On November 19, 2021, Earth will see the longest partial lunar eclipse in centuries.
Lasting 6 hours and 2 minutes from start to finish, it’s the longest since February 18, 1440.
Partial eclipses occur when the full Moon passes through Earth’s umbral shadow, but never reaches 100% obscurity.
Three simultaneous occurrences determine a lunar eclipse’s duration.
1.) The full Moon should be at apogee, its farthest from Earth.
The Moon’s elliptical orbit moves slowest at apogee: 50,000 km (30,000 miles) farther than perigee.
2.) Earth should be near aphelion.
At its most distant, the Sun casts “straighter” shadows, creating the longest-lasting lunar eclipses.
3.) The Sun-Earth-Moon alignment must be almost perfect.
The Moon should maximally pass through Earth’s umbral shadow, all without achieving totality.
November 19, 2021’s eclipse outstandingly ticks two boxes.
The full Moon is just 41 hours from lunar apogee.
The date is closer to perihelion, however.
Still, only 0.9% of the Moon remains illuminated at maximum eclipse.
Every 6585 days, the cycle almost perfectly recurs.
November 30, 2039‘s partial eclipse will be just 2 minutes shorter.
November 9, 2003‘s eclipse was longer, but achieved totality.
A longer partial eclipse won’t occur until February 8, 2669.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.