See a Half Moon on Dec 5 (It’s the Most Illuminated Half Moon This Year)

A Quarter Moon Is a Half Moon

At First Quarter and Third Quarter, half of the Moon’s face is visible from Earth.

• A First Quarter Moon marks the halfway point as the Moon’s phase waxes (increases) from New Moon to Full Moon.
• A Third Quarter Moon comes halfway as the Moon wanes (decreases) from Full Moon back to New Moon.

So, unsurprisingly, we often refer to a Quarter Moon as a Half Moon, and say it is 50% illuminated.

But this is not completely true: a Quarter Moon is always slightly more than 50% illuminated.

A Fraction More than Half

A Quarter Moon is defined as the moment when the Sun and Moon are separated by 90°, as measured from Earth.

With this arrangement, a fraction more than 50% of the side of the Moon facing Earth is illuminated—as can be seen in the illustration below.

For exactly 50% of the Moon to be illuminated for an observer on Earth, the Sun and Earth must be separated by 90°, as measured from the Moon.

This occurs when the Moon is in a slightly different position in its orbit—a little bit after Third Quarter (as shown in the illustration), or a little bit before First Quarter.

Not to Scale

The illustration above is a very rough representation of the triangle formed by the Sun, Earth, and Moon.

In reality, the distance from Earth to the Moon is about 30 times the diameter of Earth; the distance from Earth to the Sun, meanwhile, is around 12,000 times Earth’s diameter.

In other words, although our not-to-scale illustration shows a small and compact triangle, the actual triangle is extremely long and narrow.

As the Moon travels along its orbit, the moment when it is exactly 50% illuminated comes around 20 minutes after the moment of Third Quarter, or before the moment of First Quarter.

(Our illustration ignores an effect called parallax, where the Moon’s position differs by a tiny amount depending on your observing location on Earth. Moon phase calculations are based on the centers of the Earth and Moon.)

Why Is This Quarter Moon the Most Illuminated of All?

Although the Third Quarter Moon on December 5 won’t look different to any other Quarter Moon, it will also be the farthest Quarter Moon from Earth in 2023.

This is not a complete coincidence: in fact, it’s part of the reason why this Quarter Moon is the most illuminated of all. As the Earth-Moon distance gets larger, the percentage of the Moon’s face that is lit up increases.

Similarly, it is no coincidence that December 5 falls near to perihelion (early January), when Earth is closest to the Sun: a smaller Earth-Sun distance also leads to a greater percentage of the Moon’s face being lit up.

Lesser-Known Moon Dates for 2024

The Moon’s orbit around Earth is never the same from one lunar month to the next, leading to events such as Supermoons and Micromoons.

Here are six lesser-known lunar events for 2024.