On May 20th, we on the East Coast will unfortunately miss the first solar eclipse this century to reach the continental United States. A solar eclipse occurs when the moon passes directly in front of the sun and casts its shadow upon the Earth. Solar eclipses are actually not rare on Earth, with two to five eclipses occurring each year somewhere on the planet. Because the apparent sizes of the moon and the sun are nearly matching, an eclipse’s shadow is very narrow — always less than about 150 miles in width. The path of this eclipse runs from Taiwan, across the Pacific, entering the U.S. in northern California and ending in eastern Arizona.
Partial eclipses, in which the center of the moon does not cross the center of the sun, can be seen up to 3,000 miles from the center of the eclipse. Unfortunately, this eclipse will start at about 8:30 p.m. on the 20th, about 11 minutes after sunset. A partial eclipse will be visible as far east as upper Michigan.
This is a particularly interesting eclipse because it happens in the same lunar cycle as the . The , you will recall, occurs because the day of the full moon coincides with the day of perigee — the nearest approach of the moon to Earth. Because any solar eclipse must occur at the moment of new moon, and new moon occurs at the point in the moon’s orbit opposite to the full moon position (approximately), the moon will be near its furthest distance from Earth during this eclipse.
As a result, this eclipse will not completely cover the disk of the sun — this will not be a total eclipse, but rather an “annular” eclipse. At the point of maximum eclipse, the sun will appear as a bright (dangerously bright, please remember) ring in the sky. The effect (which I have never personally seen) is extraordinarily eerie, resulting in unusual shadowing effects on the ground.
The phenomenon of a solar eclipse, though rather common on Earth, is exceeding rare in the universe at large. Here on Earth, our moon has an average apparent diameter almost exactly that of the sun — allowing the moon to completely cover the sun during a total eclipse, and allowing us to see the outer regions of the sun’s atmosphere during these events.
The effect of this fact on the history of mythology, religion, and science in particular has been astounding. Prior to the 20th century, most of what was known about the solar atmosphere — the existence of the corona, of solar flares and prominences, even the cause of the aurora — had been discovered only through the observation of total solar eclipses.
The nearly exact match in apparent sizes of moon and sun requires a precise arrangement between the size and distance of the moon, and the size and distance of the sun. Our planet has an unusually large moon for its size — no other planet in our solar system has a moon anywhere near ¼ the size of its planet, and no other planet in our solar system experiences eclipses in which a moon exactly covers the sun.
Furthermore, our moon’s orbit is known to be continually increasing in size — the moon moves away from Earth, on average, about one and a half inches every year. Although this seems a small distance, over a billion years this is about 1/10th the distance to the moon. A billion years from now, total solar eclipses will no longer occur on Earth; a billion years ago, an annular eclipse would never occur.
For those of us not willing to travel to chase solar eclipses, the next solar eclipse in the United States will occur on August 21st, 2017. This eclipse will be visible over the entire country, and will be a total eclipse along a narrow strip of land passing nearest to us in Kentucky and North Carolina.