Imagine: You are outside on a sunny day, and it starts to get really dark, not just heavily overcast, but night-dark, perhaps dark enough to see stars. Birds stop singing and decide to get an early night. Dogs and cats are puzzled. You look up in the sky, and where the sun should be there is a black disc surrounded by a strangely beautiful pearly glow, its corona or crown.

This will happen on Mon., Aug. 21, a total eclipse of the sun. The moon’s shadow will pass right across the United States from Oregon to South Carolina, passing Carbondale in southern Illinois at about 1:20PM. From Naperville the sun will appear to be a narrow crescent, 90 percent covered. From Carbondale the moon will fully cover the sun for two magic minutes, and the corona will appear – a shining plasma a thousand times hotter than molten iron.

At the beginning and end of totality we may see the sun shining through one or more craters on the edge of the moon, looking like a diamond ring in the sky.

It is an odd fact, surely a coincidence, given that the sun is 400 times bigger than the moon, that they look to us almost exactly the same size, the sun being 400 times further away. Both sizes vary a bit, since neither the moon’s orbit nor the Earth’s orbit are perfectly circular, so their distances change. Sometimes the moon is smaller and does not completely cover the sun, leaving a bright golden ring, an annular eclipse.

When the sun is fully covered some stars may be seen. Look for the planet Venus, to the west of the sun. Try an app called Night Sky, allowing one to see where Venus and the other planets are at any time.

Rarely, Venus passes in front of the sun, a phenomenon called a transit, when Venus appears as a tiny black disc taking a few hours to move across. The last was in 2012. If you missed it, sorry, the next is not until 2117. Astronomers have detected planets passing in front of their own distant suns, a topic for a future column.

Do not look at the sun directly, except for those magic two minutes. Use a dark filter, or look at the shadow made by a card with a small hole.

Please, please, let it not be cloudy, as our next chance is not until 2099, total from Chicago!

Clarification, Aug. 26, 2017: In the August column I said that your next chance to see a total solar eclipse is in 2099. I meant without leaving Chicagoland, but on April 8, 2024, one will be visible from Indianapolis.

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