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Wednesday, May 4, 2016

Equinox arrives: North America falls into autumn

Friday, September 23, 2011

The autumnal equinox, the beginning of fall by the astronomical calendar, took place at 4:04 a.m. local time today.

According to the web site timeanddate.com, the equinox is the point in the year when the sun is directly over the equator at solar noon for any given location.

For the past six months, which we in the northern hemisphere refer to as spring and summer, the sun is directly over the northern hemisphere during daylight hours, bringing warmer temperatures to the northern half of the planet.

For the next six months, it will be directly over the southern hemisphere at noon, bringing warmer temperatures to the southern half of the planet while we in the northern half experience the chill of fall and winter.

It is actually the earth that is moving, of course, not the sun. The Earth is tilted at an angle as it orbits the sun. For half of its orbit, that tilt means the northern hemisphere faces towards the sun; for the other half of its orbit, the southern hemisphere faces that way.

The change in seasons affects not only the weather but many other aspects of human life.

Seasons of crime

Shelbyville Police Chief Austin Swing said his department sees a definite change in the type of calls it receives depending on the season of the year. In winter, as people are forced indoors, domestic violence calls due to family disputes increase. There are more calls related to abuse of hard liquor.

In the spring, as people move back outside, there are more cases where a party at one home disturbs the neighbors and has to be addressed. As late spring turns into summer, and as children get out of school, juvenile offenses increase. Noisy parties run later in the evening, and people are more likely to have their windows open, increasing the chance that noise at one home will disturb the neighbors. Disputes between neighbors become more common.

The fall is actually a relatively quiet period. Children return to school and loud parties become less common. But then, once the holiday season approaches, shoplifting offenses begin to rise.

Seasonal Affective Disorder, or SAD, refers to episodes of depression that occur during the winter months, associated with a lack of sunshine. Those suffering from the disease are often directed to make special efforts to go outdoors when it is light out, or are even prescribed "light therapy" in which they spend time near a very bright light which mimics the sun.

Rhonda Saylor of Contact Lifeline of the Highland Rim, a 24-hour crisis line serving Bedford and several other counties, said Contact Lifeline is already seeing a spike in calls so far in September -- some of it is related to the economy, but some is related to the change in seasons.

"All that's starting to set in," she said. She noted that at the deepest part of winter, some people leave their homes for work while it's still dark and return home in the dark, which can have an impact on mood.

Saving daylight

The equinox was first identified by Greek astronomer and mathematician Hipparchus, who lived from 190 to 120 B.C.

The dates in March and September when the sun crosses the equator are called equinoxes, while the dates when the sun reaches its farthest north or south are called solstices. So, in the northern hemisphere, we have the spring equinox (March), the summer solstice (June), the autumnal equinox (September) and the winter solstice (December). Each one marks the official beginning of its season.

As the winter approaches, the days will grow shorter.

"Even right now, we're picking up kids standing in the dark," said Jimmy Williams, transportation supervisor for Bedford County schools. By the time school lets out for Christmas break, close to the shortest day of the year, quite a few of the county's approximately 4,000 school bus passengers will be picked up in the dark, and many will be dropped off in the dark after school.

The change in the length of each day is gradual, but a man-made change to the schedule causes it to seem more dramatic. Most of the U.S. is currently under Daylight Saving Time, which artificially shifts the clock forward an hour to allocate more sunlight for evening activities during warm weather. When that ends on Nov. 6, sunset -- which will have been gradually getting earlier prior to that time -- will suddenly be an hour earlier, making the day seem that much shorter.

Not exactly half

Because the changing seasons alter the balance between day and night, many people assume that the spring and fall equinoxes represent the days when there are exactly 12 hours of daylight and 12 hours of darkness. That's close to being true, but not precisely true, according to timeanddate.com.

Sites along the equator will see the closest to a perfect 12-and-12 division, but as you move in either direction, north or south, daylight today will be just barely longer than 12 hours. This has to do with several factors, including the angle at which the sun's light hits those places on the globe, and the fact that the atmosphere bends sunlight slightly, making the sun visible a little sooner and a little later than it would be if the atmosphere were not present.

The dates of the equinoxes vary each year, because our man-made calendar is only a close approximation of the actual movement of the earth around the sun. The autumnal equinox usually falls on the 22nd or 23rd of September, but the math allows it to, in very rare cases, fall on the 21st or the 24th.