Today’s Question
Is the moon we see on Chuseok really the roundest full moon of the whole year?
Hyeongseop sat on a mat spread across his grandmother’s yard, watching a huge, golden moon slowly rise. The smell of holiday food drifted out from the kitchen, and cousins ran laughing across the yard. With the whole family gathered together after so long, the yard felt livelier and warmer than usual.
“Byeoli, Grandma said tonight’s moon is the biggest and roundest one of the whole year. Is that actually true?”
Byeoli pulled up a diagram of the Moon’s orbit on Byeol-maru’s screen.
“Well, it’s a little more complicated than that. Chuseok falls on the 15th day of the 8th lunar month, but that date isn’t always the exact moment the Moon looks most perfectly round in the sky. The Moon’s orbit around Earth isn’t a perfect circle, so the true astronomical full moon often lands a day or two before or after that date.”
“What shape is the orbit if it’s not a circle?”
“It’s a slightly squashed oval. That makes the Moon’s speed change a little as it orbits, so the time it takes to go from new moon to full moon isn’t exactly the same every month. The lunar calendar doesn’t account for that tiny difference, it just uses an average, so the real full moon and the 15th day of the lunar month can end up a day or so apart.”
“So when’s the actual roundest moon this year?”
“This year, the Moon reaches its most perfectly round shape in the early hours of September 26th, the day after Chuseok. So tonight’s moon is plenty round and beautiful already, but just one more day would make it ever so slightly more perfect.”
Hyeongseop looked up at the moon again. A difference of one day was so small it was nearly impossible to notice with the naked eye.
“Why did people long ago pick the 15th day of the 8th month for Chuseok anyway?”
“The lunar calendar is based on the Moon’s changing shape. It takes about fifteen days to go from a new crescent to a full moon. So the middle of each month, the 15th, tends to land close to a full moon. Long before precise astronomy existed, people could look at the Moon alone and know how far the season had come, and when it was time to harvest.”
Grandmother came out to the yard, setting down a plate piled high with songpyeon. “Here, everyone make a wish before you eat one.” The cousins crowded around to pick their favorites, and Hyeongseop took one too.
“Byeoli, why does everyone make wishes looking at the moon instead of something else?”
“The full moon has long symbolized abundance and completeness. It’s the fullest the month ever gets, so people hoped their own wishes might fill up just as completely. And isn’t it kind of special that everyone looks up at the very same moon at the very same time to make their own separate wishes? Even scattered far apart, everyone is connected under the same moonlight.”
Hyeongseop closed his eyes toward the moon and made a quiet wish. Even while imagining the slightly more perfect full moon that would rise the following night, the one he was looking up at right now, surrounded by family, already felt completely perfect to him.
“Chuseok’s full moon falls on the 15th lunar day, but since the Moon’s orbit isn’t a perfect circle, the true fullest moment can land a day or two off.”
How Much Did You Guess Right? (Mixed True/False)
- (True/False) Chuseok is always the roundest full moon day of the year.
- Why do the real full moon and the 15th lunar day drift apart?
- (True/False) This year’s true full moon rises the day before Chuseok.
- What did ancient people use to track seasons and harvest time?
- (True/False) The full moon has long symbolized abundance and completeness.
(Answers: 1. False, it can be off by a day or two. 2. The Moon’s orbit isn’t circular, so its speed varies. 3. False, it’s the day after, September 26th. 4. The Moon’s changing shape. 5. True.)
Curious why the moon always shows the same face? Read Why Does the Moon Always Show Us the Same Face? And for a night when the moon looks huge and orange, see The Night We Wait for the Orange Moon.
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