autumnal equinox; how leaves change color
The first day of Fall is Wednesday, Sept. 22 and the Autumnal Equinox occurs later in the night on that date with a Harvest Moon. Image: Thinkstock

The first day of fall 2010 is Wednesday. Most people may not know that Summer’s end is different from the Autumnal Equinox 2010. While the first day of fall is a date on the calendar — Sept. 22 — the Autumnal Equinox is an instant in time as Earth travels through space around the sun. Equinox means “equal nights,” and day and night are roughly equal on the Equinox in either spring or fall. As the days continue to grow shorter, the ebbing sunlight causes a chemical reaction in leaves, transforming them into brilliant colors. The first day of fall 2010 offers the bonus of a Harvest Moon.

First day of fall vs. Autumnal Equinox

The First day of Fall 2010 is Sept. 22, but the season doesn’t technically change until well into the night at 11:09 p.m. EDT, when Autumnal Equinox 2010 occurs. The seasons change because the Earth spins on an axis at about a 23-degree angle from the horizontal plane of its orbit (axial tilt). If the north and south pole were exactly vertical to Earth’s horizontal orbit, there would be no change of seasons. As the planet travels in orbit, its axis is tilted toward the sun during one portion, causing the northern hemisphere to be exposed to more sunlight (summer). During another portion of the orbit the axis is turned away and the northern hemisphere gets less light and warmth (winter). 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.

How leaves change color

At the instant of the Autumnal Equinox, the the sun is directly over the equator. Day and night are roughly equal just about everywhere on Earth. In the northern hemisphere the daylight has been getting shorter for 94 days — since the Summer Solstice. Less sunlight sends a signal to the trees to get ready for winter hibernation. They begin to shut down the process of photosynthesis, a chemical reaction in leaves that converts sunlight and water into glucose to fuel growth. Chlorophyll is the catalyst for photosynthesis. It’s the chemical that gives leaves their green color. Without enough sunlight for photosynthesis, chlorophyll disappears from the leaves. The green color goes away, and the bright colors in the leaves that were there all along, but obscured by chlorophyll, emerge from hiding.

Harvest Moon headlines skywatching show

On the first day of fall, just after sunset, the Harvest Moon will rise above the horizon. Technically, the moon won’t be full until early in the morning Thursday. This makes it the full moon closest to the Autumnal Equinox, which is known as the Harvest Moon. The nearly full moon will be accompanied by the planet Jupiter, shining like a bright beacon just below it. Jupiter made its closest approach to Earth in nearly 50 years on Monday. Skywatching with binoculars, people may be able to make out the planet Uranus, just above and slightly to the left of Jupiter. 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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