The 28 Lunar Mansions: where each one is in the sky

The Moon takes about 27.3 days to circle the sky, lodging in a new stretch of stars each night. The word xiu is usually read as "lodging": ancient astronomers cut the Sun and Moon's path into twenty-eight segments to record where the Sun, Moon and planets had reached. The oldest known object listing all twenty-eight is a lacquer chest lid from the tomb of Marquis Yi of Zeng in Hubei, from about the 5th century BC.

All 28 mansions

Determinative stars follow the usual modern table (Zi uses φ¹ Ori and Shen uses δ Ori; sources occasionally differ). The meridian month is from our sky engine: the month in which the determinative star is closest to due south at 9 p.m. on the 15th, local time.

Mansion names are given as the Chinese character, its pinyin and the usual English rendering, so 角 Jiao (Horn) is the first mansion of the Azure Dragon.

Why the mansions vary so much in width

The mansions are not twenty-eight equal slices. Each one runs from its own determinative star to the next one's. With today's star positions the widest is Jing, about 32°, and the narrowest, Gui, is only 1.5°.

Stranger still are Zi and Shen. Zi's determinative star originally came before Shen's, but precession has carried it past: by today's right ascension Zi now sits 0.7° behind Shen, the so-called "swap of Zi and Shen". Mansion widths drift over the centuries, which is one reason every Chinese dynasty had to re-measure them for its calendar.

The seven easiest mansions

Most mansions are made of faint stars and are hard to see from a city. These have bright stars or distinctive shapes and make the best starting points:

Taipei, July 15, 9 p.m.. The maps show the sky as you look up: north at the top, east on the left; the center is the zenith and the rim is the horizon. Planets and the Moon are left out because they move from year to year; use the tool on this page to see a given night.

The almanac's "mansion of the day" is a different thing

The Chinese farmer's almanac assigns each day to a mansion ("today is ruled by such-and-such mansion"). That is the twenty-eight mansions taking turns by date, one full cycle every twenty-eight days, and it has nothing to do with which mansion is on the meridian that night. Our engine puts Xin on the meridian at 9 p.m. on July 15, but the almanac's mansion for that day depends only on where it falls in the twenty-eight-day rota. Date selection uses the rota; see date selection.

Three common misconceptions