Is tonight's Mid-Autumn moon full?

Why the festival moon is not always full

The Mid-Autumn Festival, 中秋节, falls on the 15th day of the 8th month of the Chinese lunar calendar. Families get together, share mooncakes and look at the moon. Each lunar month starts on the day of the new moon, which this year was Sep 11 in China.

The time from new moon to full moon is not fixed, because the Moon moves on an ellipse. From 2000 to 2050 it ranges from 13.9 to 15.6 days, 14.8 on average. This month it is 15.56 days, longer than 95% of months in that span, so the exact full moon lands on Sep 27 at 00:49 Beijing time, two days after the festival. An old Chinese saying goes 十五的月亮十六圆, the moon of the 15th is roundest on the 16th. This year it is the 17th.

Near full the number moves slowly: about 0.1% an hour tonight and almost zero at the peak, because the lit share follows a cosine curve that is flat at the top. Even at the exact moment it tops out at about 99.9%. The Moon's orbit is tilted about 5 degrees to Earth's, and at this full moon it sits 2.9 degrees above the plane of Earth's orbit, so a thin edge stays dark. When the three line up exactly, you get a lunar eclipse instead.

Xinhua reported the same 00:49 Beijing time, and Japan's National Astronomical Observatory notes that a two day gap is relatively uncommon: Xinhua on Weibo, NAOJ.

How this page works

Positions of the Sun and Moon come from Astronomy Engine by Don Cross, running in your browser. Nothing is precomputed.

The lit part is drawn from the Moon's phase angle. It is then turned using the direction of the Sun and the parallactic angle at your chosen place, so the picture matches how the Moon sits in that sky, with straight up pointing away from the horizon.