An observer’s guide to the August 2017 eclipse.
The armies battled beneath the hot sun. Neither side seemed to be winning. It was May 28, 585 BCE, the sixth year of war between the Lydians and the Medes in what is now Turkey. Suddenly, the light grew thin and the air turned cold. A hole appeared in the sky where the sun should be, surrounded by a halo of light. Surely this was a sign from the gods that they should make peace.
A total eclipse of the sun, as those ancient armies witnessed, could be frightening if you didn’t know it was coming. That fear could even stop a war! Nowadays, we know what causes eclipses. We can predict when and where they’ll happen, down to the minute and mile. Instead of frightening people, an eclipse can bring millions of us together in a wondrous moment of awe.
What Causes an Eclipse?
Earth orbits the sun, and the moon orbits Earth. The moon passing between the sun and Earth causes a solar eclipse. Although the moon is 400 times smaller than the sun, it’s 400 times closer to Earth. That means they look the same size when viewed from Earth.
During an eclipse, the moon casts a fuzzy shadow on the Earth. The center part of the shadow is called the umbra; people in the umbra see a total eclipse. For these viewers, darkness falls as the moon entirely covers up the sun. The gray, fuzzy part of the shadow is the penumbra; people here see a partial eclipse. The farther you are from the center, the less sun is covered up.
The moon takes about 27 days to orbit the Earth, but we don’t have eclipses every month. That’s because the moon’s orbit is tilted five degrees to our orbit around the sun. So the moon is often too high or too low when it’s between us and the sun. An eclipse happens only when the sun, the moon, and Earth line up perfectly.
The moon’s orbit is an ellipse (oval), so sometimes it’s closer to us and looks bigger, and sometimes it’s farther away and looks smaller. If it appears too small to completely cover the sun, we’ll see an annular eclipse, where a bright ring of light is visible around the moon.
When the moon is on the opposite side of Earth from the sun, we may experience a lunar eclipse. If the sun, Earth, and the moon line up right, the full moon will cross into the Earth’s shadow. Because Earth’s shadow is bigger than the moon’s, we see lunar eclipses more often than solar ones.
Where and When to See the Eclipse
On the morning of August 21, 2017, a total solar eclipse will start in the middle of the Pacific Ocean. It will hit land in Oregon, run diagonally through the United States to South Carolina, and end in the Atlantic Ocean. Everyone else in North America and people in Hawaii and parts of South America will be able to see a partial eclipse. A total solar eclipse hasn’t occurred the United States since 1979!
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