Fall begins with the advent of the Autumnal Equinox the night of September 22. Although everyday life is not affected, there is a distinction between the Autumnal Equinox and the very first day of fall. The very first day of fall is an Earthbound phenomenon; the Autumnal Equinox involves the mechanics of the solar system. Latin in origin, Equinox means “equal nights,” which describes the virtually even distribution of evening and day around the very first day of fall. When evening gains the advantage, trees respond with a transformative chemistry that changes green leaves into fiery shades. A Harvest Moon makes a rare appearance to mark the occasion in this year.
Why the Autumnal Equinox follows the very first day of autumn
The 2010 calendar shows the very first day of fall as September 22, however in reality the transition doesn’t occur until the Autumnal Equinox comes about at 11:09 p.m. EDT. The temperate zones of Earth have seasons because of its “axial tilt,” which means the poles are skewed at about a 23-degree angle as the planet travels along its orbit around the sun. If the north and south pole were exactly vertical to Earth’s horizontal orbit, there would be no change of seasons. Earth’s northern hemisphere receives a majority of the sun’s rays within the orbit when the planet’s axis is oriented toward its star. During an additional portion of the orbit the axis is turned away and the northern hemisphere gets less light and warmth (winter). As Earth revolves around the sun, its axis reaches a point at the Vernal and Autumnal Equinox when it tilts along the path of the orbit, instead of away or toward the sun.
The technology powering autumn colors
On the first day of fall, the sun is at about a zero angle over the equator. Day and night are roughly equal just about everywhere on Earth. Starting with the Summer Solstice 94 days prior, days have been getting shorter. Daylight diminishes to a point where the trees sense the coming winter. Trees and shrubs begin to winterize. They turn off photosynthesis, the biochemical process in which leaves transform water and sunlight into their essence for life and growth. Chlorophyll is the catalyst for photosynthesis. Leaves are various shades of green because of the chlorophyll present in them. Trees shut down production of chlorophyll when there is no longer enough sunlight to trigger photosynthesis. With no chlorophyll to make the leaves green, they take on many other brilliant shades.
Harvest Moon headlines sky-watching show
After sunset on the first day of fall, the Harvest Moon lights up the night. The full moon doesn’t officially arrive until the morning hours Thursday, but it can be close enough Wednesday night. Being that it occurs so close to the moment of the Autumnal Equinox, this full moon is known as the Harvest Moon. The bright star that cannot be missed just below the full moon is really the planet Jupiter. Monday, Jupiter was closer to Earth than it had been since 1963. The planet was only 363 million miles away. Slightly above and left of Jupiter, the planet Uranus may be visible to sky-watchers observing with binoculars. Individuals lucky enough to have good weather can enjoy a spectacular conclusion to the first day of autumn this year.
USA.gov
answers.usa.gov/cgi-bin/gsa_ict.cfg/php/enduser/std_adp.php?p_faqid=7114
Wikipedia
en.wikipedia.org/wiki/Equinox
Space.com
space.com/spacewatch/jupiter-uranus-harvest-moon-skywatching-100921.html
Science Made Simple
sciencemadesimple.com/leaves.html
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