Tonight the 19th is the “mid-autumn famous moon” — the fifteenth night. Tokyo is forecast clear; we should be able to enjoy a vivid full moon (I tried shooting last night a day early, but a phone has its limits and it came out like a hazy moon or Halley’s comet).
“Mid-autumn” means the middle “day” of autumn on the old calendar (months 7, 8, 9) — the 15th of the eighth month. You also see “harvest moon of mid-autumn” written 仲秋 with different characters; “仲秋” means the middle “month” of autumn, that is, the eighth lunar month.
Moon-viewing is said to have come from China, but primitively people everywhere must have looked up, given thanks, or made wishes on their own. From Nara and Heian, palace moon banquets and other upper-class rites likely felt Chinese influence. At the common level it settled as a harvest festival of thanks for the season’s yield and a prayer for next year’s crop.
Did you know the fifteenth night is not always a full moon? I learned only recently, to my shame. This year — today — is a full moon, but it is the last in the 2010s; the next fifteenth night that is a full moon is September 21, 2021, a year after the 2020 Tokyo Olympics, eight years later.
The old calendar took the new-moon day (the day containing the moment the moon sits between Earth and the sun) as the first of each month. A month is 29 or 30 days, but the true cycle is 29.5, so a slight slip means the days from new moon to full moon (the day containing the moment the moon is opposite the sun as seen from Earth) are not always fifteen. Thus the fifteenth night meant for enjoying the full moon is often not full.
From 2011 three years running the fifteenth night was full; before that, in 2010, the fifteenth was September 22 and the full moon the 23rd. Next year the fifteenth is September 8 and the full moon September 9; then through 2020, seven years, the two will slip by one or two days.
Thinking that, this year’s fifteenth night feels all the more precious. Moonrise in Tokyo tonight is 17:22. Palaceside Building does not open the roof at night, so you cannot watch from there, but the moon rises east to southeast; please see the “moon and buildings” collaboration from around the National Museum of Modern Art.