Alan Stahler: Eclipse | TheUnion.com

The word “month” derives from the word “moon.” The moon orbits the Earth in (very nearly) a month of four weeks.

Monday before last, July 31, we faced the moon with our back to the sun. The nighttime faced us, and faced the sun. Lilt full-face, it was full moon.

Monday last, Aug. 7, both Earth and moon were sideways to the sun. Lit from the side, the D-shaped moon rose at midnight, putting it high in the sky, if behind the clouds, at dawn: third-quarter.

Monday next, Aug. 21, the daytime moon will have its back to the fire. Lit from behind, the new moon is, by rights, invisible. But next Monday’s moon will be very visible — in silhouette — as it eclipses the sun.

Seen here in the foothills, Monday’s eclipse will be partial. But it will be a deep partial eclipse — the sun will suffer more than four-fifths of its face hidden by the moon.

The remaining, crescent-shaped fifth of the sun will bathe the landscape in an eerie light.

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Glance at the sun for only a moment and your eyes see spots — they’re temporarily damaged. The narrow crescent of the four-fifths-eclipsed sun may be tempting to stare at, but it is still bright enough to damage the eye badly. Do not look at the sun without eye protection.

You can view the sun safely if you’re looking through an ISO-certified eclipse viewer, looking through a number 12 or higher welder’s plate, or, as eclipse-watchers have done for millennia, looking at a pinhole projection.

What is light? Is light made of waves or is it made up of particles?

Physicists have debated that question for centuries. There’s evidence for both — solar panels work only because light behaves as a particle. But there’s also good evidence that light is a wave.

Look at the edge of the shadow of a door. The shadow is not sharp, but fuzzy.

If light were made of particles, the edge of the shadow of the door would be sharp — not unlike the way we mask off an area when we’re painting. Masking tape creates a “shadow” that the particles of paint can’t get into.

Looking down from a cliff, on waves coming into shore, one sees the waves bend — they diffract — as they pass a rock, a jetty, anything with an edge.

When light passes the edge of the door, it diffracts into the shadow, making the shadow fuzzy, evidence that light is composed of waves (don’t take my word for this — it’s easy to see).

Light behaves as a wave or as a particle, depending on what sort of experiment you do.

Taking advantage of the wave nature of light, we punch a small hole in a piece of card (something the weight and size, say, of a manila folder). Light will diffract as it passes the edges of the hole, causing the hole to project a (slightly) magnified image of the sun in eclipse.

Here in the foothills, Monday’s eclipse starts a few minutes after 9 a.m.; deepest eclipse is about 10:20 a.m.; it will be over before noon.

Eclipses reveal parts of the sun impossible to see otherwise, even with the most modern equipment. And they create a tiny patch of nighttime that flies supersonically through the atmosphere. Such conditions offer unique opportunities to study both sun and Earth. More about that next week.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages — kid to adult — and may be reached at [email protected]

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