In case you missed it, Sept. 22 was the autumnal equinox, the official first day of autumn, when the sun crosses the plane of Earth's equator and makes day and night approximately of equal length.

It also explains why all the deciduous trees are doing a striptease and burying us under a blanket of leaves. We humans are such neat freaks, we don't realize this is nature's way of replenishing the soil. If we didn't rush to rake them up, they would decay and compost right on the ground where they fell.

I will admit I rake them up, too, but then I chip them into finer parts and compost them in my garden to add all the nutrients I have removed in the harvest. These composed leaves do wonders for black mud.

The autumnal equinox signals for me the best time of year. Not too cold, not too hot. It also begins the rainy season here in Oregon, but this early-on rain is a gentle kind that just sort of washes the dirty air clean.

I enjoy the chemical changes in the color of the leaves. Maples turn bright red; other leaves become yellow and orange.

Leaves are the food factories for trees. The trees take water from the ground through roots, then take carbon dioxide from the air and uses sunlight to make these two elements into glucose -- a kind of sugar that the tree use for energy. It is called photosynthesis -- putting together with light. That is why the leaves on a tree are green and change color in the dying process.

Every fall, the leaves fall, an event triggered by the autumnal equinox. Oddly this September occurrence happened on the same day in 2008, and will move up a day in 2010 to Sept. 23, but will occur on that same day in 2011. This variation is due to the Gregorian calendar. Most Western countries follow the Gregorian calendar, which has 365 days in a year and 366 days in a leap year.

Equinoxes generally occur about six hours later each year, with a jump backwards in a leap year. The extra day in a leap year is to minimize the gradual drift of the equinox date through the seasons.

The next big seasonal change comes on Dec. 21, the date of the arrival of the winter solstice, a time when the sun is furthest from the equator. Summer solstice, caused by the same phenomenon, occurs on June 21.

It was Hipparchus, a Greek astronomer and mathematician who discovered the precession, the regular motion of a spinning body such as a planet in which the axis of rotation sweeps out of a cone. He noted the slow movement among the stars of the two places where the sun crosses the celestial equator. The word equinox comes from the Latin meaning "equal night."

Hipparchus also made observations that identified for us the solstice. He is also famous for having catalogued the stars, but his mapping of the heavens has been lost. Interestingly, the September equinox is the oldest reference point denoting the seasons. You might call Hipparchus a man of all seasons.

The vernal equinox, the time when the sun crosses the celestial equator and day and night are of equal length, marking the beginning of spring, occurs in March and the autumnal equinox comes in September.

Now, if you happen to be reading this in Australia or New Zealand, just turn everything I have said upside-down. The autumnal equinox in the Southern hemisphere happens in March, and the vernal equinox comes in September.

Well, mates from down under, happy springtime. Put one on the "barbie" for me.

Bill Duncan can be reached by writing to P.O. Box 812, Roseburg, OR 97470, but he is busy this time of year raking autumnal equinox droppings.

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