My favorite thing in the night sky appears on Aug. 28 — and it proves Earth is a sphere
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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.
What’s happening and when to look
North and South America get the best view. In North America, maximum eclipse occurs late on Aug. 27 in the west and around or after midnight on Aug. 28 farther east. Europe sees the eclipse before dawn on Aug. 28, with the moon low and setting during the later stages in many places. A clear western horizon will matter there.
Knowing the schedule is key. Punch in your location to Time Date, and you’ll get timings for the eclipse’s phases, with the partial phase the one to watch. For example, the partial phase in New York will begin at 10:33 p.m. EDT on Aug. 27, peak at 96% at 00:12 a.m. EDT on Aug. 28, and end at 01:51 a.m. EDT. In Los Angeles, it will be 7:33 p.m. PDT, 9:12 p.m. PDT and 10:51 p.m. PDT on Aug. 27. From London, it will be 3:33 a.m. BST, 5:12 a.m. BST and end as the moon sets at 6:15 a.m. BST on Aug. 28.
How and when I’m watching it
The beginner’s mistake is to look at the maximum eclipse once and call that the event. Seeing and appreciating the curved edge of the umbra that reveals Earth’s shape, projecting onto — and moving across — the moon, is what I’ll be watching for. Every lunar eclipse quietly repeats the evidence: Earth casts a round shadow because Earth is round.
The best way to watch is with a telescope tracking the moon, so you can easily see the edge of the shadow creep across craters and maria, then recede over the 3 hours, 18 minutes and 5 seconds of partiality. I’ll probably use my image-stabilized binoculars to give me a close-up, but this is a sight anyone can see easily and safely with the naked eye. Besides, the naked eye gives a better sense of the moon as a whole object entering a larger object’s shadow.
You obviously need a clear sky to see it at all, but gaps in the cloud are fine — you only really need glimpses every now and then to get the full effect. If you’re in a dark-sky park or somewhere away from light pollution, you may also see the Milky Way appear as the light of the full moon is temporarily flattened, but my favorite sight in the night sky will be far closer — the full moon slowly being marked by the planet I’m standing on.
Stargazer’s corner: Aug. 21-27
The bright moon dominates this week, growing from a thickening gibbous toward full, which is bad news for faint deep-sky observing. I would also not plan a serious Milky Way-hunting session this week — though it may be wise to be somewhere as dark as possible for the eclipse (though the shadow moving across the moon can be seen from everywhere, even in the most light-polluted cities). While you moon-gaze, keep an eye out for Saturn late in the evening and into the night (it will be to the upper-left of the eclipsed full moon), and half an eye out for late Perseid stragglers — particularly during the eclipse. That said, this is a week largely for waiting for the full moon to do its one slow, peculiar thing.
Constellation of the week: Aquarius
Aquarius is not the easiest constellation to love from a bright suburban sky. It is large, faint and not especially helpful if you expect constellations to resemble their names. But it matters this week because the full moon sits within it during the eclipse. To find Aquarius, look south to southeast in the late evening from the northern hemisphere, below the Great Square of Pegasus and west of the Summer Triangle. The easiest time to find it this week is probably close to the peak of the partial lunar eclipse, when Aquarius will surround a darkened moon.
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