Saturday, September 22, 2018

Autumn Begins Saturday Evening; Harvest Moon Monday Night
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 solstices and equinox of the year.
(Image Source: ©1999, Eric G. Canali, former Floor Operations Manager of Pittsburgh's original Buhl Planetarium and Institute of Popular Science 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 Autumnal Equinox, the beginning of the season of Autumn or Fall in the Northern Hemisphere of Earth, begins Saturday evening. In Earth's Southern Hemisphere, this equinox marks the astronomical beginning of the season of Spring. And, the Harvest Moon in the Northern Hemisphere, the Full Moon of September, is on Monday evening.

                                                  September Equinox

The September Equinox occurs on Saturday Evening, 2018 September 22 at 9:54 p.m. Eastern Daylight Saving Time (EDT) / September 23, 1:54 Coordinated Universal Time (UTC).

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 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 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.

                                                    Harvest Moon

This year's Harvest Moon will occur on Monday Evening, 2018 September 24 at 10:52 p.m. EDT / September 25 at 2:52 UTC. Often, but not always (usually, two out of every three years), the September Full Moon is considered the Harvest Moon. The Harvest Moon is the Full Moon closest to the Autumnal Equinox, when the season of Autumn or Fall officially begins, usually September 22 or 23.

So, the Harvest Moon, in general, can occur from two weeks before the Autumnal Equinox to two weeks after the beginning of Fall. When the October Full Moon occurs early in the month, it is then sometimes considered the Harvest Moon. The October Full Moon, which is usually the first Full Moon after the Harvest Moon, is usually considered the Hunter's Moon, providing hunters with additional light to hunt game after sunset. When the October Full Moon is considered the Harvest Moon, some still consider it the Hunter's Moon as well, while others then consider the November Full Moon the Hunter's Moon.

Occurring in the late Summer or early Autumn, in September or October in the Northern Hemisphere, the Harvest Moon provides farmers with additional light in the early evening, during the very busy harvest time.  The Harvest Moon has the same characteristics in the Southern Hemisphere, when it occurs in March or April.

On average, throughout the year, the Moon rises 50 minutes later each day. However, this lag time between successive Moon rises shrinks to an annual minimum near the Full Moon closest to the Autumnal Equinox. This greatly reduced lag time averages between 25 and 35 minutes throughout most of the United States, and as little as 10 to 20 minutes for Canada and much of Europe, each day for several days around the time of the Full Moon. Hence, for a few days around the date of the Harvest Moon, there is little or no period of darkness between sunset and Moon rise. This provides farmers with several days of extra, uninterrupted, light after sunset, for completing the harvest (of course, weather-permitting).

In the Northern Hemisphere, the September Full Moon is also known as the Corn Moon, Fruit Moon, Wild Rice Moon, and Red Plum Moon.

The September Full Moon has been given several names in the Southern Hemisphere: Worm Moon, Lenten Moon, Crow Moon, Sugar Moon, Chaste Moon, and Sap Moon.

Internet Links to Additional Information ---

Season of Autumn or Fall: Link >>>

Equinox: Link >>>

Earth's Seasons: Link >>>

Tilt of a planet's axis: Link >>>

Urban legend of eggs and brooms standing on their own, only on an Equinox:
Link >>>

Falls Prevention Awareness Day: Link >>>
Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
              Saturday, 2018 September 22.

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Glenn A. Walsh --- < >
Electronic Mail: < >
Project Director, Friends of the Zeiss: < >
SpaceWatchtower Editor / Author: < >
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.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
  < >
* Adler Planetarium, Chicago:
  < >
* Astronomer, Educator, Optician John A. Brashear:
  < >
* Andrew Carnegie & Carnegie Libraries:
  < >

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