The reason why the moon on the Mid-Autumn Festival is extraordinarily round is because this traditional festival happens to fall on the 15th day of the eighth lunar month, which is also the time when the moon is at its fullest in the entire lunar phase cycle.

Because the Mid-Autumn Festival falls on the fifteenth day of the eighth month of the lunar calendar, the earth moves between the moon and the sun at every fifteenth day of the lunar calendar. On this day, when the sun sets in the west, the moon rises in the east. , a round moon is seen on the earth. This moon phase is called Wangyue. The moon at this time is called Wangyue, also called a full moon, so it looks round.

The fullest time of the "Mid-Autumn Moon" occurs on the 15th day of the eighth lunar month. This is because the moon's orbit around the earth is elliptical, and its revolution speed is uneven. When the moon is closer to the earth, the moon moves faster; when the moon is farther from the earth, the moon moves slower.

The revolution and rotation of the moon

The moon revolves around the earth in a circular orbit. The great circle intercepted by this orbital plane on the celestial sphere is called the "white path". The ecliptic plane does not coincide with the celestial equator, nor is it parallel to the ecliptic plane, and its spatial position is constantly changing. Period 27.32. The average inclination of the Moon's orbit (the ecliptic) to the Earth's orbit (the ecliptic) is 5°09′. However, it is known that the moon gradually moves away from the earth at an average rate of 3.8cm per year.

The moon rotates while revolving around the earth, with a period of 27.32166 days, which is exactly one sidereal month, so we cannot see the back side of the moon. We call this phenomenon "synchronous rotation" or "tidal locking", and it is almost a universal law in the satellite world of the solar system. It is generally believed to be the result of the long-term tidal effects of satellites on the planet. Libra motion is a wonderful phenomenon that allows us to see 59% of the moon.

The above content refers to Baidu Encyclopedia-Moon