Why the partial lunar eclipse of November 19, 2021 is the longest in 600 years

On November 19, 2021, Earth will see the longest partial lunar eclipse in centuries.

Lasting from roughly 6 AM to 12 PM Universal Time, the lunar eclipse of November 19, 2021 will reach its maximum at 9:03 am, where 99.1% of the Moon will be obscured. Despite not reaching totality, the unilluminated portion of the Moon should still appear red, owing to sunlight filtering through Earth’s atmosphere and landing on the eclipsed portion of the Moon. (Credit: NASA’s Scientific Visualization Studio)

Lasting 6 hours and 2 minutes from start to finish, it’s the longest since February 18, 1440.

When passing through a large amount of atmosphere, the bluer wavelengths of light are mostly scattered away, while the red light can make it through and land on the lunar surface during a total eclipse, which is why the Moon is visible, but red and dim during a total lunar eclipse. The locations of the total and penumbral phases are shown here, while the partial phases occur when the Moon is partially in and partially out of the umbral shadow. (Credit: NASA)

Partial eclipses occur when the full Moon passes through Earth’s umbral shadow, but never reaches 100% obscurity.

During the eclipse, the Moon will appear to move through the sky at its normal rate, but will spend approximately 3.5 hours in the umbral shadow of the Earth and over 6 hours in its penumbra. (Credit: NASA’s Scientific Visualization Studios)

Three simultaneous occurrences determine a lunar eclipse’s duration.

A perigee full Moon compared with an apogee full Moon, where the former is 14% larger and the latter is 12% smaller than the other. The longest lunar eclipses possible correspond to the smallest apogee full Moons of all. At apogee, the Moon is not only farther and appears smaller, but also moves at its slowest in its orbit around Earth. (Credit: Tomruen/Wikimedia Commons)

1.) The full Moon should be at apogee, its farthest from Earth.

In order for an eclipse to occur, the nodes of the Moon’s orbit must line up with the Earth-Sun plane during a new or full moon. Having this align with the Moon at either perigee or apogee and with the Earth close to either perihelion or aphelion is a very rare occurrence indeed, but the longest lunar eclipses will occur with the Earth at aphelion and the Moon at apogee. (Credit: James Schombert/University of Oregon)

The Moon’s elliptical orbit moves slowest at apogee: 50,000 km (30,000 miles) farther than perigee.

When the Earth is farthest from the Sun, at aphelion, it not only moves the most slowly in its orbit, but experiences its straightest “shadow-cone” at that time, enhancing the maximum possible duration of lunar eclipses. (Credit: Lalith Perera/University of Mississippi)

2.) Earth should be near aphelion.

From early November through late February, Earth moves at its slowest in orbit around the Sun, and hence the Sun appears to migrate through the sky the most slowly during this time. This slightly increases the maximum duration that a lunar eclipse can possess during this time period, as the Equation of Time shows. (Credit: Rob Carr/Wikimedia Commons)

At its most distant, the Sun casts “straighter” shadows, creating the longest-lasting lunar eclipses.

When the Earth is farthest from the Sun and the Moon is farthest from the Earth, the longest-duration lunar eclipses can occur. The longest-duration partial eclipses also require that the alignment be imperfect, and that 100% of the Moon never pass through the Earth’s umbral shadow all at once. (Credit: E. Siegel/Beyond the Galaxy; eclipse sequences by Zaereth and Javier Sanchez)

3.) The Sun-Earth-Moon alignment must be almost perfect.

Animation showing the umbral phase of the November 19, 2021 partial lunar eclipse. At 9:03 AM UT, maximum eclipse is reached, where only 0.9% of the Moon remains illuminated by direct sunlight. The umbral phase lasts over 3.5 hours: the longest this century for a partial eclipse. (Credit: NASA’s Scientific Visualization Studio)

The Moon should maximally pass through Earth’s umbral shadow, all without achieving totality.

The various regions of Earth where the various phases of the November 19, 2021 eclipse will be visible. The entire continent of North America is well positioned to view the lunar eclipse, with some timezones experiencing the eclipse’s start on the night of November 18, 2021, instead. (Credit: NASA)

November 19, 2021’s eclipse outstandingly ticks two boxes.

The lunar eclipse of November 19, 2021 will enter Earth’s penumbra (P1) at about 6 AM UT, will then enter Earth’s umbra (U1) about 76 minutes later, but will never fully enter Earth’s umbra, remaining partially in and partially out until its exit at 10:47 AM UT, followed by its exiting the penumbra (P4) at 12:03 PM UT. (Credit: Fred Espenak)

The full Moon is just 41 hours from lunar apogee.

At its most distant from Earth, the full Moon is known as a Micromoon, the opposite of a (perigee) Supermoon. A Supermoon is 14% larger and 30% brighter than a Micromoon, but Micromoons move the slowest in orbit around Earth, leading to the longest lunar eclipses. (Credit: NASA/JPL-Caltech)

The date is closer to perihelion, however.

The Sun and Moon vary in distance from Earth by a few percent, with the longest lunar eclipses occurring when the Sun is close to aphelion and the full Moon is close to apogee. (Credit: Ehsan Rostamizadeh/Astrobin)

Still, only 0.9% of the Moon remains illuminated at maximum eclipse.

This diagram shows the path of the Moon through Earth’s penumbral (outer) and umbral (inner) shadows during the lunar eclipse of November 19, 2021. It is the closest eclipse to totality that doesn’t quite achieve it all century. (Credit: SockPuppetForTomruen/English Wikipedia)

Every 6585 days, the cycle almost perfectly recurs.

After centuries of total lunar eclipses, the Lunar Saros cycle (number 126) is about to transition to a series of partial lunar eclipses. The one on November 19, 2021, will be the longest lunar eclipse in nearly 600 years. (Credit: English Wikipedia)

November 30, 2039‘s partial eclipse will be just 2 minutes shorter.

When near-totality is reached, which November 19, 2021’s and also November 30, 2038’s lunar eclipse will do, it’s possible to observe a blue band opposing the red side just before the directly illuminated thin portion is reached. This is colloquially known as the ‘Japanese Lantern’ effect. (Credit: Larry Johnson/Wikimedia Commons)

November 9, 2003‘s eclipse was longer, but achieved totality.

This photograph, taken during totality during the lunar eclipse of November 2003, shows the Moon illuminated only by the filtered sunlight that’s passed and refracted through Earth’s atmosphere. This eclipse, like many total eclipses, was longer than 6 hours and 2 minutes in duration, but a partial eclipse won’t exceed that duration for ~600 years. (Credit: Tomruen/Wikimedia Commons)

A longer partial eclipse won’t occur until February 8, 2669.

Partial lunar eclipses are often offset by a total solar eclipse by 14 days. The total solar eclipse of December 4, 2021, or that of February 22, 2669, are both examples of a total eclipse occurring on the very next new Moon after an almost-total lunar eclipse during the prior full Moon. (Credit: Joe Sexton/Jesse Angle)

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