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The Chushu Moon (September, 2018)

Night Sky in Tokyo around 9 p.m. on Sept. 24, 2018
Medium resolution (2000 x 1265) High resolution (5500 x 3480)

Let’s Enjoy Otsukimi (Moon-Viewing)!

This year’s Chushu Moon is on September 24.

In the “luni-solar calendar (Note 1),” the moon on the evening of the 15th day of the 8th month is called the “Chushu Moon.” In Japan, it is also referred to as the “Imo-meigetsu” in connection with agricultural events. From ancient times, people in Japan used to serve taros on the Imo-meigetsu. The tradition of viewing the Chushu Moon is said to have been transmitted from China during the Heian Era. In addition, the 13th night of the 9th month in the luni-solar calendar is known as “the 13th Evening.” In Japan, it is traditional to also conduct moon viewing on that evening. The 13th Evening Moon is also called “the Latter Moon,” “the Mame-meigetsu,” or the “Kuri-meigetsu.” This year the 13th Evening falls on November 1.

More than a few people think, “We can see a beautiful moon during the Chushu Moon; and the Chushu Moon is the full moon.” But, September 24 is the Chushu Moon this year and September 25 is the full moon. So, the dates of the full moon and the Chushu Moon are one day apart. In fact, the dates of the full moon and the Chushu Moon are often different.

How does this happen? In the luni-solar calendar, the “first day” is the day including the moment when the Moon passes new phase. The Chushu Moon indicates the Moon on the 15th day of the 8th month in the luni-solar calendar.

This year, September 10, which includes the moment when the Moon passes new phase (3:01 a.m.), is the first day of the 8th month in the luni-solar calendar. Therefore, September 24 is the 15th day of the 8th month in the luni-solar calendar.

On the other hand, the full moon is defined astronomically by the positions of the Sun, the Earth, and the Moon. The full moon refers to the Moon when the Moon and the Sun are in opposite directions as seen from the Earth (the Moon is exposed to sunlight directly from the front so that seen from the Earth, the Moon looks round.) We will have the moment of full moon at 11:52 a.m. on September 25.

The dates of the Chushu Moon and full moon can be different. However, the Moon is very bright for several days before and after the full moon. So it should be impressive.

If the weather is fine, we can enjoy a beautiful Chushu Moon this year too.

(Note 1) This calendar was used in Japan until the Meiji Era. The days of each month were determined based on the waxing and waning of the Moon. Back

Reference: Ephemeris Computation Office

With the “Sky Viewer” you can easily explore the appearance of a typical urban night sky (planets and constellations are visible). The Celestial Phenomena section of the glossary explains the planetary phenomena terms: greatest elongation, opposition, conjunction, stationary, etc.