This Month’s Full Moon Will Only Be 99.82% Full

A Special Full Moon

As we recently reported, this month’s Full Moon will rise and set at its most extreme northerly points on the horizon. The next time the Full Moon rises this far north on the horizon will be December 2043.

Contrary to what we expect, the Moon’s face is almost never 100% lit up at Full Moon. Why is this?

The Moon Gets Out of Line

A Full Moon occurs when the Moon lies on the opposite side of Earth to the Sun. As seen from the Earth, the Moon appears fully illuminated—as shown in the illustration below.

But there’s a catch. The Moon’s orbit around Earth is slightly tilted with respect to Earth’s orbit around the Sun.

This tilt means that the Moon does not always lie precisely opposite the Sun.

Look again at the illustration above: the Sun, Earth, and Moon all lie on the flat screen of the computer or mobile device you’re using to read this article.

But, in reality, the Moon’s tilted orbit means that the Moon can lie a little bit in front of the screen (toward you) or a little bit behind the screen (away from you).

As a result, the alignment of the Sun, Earth, and Moon is not exactly perfect—and, as seen from Earth, the Moon is a tiny fraction less than 100% fully illuminated.

No 100% Full Moons in 2024

It’s important to stress that we’re talking about a tiny fraction. When we look at a Full Moon in the sky, it always appears entirely full.

However, if we look at the data, the numbers tell us that some Full Moons are fuller than others.

There are 12 Full Moons in 2024. The following table gives the date of each Full Moon (in UTC), along with a percentage number that tells us how much of the Moon’s disk is illuminated.

None of this year’s Full Moons are 100% lit up. The least illuminated Full Moons are in June and December, when 99.82% of the Moon’s disk it lit up.

In the table, we’ve rounded the percentages to two decimal places—in fact, the percentage for December 15 is a whisker less than the percentage for June 22.

Illumination of Moon’s Disk at Full Moon, 2024

Moon’s Tilt Also Causes Extreme Moonrises

It’s no coincidence that the June and December Full Moons featured in our recent news story about extreme moonrises and moonsets.

Wherever you are in the world, the December 2024 Full Moon rises and sets in its most extreme northerly direction. Six months previously, the June 2024 Full Moon rose and set in its most extreme southerly direction.

In both these cases, the extreme moonrise direction happens because the Moon is close to its maximum tilted position around the moment of Full Moon. If we go back to the illustration above, on December 15 the Moon will be at its furthest distance in front of the screen (toward you). Meanwhile, on June 22 it was at its furthest distance behind the screen (away from you).

This is the same reason why these Full Moons have the lowest illumination percentages.

The 99.99% Full Moons

What about the two Full Moons with the highest illumination percentages in the table above? The Full Moons on March 25 and September 18 were 99.99% illuminated—just short of 100%.

In both these months, the Sun, Earth, and Moon were closely aligned at Full Moon. So much so, in fact, that on both occasions, we had a lunar eclipse.

Nevertheless, we can see from the not-quite-100% illumination figures that the Sun-Earth-Moon alignments on these dates still weren’t completely perfect.

This is why the March 25 lunar eclipse was a penumbral eclipse, and the September 18 lunar eclipse was a partial eclipse. On both occasions, the Moon passed through the outer parts of Earth’s shadow—but not the center.

100% Full Moons?

So, is the Moon ever 100% illuminated at Full Moon?

For this we would need a perfect Sun-Earth-Moon alignment at Full Moon. This would produce not just a penumbral or partial eclipse, but a total lunar eclipse, where the Moon passes through the center of Earth’s shadow.

We have two total lunar eclipses coming up in 2025, on March 14 and September 7.

Sure enough, if we look at the data for both these dates, the Full Moon has an illumination figure of 100.00%—a “perfect” Full Moon.