March 25, 2024’s full moon portends April 8th’s solar eclipse

On April 8, 2024, a spectacular total solar eclipse arrives.

This photograph, taken during the 2017 total solar eclipse, shows the Sun being eclipsed by the Moon during totality. Note how, although the sky is darkened closest to the Sun, the horizon is still illuminated by direct sunlight. The closer you are to the center-line of totality and the longer the duration of the eclipse, the darker the overall sky becomes, allowing observers to see fainter, dimmer objects. You will miss details such as this if you spend all of totality’s time attempting to photograph the eclipse.

But two weeks prior, on March 25th, the full moon signals its arrival.

The full moon as seen on July 31, 2015, will appear very similar to March 25, 2024’s full moon, but with one important exception: the March 25 full moon will feature a penumbral eclipse, which foreshadows the upcoming total solar eclipse on April 8.

Solar eclipses can only occur when the Sun, Moon, and Earth all align.

When the Moon passes directly between the Earth and the Sun, a solar eclipse occurs. Whether the eclipse is total or annular depends on whether the Moon’s angular diameter appears larger or smaller than the Sun’s as viewed from Earth’s surface. Only when the Moon’s angular diameter appears larger than the Sun’s are total solar eclipses possible, a situation that will no longer be possible about 600-650 million years from now. Eclipses have been predictable phenomena for nearly 3000 years: since the time of the ancient Babylonians.

Conditions are favorable just twice a year: when the new moon passes through the Earth-Sun plane.

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-duration lunar eclipses will occur with the Earth at aphelion and the Moon at apogee, while the longest-duration solar eclipses require a perigee Moon in the new phase.

The Moon orbits Earth in an ellipse that’s inclined 5.2° to the Earth-Sun plane.

Because the Moon-Earth orbital plane is not identical to the Sun-Earth orbital plane, but rather is inclined to it by 5.2 degrees, eclipses do not occur with every new Moon and every full Moon. Instead, only when the Moon happens to pass through the Earth-Sun plane coincident with the new/full phase are eclipses possible.

Because the Moon and Sun are each just ½° wide, solar eclipses are only possible when “nodes” align.

Right now, the largest (perigee) full Moon appears bigger than the Sun at all times of the year. However, over time, the Moon will migrate away, causing its angular diameter to shrink. When the perigee full Moon is smaller than the aphelion Sun, no total solar eclipses can occur anymore. The Moon varies by a much greater amount in angular size than the Sun owing to its more eccentric orbit around the Earth compared to Earth’s around the Sun.

This recurs roughly every six months, with the Moon reaching maximal misalignment every ~3 months between them.

Although the Moon is tidally locked to the Earth so that the same side always faces our planet, the fact that the Moon’s orbit is elliptical and follows Kepler’s laws of motion ensures that it appears to rock back-and-forth while growing and shrinking in apparent size over the course of a month: a phenomenon known as lunar libration. Overall, 59% of the total lunar surface, not 50%, is visible from Earth over time, and eclipses can occur during the new or full phase if the Moon happens to be passing through the Earth-Sun plane at that time.

Every two weeks, the Moon migrates about ½° up-or-down relative to the Earth-Sun plane.

Because the Moon-Earth orbital plane and the Earth-Sun orbital plane are inclined to one another at 5.2 degrees, eclipses are only possible when either the “new” or “full” phase of the Moon coincides with the Moon crossing the nodes of the Earth-Sun orbital plane. This means that solar and lunar eclipses can only occur in groups every ~6 lunar months, with a slight misalignment of 0.5 degrees during a full or new moon often leading to a total solar or lunar eclipse, respectively, two weeks later.

On March 25, 2024, the Moon will be full, but near apogee: a micro-Moon.

At its most distant from Earth, or apogee, 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. The penumbral eclipse of March 25, 2024 occurs near apogee, ensuring that the solar eclipse of April 8, 2024 will occur near perigee, resulting in long totality times.

An observer on the Moon would see a partial eclipse, as Earth partially blocks the Sun’s light.

In November of 1969, the Apollo 12 spacecraft left Earth for the Moon. During its journey, the disk of the Earth was seen partially blocking the Sun, which is when this photograph was taken. From portions of the Moon’s surface, during a penumbral eclipse, a similar sight would arise, as the Earth will partially, but not completely, block out the Sun’s disk.

From Earth, this creates the most boring species of eclipse: a penumbral eclipse.

When the Moon passes through Earth’s penumbral shadow but not through the umbral shadow, the entirety of the Moon’s disk can still be seen illuminated by sunlight from Earth, but part of the Moon will darken as seen from Earth.

The Moon’s limb temporarily darkens, as Earth’s umbral shadow just barely misses it.

During a penumbral lunar eclipse, the Moon just misses passing through Earth’s umbral shadow, but passes at least partially through Earth’s penumbra, causing a severe limb-darkening effect on one region of the fully illuminated Moon temporarily. Penumbral lunar eclipses often precede or follow more spectacular partial, annular, or even total solar eclipses.

Two weeks later, it migrates another ½° “downward,” while nearing perigee: closest approach to Earth.

This photograph of the eclipsed Sun during totality shows the asymmetric corona and the last remnant of a tiny bit of sunlight poking through a crater on the Moon: one of Baily’s beads.

As a result, March 25th’s full moon portends April 8th’s long, perfectly aligned total solar eclipse.

This map shows the path of totality during the April 8, 2024 total solar eclipse. The red, central line is the only location where totality will occur, with a maximum duration of about 4 minutes and 30 seconds occurring in the Pacific Ocean just off the coast of Mexico. The southwest locations are more likely to be cloud-free than the northeast locations.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.