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Al Stahler: Sunday eclipse

Seen from Earth, the moon goes through phases.

Same trick backwards: Seen from the moon, Earth, too, goes through phases – new to full, full to new.

New Earth – like new moon – is normally black as the night: Invisible. This Sunday night, however, cameras on the moon would see the new Earth rimmed by a glowing ring of red light.

For those of us on Earth that night, the full moon will glow, not bright white, but a dark, somber red.

Suppose you and I were to play catch … in a room full of running, jumping, dancing kids … little kids who get a kick out of intercepting our ball.

We start playing catch with ping pong balls, but the kids have no problem intercepting them, then tossing them every which way. Not one ping pong ball gets from me to you.

So we try tossing tennis balls. Those are a bit harder for the kids to catch … but not hard enough.

Baseballs? Even more get through, but not all.

We resort to playing catch with bowling balls. Every ball we toss … well, roll … makes it across the room.

The sun plays catch with the planets, flinging photons – particles of light – out into space. Before those photons can reach Earth’s surface, however, they must get through the running, jumping, dancing molecules of air in our atmosphere.

Molecules in the air see photons of blue light as ping pong balls … easy to catch, easy to fling away, this way and that. Flinging blue photons every which way, air molecules light up the sky, making the sky blue.

At high noon, the sun throws photons at us more-or-less straight down. But as the sun drops lower in the sky, its light comes in at an ever-greater angle, forcing that light to travel through ever-more air. As more and more air molecules get a chance to catch and fling that sunlight about, other photons – green, yellow, orange … like tennis balls and baseballs –also get caught, and are flung this way and that … few of those photons – those colors – reach our eyes.

Bowling ball red photons do get through, though, so we see the setting sun as red.

Anything standing in the sun casts a shadow. Earth’s shadow stretches out into space, and we never notice it, save when we’re watching, say, the International Space Station pass overhead, lit by the sun … until it disappears in Earth’s shadow.

Or when the moon’s orbit carries the moon into our shadow, as will happen next Sunday night.

With Earth blocking the sun’s light from reaching the moon, a camera on the moon would still see light – red, bowling ball light – ringing the edge of our planet – the red light of every sunset … and every sunrise … on Earth.

The full moon looks bright white, when it’s lit by direct sunlight. But within Earth’s shadow, the moon is cut off from direct sunlight.

Put a spoon in a half-glass of water and the spoon looks broken. When light moves from water to air, or air to water, it bends. We see light coming from two different directions depending on whether the spoon is in water, or air.

Light moving from the vacuum of space, into Earth’s atmosphere, bends, too … bends into Earth’s shadow. So even when the moon is in Earth’s shadow, some of the sun’s light reaches the moon.

Not all colors of sunlight reach the moon, though … only bowling ball red, from all Earth’s sunrises, all Earth’s sunsets.

The moon – not quite totally eclipsed – will rise, next Sunday, around sunset … around 8 p.m. Here in the foothills, we’ll then have to wait for the moon to rise above the hills, above the trees. By the time it’s high enough to see (and you’ll still want a good view to the east), the moon will likely be entirely within Earth’s shadow, completely eclipsed … and red. The color will get easier to see as the sky grows darker.

Totality will end – the moon will begin slipping out of Earth’s shadow – at 9:53 p.m. Over the next hour, the moon will grow from crescent to full.

Local astronomers will set up scopes, Sunday night, at 8 p.m. – NOT at our usual site, but, for a better view to the east, at the overlook on the road down to the town of Washington, 1.8 miles off SR20 (thanks to Paul Bacon, for scouting out a good location). The intersection with SR20 is about 13 miles up from Nevada City. It’s free … bring a sweater.

Al Stahler enjoys sharing science and nature with friends and neighbors, in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and may be reached at a.a.stahler11@gmail.com

Crescent Earth, hovering above the moon.
Photo courtesy Apollo 17, NASA; Restoration: Toby Ord

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