Al Stahler: Playing tricks with light and the moon |

Al Stahler: Playing tricks with light and the moon

Lit by the sun from the side, the moon, last Wednesday, shone in the shape of a letter ”D”: First quarter.

Tonight, with the moon beyond first-quarter, a bit more of its face is sunlit.

Next Tuesday night (and Wednesday before dawn), the moon will be lit full-face: Full moon.

The moon orbits Earth in an ellipse – a slightly-squashed circle – so sometimes the moon is closer to Earth, other times father away. Tuesday night’s full moon will be closer to Earth than any other full moon of the year: A supermoon. The moon near the horizon always looks large, but, to my eye, a good supermoon – even well up from the horizon – seems a bit larger than usual.

The moon is full – lit full-face – when sun, moon and Earth fall into a straight line, Earth in the middle. Three bodies in a straight line are in syzygy (SIH-zih-gee … my apologies to readers of a previous column, in which I allowed spell-check to replace “SIH” with “SIGH”).

But the sun-Earth-moon line is rarely perfectly straight – few full moons occur in perfect syzygy.

A lunation is the time between one full moon and the next – twenty-nine-and-a-half days, plus some minutes and seconds. Syzygy comes close to perfect every six lunations.

Last November, the Sun-Earth-moon syzygy was close enough to perfect that the moon brushed the outer edges of the shadow Earth casts into space.

That was 177 days ago – six lunations – next Wednesday. Next Wednesday, before dawn, syzygy will again be close to perfect … close enough that the moon will enter completely into Earth’s shadow: The moon will suffer (that’s the word) a total eclipse, just before dawn.

With the moon in Earth’s shadow, we would not be able to see the moon in eclipse … but for the fact that we live within a giant lens.

Driving down the road, on a hot summer’s day, the wind is dry-as-dust … and yet … the road ahead … looks WET!

It’s always the road ahead. The road just in front is dry … but the road ahead … looks wet.

It’s a mirage … the same mirage that taunts the poor guy in the cartoon, as he crawls, clothes in tatters, through the desert … taunts him into thinking there’s water just ahead.

We figure out where something is at – where to aim a ball, say – by presuming that light travels in straight lines. It usually does … but not always. Light bouncing off a mirror is bent big-time. Light also bends – more subtly – when it crosses a boundary, from one thing into another.

Sometimes the bend is not-so-subtle. A spoon in a half-glass of water doesn’t look bent … it looks broken.

Light travels fast – 186,000 miles per second – seven-and-a-half trips around the world in one second.

But that’s in a vacuum … out in space. When light leaves a vacuum and crosses into something else, it immediately slows down. Hitting air, light slows just a tad. Moving from air to water, light slows again, slows much more … down to 140,000 miles per second.

When light slows down, it bends. The more it slows, the more it bends.

Same trick backwards: When light moves, say, from water to air, it speeds up … and, again, it bends … bends back, the other way.

When light reflects off a spoon, in a half-glass of water … and then leaves the water, for the air … the light bends. Our eyes – and brains – assume the light is traveling in straight lines. We interpret the light rays coming from under-the-water and over-the-water as coming from different places. The spoon looks broken.

Cold air is denser – thicker – than hot air. Cold air slows – and bends – light more than hot air. Over a sun-baked parking lot, light moves from hot air (rising), into cold air (sinking) … then, back into hot air, then back into cold. Speeding-up and slowing-down … bending this way and that … makes the air shimmer (and, high overhead at night, makes the stars twinkle).

Back to that mirage. The air over the road we’re driving (or over the desert floor) is super-hot (and thin), where it touches ground; it cools as we move upward. Skylight moving down toward the road hits, first, the cool air … then the warm … then the hot. Each layer of air bends the skylight upward … bends the light toward our eyes. Looking down at the road ahead, we see skylight … shimmering, like a puddle.

Air bends light. Surrounded by light-bending air, Earth is a gigantic lens … the sort of lens we use as a magnifier … or a fire-starter. And we live within it.

The moon in Earth’s shadow should, indeed, be invisible. But sunlight slicing through the air at the edges of the globe is slowed, and bent … bent inwards, into the shadow, where the red light of all the world’s sunrises, and all the world’s sunsets, will paint the eclipsed moon’s face.

The upcoming eclipse will be total, but just barely … the moon will not go deeply into Earth’s shadow. This will affect how the moon looks, so I make no predictions … but I can cross my fingers. With luck, all that light bending into the shadow will paint the moon – for fourteen minutes – a deep, dark red.

This eclipse occurs just before dawn … just before moonset. So to see it, we’ll need a good view toward the western horizon – if you’ve got a place to view the sunset, that’s where you’ll want to be. The moon will begin its entry into Earth’s shadow, Wednesday morning, at 2:45 AM. If you’d like to get a bit more sleep, be out by 3:45, to see the last sliver of sunlit moon slide into Earth’s shadow, marking the start of totality, at 4:11. Fourteen minutes later, totality ends as the moon begins sliding out of the shadow.

No eye protection is needed to view a lunar eclipse. Staring at the moon has, though, been blamed for attacks of lunacy.

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 can be reached at

Earth and its moon, captured by the spacecraft Galileo looking back as it heads for planet Jupiter.
Photo courtesy NASA

Support Local Journalism

Support Local Journalism

Readers around Grass Valley and Nevada County make The Union’s work possible. Your financial contribution supports our efforts to deliver quality, locally relevant journalism.

Now more than ever, your support is critical to help us keep our community informed about the evolving coronavirus pandemic and the impact it is having locally. Every contribution, however large or small, will make a difference.

Your donation will help us continue to cover COVID-19 and our other vital local news.


Start a dialogue, stay on topic and be civil.
If you don't follow the rules, your comment may be deleted.

User Legend: iconModerator iconTrusted User