Alan Stahler: This summer’s eclipse | TheUnion.com
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Alan Stahler: This summer’s eclipse

Our eyes look for clues to judge distance: Distant objects look hazy — the farther the distance, the hazier they look.

Distant objects seem to move slowly. Think of cars on a far-off freeway, or a jumbo-jet, way up high.

Most of all, distant objects look small.

The moon measures roughly 2,000 miles across; the sun is 400 times larger. Placed side-by-side, the sun would look way larger than the moon.

But the moon is only a quarter million miles away; the sun is 400 times farther, so distance makes the sun shrink 400 times more than the moon.

The sun is 400 times larger across than the moon, but 400 times farther away. The two "400s" cancel each other out — when we look at them, sun and moon look to be the same size.

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When the moon comes between Earth and sun on August 21, it will cover the same-size sun, in a total eclipse.

To see the eclipse as total, you'd have to be right on the line connecting sun and moon, center-to-center. The Sierran foothills will be just a bit off that line, making the eclipse here not total, but partial — the moon will only partially cover the sun — about 3/4 of it.

To see totality, you'd have to travel north, or east. For a map of where you might go, visit our local astronomers' website: ncastronomers.org (thanks to Bill Thomas, webmaster).

During a partial eclipse, the sun looks as if a large bite has been taken out of it, leaving a crescent sun. The sunlight that reaches Earth seems kind of odd — neither as bright, nor as warm, as usual. Speckles of sunlight, coming through the leaves and projecting onto the ground, form crescents.

During the total eclipse, as the moon hides the sun completely, things get even stranger: for two-plus minutes, it becomes night again — the stars come out, and temperatures drop.

The sun has an atmosphere — a corona, or "crown ." The corona glows, but it's very, very dim — so dim, it's normally invisible, blotted out by the sun's glare. But during total eclipse, for the two minutes the sun is covered by the moon, the corona becomes visible … and you can look at it with the naked eye.

The sun's corona is a mystery. It's far from the center of the sun — far from the sun's energy source, its 27-million-degree-Fahrenheit fusion furnace.

Move away from the campfire and you get cold. Move away from the sun's central furnace — climb up from the core — and the temperature drops. When you reach the sun's surface, the temperature is a (relatively) chilly 9,600 degrees.

But keep moving out, out into the corona, and the temperature climbs back up, up to something like two million degrees Fahrenheit.

Something — something electromagnetic — must be focusing the sun's energy, concentrating it within the corona. So far — after years of trying — no one's been able to completely explain it.

More immediately, local astronomers will set up scopes and invite our neighbors to enjoy the night sky, at the junction of the old Downieville Highway and Highway 49, this Saturday night, beginning at 8 p.m. We'll explain what we're seeing, and why Venus, which was the evening star last summer, is now the morning star; why the moon goes through phases, from new to quarter to full to quarter to new; and why sun and moon suffer eclipses.

As always, this sky watch is free, and kids are especially welcome. Bring a sweater.

Al Stahler shares his enjoyment of nature with students of all ages. His science stories can be heard over-the-air on KVMR (89.5 FM). Al may be reached at stahler@kvmr.org