Earth rotating during the Autumnal Equinox.
Incoming solar energy is equal in both the Northern
and Southern Hemispheres.
(Image Source: NOAA)
By Glenn A. Walsh
Reporting for SpaceWatchtower
The Autumnal Equinox, the beginning of the season of Autumn or Fall in the Northern Hemisphere, began at the moment of the posting of this blog post: Monday Evening, 2014 September 22 at 10:29 p.m. EDT / September 23, 2:29 Coordinated Universal Time (UTC).
In the Southern Hemisphere, this marks the astronomical beginning of the season of Spring.
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 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.
September 23, the first full day of Fall, is designated the seventh annual Falls Prevention Awareness Day.
More on the Autumnal Equinox: Link >>> http://scienceworld.wolfram.com/astronomy/AutumnalEquinox.html
More on the season of Autumn or Fall: Link >>> http://en.wikipedia.org/wiki/Autumn
More on an Equinox: Link >>> http://en.wikipedia.org/wiki/Equinox
More on Earth's Seasons: Link >>> http://en.wikipedia.org/wiki/Season
More on the tilt of a planet's axis: Link >>> http://en.wikipedia.org/wiki/Axial_tilt
More on Falls Prevention Awareness Day: Link >>> http://www.ncoa.org/improve-health/center-for-healthy-aging/falls-prevention/falls-prevention-awareness.html
Special Thanks: Eric G. Canali, former Floor Manager of Pittsburgh's original Buhl Planetarium and Institute of Popular Science and Founder of the South Hills Backyard Astronomers amateur astronomy club.
Source: Glenn A. Walsh, Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
2014: 75th Year of Pittsburgh's Buhl Planetarium
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