A slightly special moon will rise in the night sky this Chuseok. It will be higher in the sky than usual and linger longer. That is because of the so-called "major lunar standstill," the first in about 19 years. It began in 2024 and will continue until early 2026. Of the three years, this year's Chuseok full moon will stay in the sky the longest.
The moon does not rise and set from the same place every day. Day by day it rises from a slightly different position and sets in a different direction. But every 18.6 years comes a time when the range of the moon's motion in the sky becomes the widest. During this period, the moon rises and sets much farther to the north or south than usual.
Even though this is when the moon's range of movement is widest, the moon appears to pause when it reaches the northern or southern extremes. Like a swing that lingers for a moment at its highest point before reversing direction, the moon slows down and gives the impression of standing still. Astronomers therefore call this period the "lunar standstill."
Jeon Young-beom, a principal researcher at the Korea Astronomy and Space Science Institute (KASI), said, "The position where the moon rises changes slightly every day, but during this period the difference is hardly noticeable, so it looks as if it is standing still," and added, "By the full moon standard, it can look as if it rises from the same spot for one to two years."
This phenomenon occurs because the orbits of Earth and the moon do not align perfectly. Earth orbits the sun tilted at 23.5 degrees, and the moon's orbit is offset by about 5 degrees from the ecliptic, the path the sun takes. As these two tilts interact, the moon's vertical range in the sky expands and contracts on an 18.6-year cycle.
The standstill is unfamiliar to us, but ancient people observed this cycle to read the order of nature. Some stones at Stonehenge in England may have been erected to mark the lunar standstill, and there have been claims that the Carnac alignments in Brittany, France—rows of 3,000 megaliths—and the 53 standing stones of Callanish in Scotland marked the moon's position during the standstill.
Jeon noted, "The standstill appears most dramatically around the summer or winter solstice, when the moon is farthest from the ecliptic, but its effects linger around Chuseok as well," adding, "This Chuseok, the moon will climb close to overhead, stay longer, and may feel brighter than usual."
This year's Chuseok full moon, based on Seoul, will rise in the eastern sky at 5:32 p.m. on the 6th, reach its highest point at 11:50 p.m., and set in the west at 5:07 a.m. the next day. The moon will remain in the sky for about 11 hours and 35 minutes, about one hour longer than last year's 10 hours and 29 minutes. Next year it will be 10 hours and 59 minutes.
In particular, this Chuseok full moon coincides with the standstill and a supermoon. A supermoon refers to a full moon that rises when the moon is within 90% of perigee, its closest point to Earth. According to the National Aeronautics and Space Administration (NASA), the moon is 363,105 kilometers from Earth at perigee and 405,696 kilometers at apogee, its farthest point.
The smallest full moon of the year rose in April. At that time the distance between the moon and Earth was the shortest. This Chuseok's supermoon will appear about 15% larger and 30% brighter than the April full moon.
Jeon added, "When the moon shifts northward, it can receive sunlight from the side at a slant, making the supermoon look slightly squashed," noting, "Of course, it is a difference that is almost imperceptible to the eye, but knowing this may make moon viewing this Chuseok more fun."