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The Moon's Shadow Approaches

By Doug Furton, GVSU Physics Department

About midday on a plain old Monday near the end of August, a darkness will sweep across the United States, a darkness so deep that birds will chirp as if night suddenly fell, a chill will settle in, and the sky will fill with stars for those in the darkness’ path. If that sort of thing sounds scary to you, then you should try to stay out of the way; but if it sounds exciting, you might start planning now to be in the right place at just the right time.

On the 21st of August this year the moon will pass exactly between the sun and Earth, producing what we call a solar eclipse. During a solar eclipse the moon completely blocks out the sun, turning day into night and revealing to an observer in the right place the sun’s tenuous, wispy outer atmosphere, which is normally lost in the sun’s daytime glare.

Solar eclipses are not all that uncommon. Each year there are at least 4 solar eclipses. And during some years there are even more. At first blush, we might expect as many as twelve solar eclipses each year: one each month, when the moon, in its orbit around Earth, slips between Earth and the sun. But the moon’s orbit is slightly inclined (about 5 degrees) to the plane of Earth’s orbit around the sun, causing the moon to more often than not drift unspectacularly above or below the sun from our Earthbound perspective. Yet still one might wonder: if there are at least 4 total solar eclipses every year, why haven’t I seen one? Ask around… you’ll find that very few people have ever seen a total solar eclipse. Surely such a spectacle cannot go unnoticed.

To understand why total solar eclipses are so rare it is helpful to consider the situation from a different perspective. The moon casts a shadow, as does every opaque body that is illuminated by the sun. The central part of any shadow—called the umbra—is very dark. Surrounding the umbra of a shadow, however, is a darkened, but not completely dark region called the penumbra.

Figure 1

Figure 1: The central part of any shadow, called the umbra, is very dark. In the umbra of a shadow, the light source is completely obscured. The fuzzy part of a shadow is called the penumbra. In the penumbra of a shadow, the light source is only partially blocked. Photo by the author.

The moon’s umbral shadow is shaped like a slender tapering cone that points away from the sun. By happenstance, the sizes of and distances to the sun and moon conspire to make it so the moon’s umbral shadow just reaches Earth. So perfectly balanced are size and distance that on Earth’s surface the diameter of the moon’s dark umbra is only about 60 miles.

Figure 2

Figure 2: An image rendered to scale showing the extent of the moon’s umbral and penumbral shadows. The image is from a frame of an animation produced by NASA’s Scientific Visualization Studio.

Viewed from this perspective it becomes apparent why total solar eclipses are so rare. In order to see a total solar eclipse you have to be in just the right place precisely when the tiny tip of the moon’s umbral shadow sweeps over you!

During the solar eclipse on August 21st, the moon’s shadow will sweep from northwest to southeast diagonally across the full extent of the continental United States. The shadow makes landfall very nearly at the start of the eclipse near a town called Lincoln City, Oregon, just west of Salem, at about 1:20 pm EDT, and later slips off to sea near Charleston, South Carolina at about 2:50 pm EDT. The 60-mile wide blotch of darkness covers the almost 2,600-mile distance from Oregon to South Carolina in an hour and a half, moving then at a speed of about 1,700 miles per hour! For comparison, that is about equal to the top speed of an F-16 jet.

Figure 3

Figure 3: The path of the moon’s umbral shadow across the continental United States on August 21st. Image courtesy NASA’s Scientific Visualization Studio.

The area across the U.S. swept out by the moon’s shadow during the August 21st eclipse—60 miles wide by 2,500 miles long—is about 150 thousand square miles. The total surface area of Earth is about 200 million square miles. So, with a little bit of area added to each end of the eclipse path, it only covers about 0.1% of Earth’s surface. That’s why solar eclipses are so rare: you have to be in just the right place at just the right time to see one.

If you don’t want to travel into the path of totality during the August 21st eclipse, the day here in Michigan will still be very unusual. Observers here will see a partial solar eclipse—the moon will only partially block the sun. But Michigan is deep within the moon’s penumbral shadow, and at mid-eclipse, about 2:30 pm EDT, the moon will block just over 80% of the area of the sun’s surface.

Figure 4

Figure 4: The moon’s shadow at mid eclipse. In Michigan, at this time, the moon will block just about 80% of the sun’s surface. The image is from a frame of an animation produced by NASA’s Scientific Visualization Studio.

If you want to learn more about the August 21st eclipse, there are many on-line resources. Three of the most comprehensive are listed below.

NASA’s Eclipse 2017 website at https://eclipse2017.nasa.gov/ 

NASA’s Scientific Visualization Studio: https://svs.gsfc.nasa.gov/Gallery/suneclipse2017.html

A vibrant website at http://www.eclipse2017.org/