My favorite thing in the night sky appears on Aug. 28.

What's your favorite thing in the night sky? Maybe it's the Pleiades, that sparkling open cluster that dominates winter in the Northern Hemisphere. Perhaps it's a sight of the Milky Way arching across the summer sky. Or could it be Saturn, a close-up sight of which has many people reaching for their wallets to buy a bigger telescope.

My favorite sight? It's us or, rather, our shadow. It's something that's only observable during a partial lunar eclipse — and there's one coming up very soon. Overnight on Aug. 27-28, an unusually deep partial lunar eclipse will be visible across Europe and North America, during which I will be watching Earth's shadow do something spectacular for precisely 3 hours, 18 minutes and 5 seconds.

A lunar eclipse happens when the sun, Earth and a full moon — in this case, August's Sturgeon Moon — line up closely enough for the moon to pass through Earth's shadow, a vast cone of darkness stretching away into space. Because the sun is a disk rather than a point of light, Earth's shadow tapers as it travels, narrowing like the beam of a flashlight in reverse. It creates two distinct shadows:

Umbra: its dark inner section, which extends roughly 870,000 miles (1.4 million kilometers) into space, four times farther than the moon. Earth passes through it during a partial and total lunar eclipse.

Penumbra: a paler outer shadow where Earth blocks only part of the sun. During a penumbral eclipse, the moon merely brushes this outer zone and often appears only subtly dimmed.

Those shadows create, in turn, three phases — penumbral, partial and total — as the moon passes through Earth's shadow. What we'll get on Aug. 27-28 is a penumbral phase, followed by a partial phase, followed once again by a penumbral phase. No totality, no drama? Not quite. During a partial lunar eclipse, part of the moon enters the umbra, allowing us to see the curved edge of our planet's shadow projected onto the lunar surface. It is one of astronomy's most direct visual proofs that Earth is a sphere.

During the Aug. 27-28 event, just over 96% of the moon will pass into the umbra, which is why it will look almost, but not quite, total. It may even make the moon look pinkish close to that maximum eclipse, because the umbra is not completely black. Sunlight bent and filtered through Earth's atmosphere leaks into the shadow, carrying the red-orange glow of every sunrise and sunset around the world.

Total lunar eclipses get the "blood moon" nickname, leaving partial lunar eclipses sounding like leftovers. But a deep partial eclipse can be stranger because one bright sliver remains outside the darkest part of Earth's shadow while the rest of the moon turns dim and rusty. You also get more of an impression of the moon moving slowly through a circular shadow in space.

Source: www.space.com

Share: