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.
(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
The Harvest Moon, this year the Full Moon of September, occurs early Monday evening. Fall or Autumn begins less than two days later on Wednesday afternoon.
For the year 2021, the Harvest Moon will be the Full Moon of Monday Evening, September 20, at 7:54 p.m. Eastern Daylight Saving Time (EDT) / 23:54 Coordinated Universal Time (UTC). For farmers eager to finish harvesting their crops, the bright Full Moon which shines on their farms for the several evenings closest to the Autumnal Equinox (the Autumnal Equinox is the astronomical beginning of the season of Autumn or Fall in Earth's Northern Hemisphere) is called the Harvest Moon.
The Harvest Moon is one of the signature astronomical events near the beginning of, or shortly after the beginning of, the Fall season. It is an event particularly anticipated by farmers of both the past and the present. As many crops reach the time of harvest in late Summer and early Autumn, often the work of the harvest has to continue past sunset, which comes earlier and earlier each evening.
Nature has come to the rescue of these farmers, with a bright Full Moon (weather-permitting), which arrives just around the time of sunset, that allows farmers and their staff to continue the harvest after the Sun's light has dissipated. Hence, long-ago this Full Moon came to be known as the Harvest Moon.
For a similar reason, the Full Moon of October is often known as the Hunter's Moon, which allowed Native Americans to continue the hunt after sunset, to begin to store meat for the coming Winter months. However, the Harvest Moon is designated as the closest Full Moon to the Autumnal Equinox, and such a Full Moon does not always occur in September. Every few years the Harvest Moon occurs in October, shortly after the Autumnal Equinox. During those years, the Hunter's Moon occurs in November.
On average, the Moon rises about 50 minutes later each day. However, during the days near the Autumnal Equinox, the Moon rises each day only about 25-to-35 minutes later each day in the U.S.A., and only 10-to-20 minutes later in much of Canada and Europe. Thus, for several days around the time of the Autumnal Equinox, the Harvest Moon appears to rise around the same time each evening (roughly coinciding with local sunset), providing light at the time most needed by farmers.
The reason for this is due to the ecliptic, the apparent path of the Sun, Moon, and planets through Earth's sky, which makes a narrow angle with the horizon this time of year. It is this narrow angle which provides that moonrise occurs around the time of sunset, near the time of the Full Moon of September (or sometimes October). Hence, several days appear to have a rising Full Moon.
Also, at this time of year when farmers need moonlight the most, the Harvest Moon appears larger and more prominent, due to the mysterious but well-known "Moon Illusion" that makes the Moon seem larger when it is near the horizon. And, while near the horizon, the Moon is often reddened by clouds and dust, creating the appearance of a large, rising red ball.
Some even liken a rising Harvest Moon to a rising "Great Pumpkin," of Peanuts comic-strip fame! In the Peanuts' network-television cartoon just before Halloween each year (originally aired on CBS-TV on 1966 October 27) titled, "Its the Great Pumpkin, Charlie Brown", the “Great Pumpkin” rises over the pumpkin patch to provide gifts to all good little boys and girls.
In China and Vietnam, a popular harvest festival is celebrated on the date close to the Autumnal Equinox of the Solar Cycle, as well as close to the Harvest Moon. This Mid-Autumn Festival / Moon Festival dates back more than 3,000 years to Moon worship in China's Shang Dynasty.
Native Americans also called the Full Moon of September the Corn Moon, as Corn was one of their main crops. Sometimes, the September Full Moon in the Northern Hemisphere is also known as the Fruit Moon.
In the Southern Hemisphere, where Winter is about to turn to Spring, the September Full Moon is known as the Worm Moon, Crow Moon, Sugar Moon, Chaste Moon, or Sap Moon. The Harvest Moon in the Southern Hemisphere occurs in March or April, with the same advantages to Southern Hemisphere farmers as the Harvest Moon in the Northern Hemisphere.
Beginning of Autumn
The Autumnal Equinox (also known as the September Equinox), the beginning of the season of Autumn or Fall in the Northern Hemisphere of Earth, occurs Wednesday Afternoon, 2021 September 22 at 3:21 p.m. Eastern Daylight Saving Time (EDT) / 19:21 Coordinated Universal Time (UTC). In Earth's Southern Hemisphere, this equinox marks the astronomical beginning of the season of Spring.
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 ecliptic, 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 about a half-year later during that hemisphere's season of Winter. As mentioned, during an Equinox [about half-way between Summer and Winter (Autumnal Equinox), and about half-way between Winter and Summer (Vernal Equinox)] 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, after Autumn or Fall has begun.
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 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 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 ---
Harvest Moon: Link >>> https://science.nasa.gov/science-news/science-at-nasa/2013/16sep_harvestmoon/
Native American Full Moon Names: Link >>> https://earthsky.org/astronomy-essentials/full-moon-names/
Mid-Autumn Festival / Moon Festival: Link >>> https://en.wikipedia.org/wiki/Mid-Autumn_Festival
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 >>> https://nationaltoday.com/falls-prevention-awareness-day/
Monday, 2021 September 20.
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Glenn A. Walsh, Informal Science Educator &
Link >>> http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/gaw/
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Project Director, Friends of the Zeiss: Link >>> http://buhlplanetarium.tripod.com/fotz/
SpaceWatchtower Editor / Author: Link >>> http://spacewatchtower.blogspot.com/
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
* Other Walsh Authored Blog & Web-Sites: Link >>> https://buhlplanetarium.tripod.com/gawweb.html