September 16, 2004
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The leaves on the trees won't notice it, and the clouds in the sky won't notice it, but at 12:30 p.m. Eastern Daylight Time on Wednesday, September 22, 2004, fall will begin in Earth's Northern Hemisphere astronomically speaking. At that moment the Sun passes over Earth's equator heading south, an event called the autumnal equinox.
Why do we say summer ends and fall begins at an exact moment, when the natural events of the changing seasons happen gradually? Some cultures recognize only two seasons: rainy and dry. Others observe three: growing, harvesting, and winter. Close watchers of nature could easily mark 10 or more seasons.
The four that we use winter, spring, summer, and fall would be just as arbitrary, except for one thing. Their beginning and ending points have been defined as actual key moments in the Earth's annual orbit around the Sun or equivalently, from our point of view, the Sun's annual motion in Earth's sky.
The Sun appears farther north or south in our sky, depending on the time of year, because of what some might consider an awkward misalignment of our planet. Earth's axis is tilted about 23½° with respect to our orbit around the Sun. So when we're on one side of our orbit, the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. Six months later, when we're on the opposite side, the Northern Hemisphere is tipped away from the Sun, the slanting solar rays heat the ground less, and we get winter.
For a skywatcher at north temperate latitudes, such as in the continental United States, the effect is to make the Sun appear to creep higher in the sky each day from late December to late June, and back down again from late June to late December. An equinox comes when the Sun is halfway through each journey.
This celestial arrangement makes several other noteworthy things happen on the equinox date:
Day and night are almost exactly the same length; the word "equinox" comes from the Latin for "equal night." (A look in your almanac will reveal that day and night are not exactly 12 hours long at the equinox, for two reasons: First, sunrise and sunset are defined as when the Sun's top edge not its center crosses the horizon. Second, Earth's atmosphere distorts the Sun's apparent position slightly when the Sun is very low. Have these facts on hand when you get the inevitable calls at the equinox from people saying your sunrise and sunset times must be wrong because they are not 12 hours apart.)
The Sun rises due east and sets due west (as seen from every location on Earth). The fall and spring equinoxes are the only times of the year when this happens.
If you were standing on the equator, the Sun would pass exactly overhead at midday. If you were at the North Pole, the Sun would skim completely around the horizon as the six-month-long polar night begins.
In the Southern Hemisphere, September's equinox marks the start of spring, and the March equinox marks the start of fall. (Summer for kangaroos begins in December, their winter in June). Not even the most diligent jet setter could manage to live in an endless summer, but by traveling between hemispheres at just the right time, you could live in an endless spring and summer.
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