Graham Jones of Timeanddate.com joins with Deborah Byrd of EarthSky to clarify why the December 4-5 quarter moon – aka a ‘half moon’ – is greater than 50% illuminated. Plus, we share some key moon dates for 2024, from the closest moon to the shortest lunar month. Original article from Timeanddate.com on November 27, 2023. Reprinted with permission. Edits by EarthSky.
Quarter moon or half moon?
From our perspective on Earth, the 3rd quarter moon on the evening of December 4-5, 2023, would be the most-illuminated quarter moon of this 12 months. It’ll be 50.137% lit, as seen by us on Earth.
A quarter moon seems half-illuminated in Earth’s sky. Some folks even name it a half moon. And we in astronomy usually say a quarter moon – aka a half moon – seems from Earth to be 50% illuminated.
Nevertheless it’s not true. As an alternative, each quarter moon (half moon) is at all times barely greater than 50% illuminated.
And the December 4-5, 2023, quarter moon is essentially the most illuminated of all of the quarter moons this 12 months.
A fraction greater than half?
Who knew {that a} quarter moon is at all times extra than half illuminated? That’s not widespread information, even amongst astronomers. Nevertheless it’s true.
Right here’s why. In astronomy, we outline the moment of quarter moon as when the sun and moon are separated by 90 levels, as seen from Earth.
However, to ensure that the moon to look precisely 50% illuminated for an observer on Earth, the sun and Earth have to be separated by 90 levels, from the attitude of the moon.
See? It’s a tiny distinction, however a measurable one. From Earth, at each quarter moon, a fraction greater than 50% of the Earth-facing aspect of the moon is illuminated.
To not scale
The illustration above is a really tough illustration of the triangle fashioned by the sun, Earth, and moon. In actuality, the gap from Earth to the moon is about 30 instances the diameter of Earth. The gap from Earth to the sun, in the meantime, is round 12,000 instances Earth’s diameter.
In different phrases, though our not-to-scale illustration exhibits a small and compact triangle, the precise triangle is extraordinarily lengthy and slender.
Because the moon travels alongside its orbit, the second when it’s precisely 50% illuminated comes round 20 minutes after the second of third quarter, or earlier than the second of first quarter.
(Our illustration ignores an impact known as parallax, the place the moon’s place differs by a tiny quantity relying in your observing location on Earth. Moon phase calculations are based mostly on the facilities of the Earth and moon.)
December 5 can also be the farthest quarter moon
Though the third quarter moon on December 5 received’t look completely different to some other quarter moon, it’ll even be the farthest quarter moon from Earth in 2023. This isn’t an entire coincidence: Because the Earth-moon distance will get bigger, the proportion of the moon’s face that’s lit up will increase.
Equally, it’s no coincidence that December 5 falls close to to perihelion (early January), when Earth is closest to the sun. A smaller Earth-sun distance additionally results in a better proportion of the moon’s face being lit up.
Lesser-known moon dates for 2024
The moon’s orbit round Earth is rarely the identical from one lunar month to the subsequent, resulting in occasions similar to Supermoons and Micromoons.
Listed here are six lesser-known lunar occasions for 2024.
So, once more, in 2023, the most-illuminated quarter moon would be the third quarter moon of December 5. And, in 2024, the most-illuminated quarter moon would be the third quarter moon of December 22.
January 25: Longest moon phase of 2024. There can be 8.225 days between full moon on January 25 and third quarter moon on February 2.
March 10: Closest moon of 2024. Essentially the most excessive Earth-moon distances happen at new moon or full moon. At new moon on March 10, the moon can be 221,764 miles (356,895 km) away.
Could 30: Shortest lunar month of 2024. The moon will cycle by means of all its phases – from third quarter moon on Could 30 to 3rd quarter moon on June 28 – in 29.195 days.
August 19: Shortest moon phase of 2024. The moon will take 6.625 days to wane from full moon on August 19 to 3rd quarter moon on August 26.
October 2: Farthest moon of 2024. At new moon on October 2, the gap to the moon can be 252,597 miles (406,516 km).
November 23: Longest lunar month of 2024. It would take the moon 29.868 days to go from third quarter on November 23 to 3rd quarter on December 22.
December 22: Most-illuminated quarter moon of 2024. And so it comes round once more!
Backside line: Some quarter moons are fuller than others! Essentially the most-illuminated quarter moon of 2023 is on December 5. Study extra distinctive moon dates for 2024.
This occurs when the moon is in a barely completely different place in its orbit … just a little bit after third quarter (as proven within the illustration), or just a little bit earlier than first quarter.
