Orbital mechanics, photosynthesis and the first day of fall

Photo of an autumn picture.

It is officially the first day of fall. CC by Mulberry24/Flickr

Late Wednesday night, Summer officially ends. Most people may not know that Summer’s end is different from the Autumnal Equinox 2010. The Autumnal Equinox is about the interaction of a planet and its sun, while the first day of fall is about life on Earth. Latin in origin, Equinox means “equal nights,” which describes the virtually even distribution of night and day around the first day of fall. As the days continue to grow shorter, the ebbing sunlight causes a chemical reaction in leaves, transforming them into brilliant colors. This year, the first day of fall has the added attraction of a Harvest Moon.

The truth about the first day of fall

The 2010 calendar shows the first day of fall as Sept. 22, but 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. Eventually Earth reaches a point in its orbit when the axis tilts away from the sun. Fall and then winter arrive in the northern hemisphere as a lesser area of that half of the planet gets direct sunlight. The Vernal Equinox and Autumnal Equinox occur at the exact transition when the tilt of the Earth’s axis is inclined neither away nor toward the sun.

The science behind fall 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. Photosynthesis, the biological engine in the leaves of trees, is ratcheted down. Chlorophyll is the substance trees utilize to make photosynthesis possible. Leaves are various shades of green because of the chlorophyll present in them. Without enough sunlight for photosynthesis, chlorophyll disappears from the leaves. The leaves lose their various shades of green. What is left are the brilliant colors liberated by the absence of chlorophyll.

Harvest Moon shines on first night of fall

After sunset on the first day of fall, the Harvest Moon lights up the night. The moon will be full, although it doesn’t reach that designation from a technical standpoint until the wee hours of Thursday morning. This makes it the full moon closest to the Autumnal Equinox, which is known as the Harvest Moon. The planet Jupiter appears just below the full moon in the sky, brighter than all the stars. Monday, Jupiter was closer to Earth than it had been since 1963. The planet was only 363 million miles away. Using binoculars, sky-watchers may also be able to see, at about 10 or 11 o’clock from Jupiter, the planet Uranus. For those blessed with a warm night and clear weather, the celestial show marks an end of a first day of fall to remember.





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