec. 20.
Saturday, 2021 March 20.
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Glenn A. Walsh, Informal Science Educator &
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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: Link >>> http://www.planetarium.cc Buhl Observatory: Link >>> http://spacewatchtower.blogspot.com/2016/11/75th-anniversary-americas-5th-public.html
* Adler Planetarium, Chicago: Link >>> http://adlerplanetarium.tripod.com
* Astronomer, Educator, Optician John A. Brashear: Link >>> http://johnbrashear.tripod.com
* Andrew Carnegie & Carnegie Libraries: Link >>> http://www.andrewcarnegie.cc