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Changes in daylight, manmade and natural

By Skip Rigney

Daylight savings time begins this Sunday, March 14th. Of course, no daylight will be saved. The amount of daylight tomorrow will be 11 hours and 57 minutes in Picayune, regardless of whether you reset your clocks or not. With daylight savings time, we simply shift our clocks forward so that there is an additional hour of daylight after noon. The price we pay, in addition to the annoyance of resetting our clocks, is that we have one less hour of daylight when our clocks say “A.M.”

Although it has nothing to do with daylight savings time, over the next two weeks in the Northern Hemisphere, days are getting longer and nights are getting shorter more rapidly than at any other time of the year. Exactly how rapidly depends on how far north of the equator you are.

At the latitude of Pearl River County over the next week, daylight time will increase (and nighttime darkness decrease) about one minute and 50 seconds each day. The farther north you go, the more dramatic the change. For example in the northern state of Maine, each day this week will have over three minutes more daylight than the day before.

The most daylight will be added on the day of the spring equinox, also known as the first day of spring, which is next Saturday, March 20th. After that, days will continue to lengthen, but each day the increase will be a little less than the day before. By mid-June, each new day in Pearl River County will have only a few seconds more daylight than the day before. By then each day’s light will be just over 14 hours long.

After the summer solstice on June 20th, the process will reverse. The amount of daylight will shrink each day, slowly at first, but then more and more rapidly the closer we get to the autumnal equinox in September.

Over the next three months, the increased solar energy from longer days will help turn early spring into full-blown summer. The other critical factor in warming us up will be a change in the angle of the sun’s rays. The less slanted and more directly overhead the sun’s rays, the more energy the earth absorbs.

The angle between the horizon and a point directly above us in the sky, straight overhead, measures 90 degrees. Next Saturday, the first day of spring, at noon in Picayune, the sun will be approximately 60 degrees above the horizon, two-thirds of the way to being directly overhead. (For the mathematically curious, that’s equivalent to 90 degrees minus Picayune’s latitude, which is approximately 30 degrees north of the equator.)

By the summer solstice in June, the sun at noon will be at an angle 83 degrees above the horizon, pretty close to being directly overhead. After that it will sink a little lower each day on its way back to a 60 degree angle at noon on the first day of fall, September 22nd.

Compare that to Maine. There the sun’s angle at noon varies from 45 degrees in March up to only 68 degrees in June, then back to 45 degrees in September.

That means a lot more solar energy is available for the Earth to absorb in Mississippi than in Maine.

No wonder it gets so much hotter here.