This year, “the moon on the fifteenth day is seventeen round” is only 14 times in the 21st century.

Happy Mid-Autumn Festival!

When is the moon at its roundest? A commonly heard saying is “the moon on the fifteenth day is sixteen round”.However, the moon during this year’s Mid-Autumn Festival is “seventeen round”, with the roundest moment appearing at 0:49 on September 27.

“The moon on the fifteenth day is seventeen circles” is not a visual error, nor is it an error in traditional sayings. It is a wonderful phenomenon created by the natural laws of the movement of the earth, moon, and sun, and contains exquisite astronomical knowledge.

This is the Mid-Autumn Moon taken in Haidian, Beijing on September 29, 2023. Published by Xinhua News Agency (Photo by Wang Yiyin)

Before discussing “When will the moon be full?”Let’s first clarify the explanations of two nouns, one is “朔” and the other is “王”.In layman’s terms, luo means that the moon moves between the sun and the earth, with the backlit side of the moon facing the earth; lunar means that the earth is between the sun and the moon, and the illuminated side of the moon completely faces the earth, which is a full moon.

Lai Dihui, a member of the Chinese Astronomical Society and director of the Tianjin Astronomical Society, explained that the lunar calendar is a calendar arranged based on the profit and loss changes of the moon phases. The first day of each month in the lunar calendar is the new moon, and the corresponding moon phase is the new moon. The synodic month refers to the complete cycle from new moon to the next new moon, which averages about 29.53 days. The moon revolves around the earth in an elliptical orbit, and its revolution speed is not constant: when the moon is closer to the earth, its revolution speed becomes faster; when it is farther from the earth, its revolution speed slows down. Affected by the characteristics of this movement, the arrival time of the sun is not fixed.