2017 Eclipse: A Cosmic Coincidence Turns Day into Night

First in a series of articles on the Great American Eclipse of 2017, which will occur on Aug. 21.

Imagine standing in an open field with the summer sun beating down on you. Then, slowly but steadily, the sun blackens. The temperature drops, birds become quiet, and crickets begin their evening chirps. In less than a minute the bright daytime sky is replaced by twilight. Day has turned into night.

This is a total solar eclipse and one is about to happen in our area. On Aug. 21, the “Great American Eclipse” will be visible along a narrow band that stretches from Oregon, across the central United States, and out through our home state of South Carolina.

Solar eclipses are those rare cosmic events when the moon passes between the sun and the earth. For those fortunate enough to find themselves deep within the moon’s shadow, the sun appears completely blocked and day becomes night for a few short moments.

But it did not have to be this way. Total solar eclipses are the result of an unexpected cosmic coincidence. By chance the sun is about 400 times larger than the moon, but it is also approximately 400 times farther away. From our viewpoint on earth, this means the sun and moon appear to be about the same size on the sky. When they align, the moon fits nicely in front of the sun.

In those rare moments when the moon finds itself between the sun and earth, our view of the sun is blocked, making it appear black, and we experience the strange wonders of a total solar eclipse.

But why are solar eclipses so rare?

To answer that question we have to think about the relationship between the three celestial bodies in space.

The sun sits at the center of our solar system. Every planet orbits about the sun in a nearly, but not perfectly, circular orbit. Some of the planets carry along with them smaller companions. Moons can vary in size from tiny, irregular shaped mounds, to large spherical objects. Earth’s moon is very unusual in its size. While our moon is only the fifth largest in the solar system, it is the largest compared to its parent planet at just over a quarter the size of the earth.

The moon takes about a month to complete a trip around our planet. A view from far above the earth’s North Pole would show the moon sometimes in between the sun and earth. A week later the moon is off to the side of the earth. Another week finds the moon on the far side of the earth from the sun. One more week finds the moon to the side again. Finally, after about a month, the moon is back between the sun and the earth. During this trip the moon changes its appearance to use by going through a set of lunar phases.

Solar eclipses occur when the sun, moon, and earth line up, which should occur once a month. But the moon’s orbit is slightly tipped compared to earth’s orbit around the sun. This means that most of the time when the moon is between the sun and earth, its shadow passes above or below our planet and we do not have a solar eclipse.

However, twice a year there is a chance for alignment. This time is known as eclipse seasons which occur about every 6 months. During these special occasions, the moon can find itself perfectly lined up, if the timing is right. As it turns out, the timing does work out about once every 18 months. During these special moments there is a total solar eclipse visible somewhere on earth.

The true rarity in a solar eclipse is how long we have to wait for an eclipse to occur twice at the same location. The answer varies by location, but on average it is a few centuries.

During an eclipse, the moon’s shadow covers a small portion of the earth’s surface. The region of totality is about 70 miles across. With every eclipse this region changes due to the specifics of the celestial alignment. For this reason, we you may take a few generations between when a solar eclipse is seen at the same homestead.

The upcoming solar eclipse marks the first time since Feb. 26 1979 that a total solar eclipse will be visible anywhere in the contiguos United States. The last time an eclipse crossed the continental United Sates was 1918. That year the moon’s shadow passed from Washington, through Florida, and finally out to Bermuda. The next solar eclipse will mark the first time in our nation’s history that the eclipse will be exclusive to the United States alone, making it the “Great American Eclipse” of 2017.
On the afternoon of Aug. 21, take a moment, head outside and enjoy the Great American Eclipse. As you do so, reflect on the cosmic quirk in the sizes and distances of the sun and moon that cause the day to turn into night.

Dr. Louis J. Rubbo is an associate professor of physics and astronomy at Coastal Carolina University. He has a Ph.D. in physics from Montana State University. During the month leading up to the Aug. 21 eclipse, he will write weekly articles surrounding different aspects of the eclipse. He will also give a public lecture on the science of the eclipse at the Strand Theatre in Georgetown on Aug. 20 at 6 p.m.

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