From the December 2014 issue

Shadow secrets

What we learn when light is blocked.
By | Published: December 29, 2014 | Last updated on May 18, 2023
Once a decade or so, around Groundhog Day, we explore shadows. The ones cast by woodchucks can be instructive, but let’s “think big” and focus on those cast by celestial bodies.

The Moon’s shadow can actually elicit tears of joy. But next month seeing such an inspirational total solar eclipse may be problematical because the lunar shadow only hits the cloudy North Atlantic. The March 20 event unfolds on the vernal equinox, so the shadow encounters Earth’s edge when it reaches the North Pole and will proceed no farther, but instead sweep invisibly out into space.

Fully dark umbral shadows are what mostly interest us. Earth’s umbral shadow extends for a million miles and strikes the Moon twice this year, on April 4 and again September 27, with that second total lunar eclipse visible throughout North America. Add the equinoctial solar event, and we’ve got three 2015 totalities. A shadow carnival. And next year, Mercury throws its own shadow on Earth.

Every object not in total darkness casts both an umbral and a penumbral shadow. These fancy words have a simple distinction. Hold out a grape at arm’s length, and close one eye. If you position the fruit so that it completely blocks the Moon, you’ve placed your eye within the grape’s umbral shadow. Now slightly shift it so you can see part of the Moon. Your eye now lies within the grape’s penumbral shadow. So the umbra is simply where the light source is completely blocked. This zone tapers like a chopstick and gets narrower as you go farther from the grape. If the grape were 20 feet (6 meters) away, it would appear too small to totally block the Moon or Sun. An umbral shadow ends at a certain distance.

But a penumbral shadow, the region where the foreground object blocks only part of the Moon or Sun, keeps going forever. The Moon’s umbra extends just enough to barely reach Earth, and sometimes falls short. But our satellite’s penumbral shadow hits us at every syzygy — Sun-Moon-Earth alignment — at least twice a year.


Penumbral shadows have enormous darkness variations. The next eclipse you observe, probably this September’s, notice the Moon’s uneven lighting soon after the partial phase begins, say around 8:15 p.m. EST. A colonist living on the left, or eastern, side of the Moon would see Earth blocking the entire Sun. Meanwhile on the Moon’s pricey western region, lunar citizens would observe just a little of the Sun blocked, with virtually no darkening of their lunar terrain. This fascinating and dramatic gradation is obvious at a glance, though often overlooked by lunar eclipse observers.

Step into sunlight, and look at your own shadow. The penumbral portion is the fuzzy region on its outer edge. Now observe more closely. Your ankles cast a much sharper-edged shadow onto the sidewalk than your head does. This is unrelated to what you did last night. Your head’s shadow always has a blurry edge. That’s because there’s a longer distance from your head to the ground, which creates a much wider penumbral zone. Your ankles are quite close to the shadow they cast, and when the shadow’s “throw distance” is small, the penumbra is skimpy. An ant could quickly cross the narrow region of sidewalk in which the Sun appears partially blocked by your ankle.

A transit happens when the observer’s zip code lies beyond the umbral shadow’s limit. It’s really a type of penumbral eclipse, a partial blockage. For example, Mercury’s umbral shadow never reaches Earth. But on May 9 next year, we’ll telescopically see it partially obscure the Sun if we use a safe solar filter.

Cool shadow events are not limited to the solar system. High overhead at nightfall hovers the variable star Algol, whose name famously means “the ghoul.” The ancient desert dwellers disconcertedly watched it lose half its light every 2 days, 20 hours, and 49 minutes. The entire outline of Perseus then gets an odd makeover. Science has figured it all out. A dimmer companion eclipses the brightest member of this double star system. That star’s shadow visibly changes the winter sky.

Just before our ancestors started walking upright, Algol was 10 times closer to us than it is today and appeared as bright as Jupiter. Passing within 10 light-years of us, this massive system may have perturbed the Oort Cloud of comets and probably sent a few crashing into Earth. So the ghoul with its shadow play may have actually affected us physically.

That guy who holds Punxsutawney Phil up in the air this month probably doesn’t think about whether he’s creating umbral or penumbral groundhog shadows. Or a marmot transit across the Sun. You can write and tell him.

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