A deep partial eclipse of the moon is one of astronomy's many near-misses to cherish.
(Image credit: Apostoli Rossella via Getty Images)
Stargazers have trigger words — total, bright, peak, maximum, best. But observing the sky is often more interesting when something seems neither one thing nor the other.A case in point was the almost-total partial lunar eclipse we just witnessed. From where I watched, early on Aug. 28, Earth's shadow reached its maximum across the lunar surface, covering 96.2% of it, as the moon began to sink in the west. A thin part of the lunar surface remained outside the darkest part of Earth's shadow, and the surface never did quite go red. At maximum eclipse, the moon looked like a sliver — totally unlike a crescent moon, and something very odd to behold — and came within a fingernail of becoming a totally eclipsed "blood moon."Astronomy and stargazing are full of this sort of near-miss. A comet that nearly becomes bright enough to tell your neighbors about, but ultimately becomes nothing more than a smudge in binoculars that few can even find, let alone be impressed by. A meteor shower that's hyped as the strongest for years that doesn't live up to expectations. Or the worst kind — a cloud deck that clears a few minutes after a total solar eclipse comes to an end. We pretend these moments are failures because they do not match the headline version of the sky. But it's often the "almost" part that keeps the imagination alive.Beyond the headline actA total lunar eclipse happens only when the entire moon passes into Earth's dark umbral shadow. On Aug. 27-28, the deep partial eclipse missed totality, but the bright uneclipsed sliver made the shadow easier to see and appreciate. Its almostness gave the event an unusual tension.During a total lunar eclipse — also called a "blood moon," which gets headline writers very excited — the red color comes from sunlight filtering through Earth's atmosphere. Shorter blue wavelengths scatter more efficiently, while longer red and orange wavelengths are bent into the shadow and onto the moon. In other words, the totally eclipsed moon is lit by the combined glow of sunrise and sunset around the rim of Earth. That sounds like a poetic exaggeration, but it's just physics.A partial lunar eclipse is equally just physics. With the headline act of totality not on the agenda, I watched Earth's curved shadow creep across the lunar surface, almost reach across the entire orb, then gradually retreat. You could see the geometry working. You could see the moon move through a circular shadow — our planet's shadow. A lunar eclipse always gives you time to think because it unfolds slowly, but this gave me a chance to focus on the one thing we all ignore when we obsess over the "blood moon" moment — the moon moving through Earth's giant shadow in space.
