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Rudi Kiefer: A year-end primer on earth’s equinoxes, solstices and seasons
Rudi Kiefer
It’s Dec. 31, the last day of 2017. Exotic terms are floating around: equinox, solstice, perihelion, aphelion. All of them have something or other to do with the sun and the Earth.

But where does Dec. 31 fit in? The answer is “nowhere.” Today’s date, and Monday’s start of the new year, seems rather arbitrary when you consider the important milestones on the path that the Earth takes around the sun.

We all use the Gregorian calendar, established by Pope Gregory XIII in 1582. It consists of 365.2425 days. That leaves us with a little under six hours to dispose of annually. Every four years, we add a day to February and deal with the smaller leftovers in larger cycles.

The positions of the Earth with respect to the sun are a little easier to understand. The closest one happened 10 days ago, the winter solstice.

Like a motorcyclist rounding a steep curve, the globe is tilted on its orbit. On Dec. 21, this tilt has North America, Europe and Asia leaning away from the sun, giving us short days and long nights.

During the equinoxes March 20 and Sept. 22, both hemispheres get equal amounts of daylight and nighttime. Farther along in its orbit, the Earth’s northern hemisphere was tilted toward the sun June 21.

Seasons are controlled by solstices and equinoxes, not by the change to a new year, and not by the globe’s distance from the sun. During perihelion on Jan. 3, 2018, the Earth will be closest to the sun, but this doesn’t promise a warm day. Likewise, aphelion on July 6 will have us farthest away, but it won’t be chilly in North Georgia.

Another problem with the calendar is the Earth’s rapid rotation. The town of San Antonio de Pichincha, Ecuador, for example, is located exactly on the equator, so it moves at the speed of 1,038 miles per hour.

Earth’s rotation makes it necessary to divide the world into hourly time zones. Traveling westward, we turn the clock backward. It’s the opposite if we travel east.

By necessity, when one crosses the International Date Line in the middle of the Pacific Ocean, the clock advances for travelers heading east, but the calendar snaps back by a full day. Confusing enough, but it’s the only way to keep track of the “where” and “when” worldwide.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at

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