Chinese astronomy calculator
Lunar Mansion Calculator
Enter a birth date, time and time zone to see in which of the 28 xiu (二十八宿, èrshíbā xiù), the lunar mansions of Chinese astronomy, the Moon stood. We calculate where the Moon really was, not the mansion that a Chinese almanac prints for that day.
The 28 xiu
The 28 xiu (二十八宿, èrshíbā xiù) are 28 stretches of the sky along the ecliptic and the celestial equator. The Moon moves through all of them in one sidereal month, 27.3217 days, so she is in a different xiu about every day. The system, the four symbols and the determinative stars are explained on the page ‘Chinese astronomy’.
How we calculate it
From your birth date, time and time zone we find the exact moment. For that moment the Astronomy Engine gives the position of the Moon in right ascension, as seen from the centre of the Earth. A xiu runs from its determinative star (距星, jùxīng) to the determinative star of the next xiu; we use the stars of the list in the Chinese Wikipedia, in their positions on the day of birth.
The xiu are of unequal width: in this list from about 1.5° (Guǐ) to about 32° (Jǐng). The Moon moves about 13° a day, so she can change xiu once, or even twice, in a day. That is why the birth time matters. Without one, we show the changes of the whole day.
Two lists of stars
The lists of determinative stars differ for four xiu: Xīn, Kuí, Zī and Shēn. We use the list in the Chinese Wikipedia. When the second list would put your Moon in another xiu, the result shows both. In the Chinese list Zī has no width, so a Moon there is counted in Shēn. We have not checked either list against Sun and Kistemaker, The Chinese Sky during the Han (1997).
Not the mansion of the day
Chinese almanacs print a ‘mansion of the day’ (èrshíbā xiù zhírì) that goes through the 28 xiu in a fixed order, one a day. That is arithmetic of the calendar, not a position of the Moon: the Moon needs 27.32 days, not 28, to return to the same stars. This calculator gives the xiu in which the Moon actually stood.
We have not been able to confirm from a printed source how the almanac counts these days, so we do not describe the rule here.
What this calculator does not do
It gives no meaning, element, lucky days or character for a xiu; those belong to astrology, and we do not give them here. It does not give your Chinese zodiac animal either; for that, use the Chinese zodiac calculator. It works for dates from 1900 to 2100 and shows the Moon as seen from the centre of the Earth, not from your birthplace.
Sources
- Chinese Wikipedia, ‘二十八宿’ (revision 95216397): the 28 xiu and the determinative stars of the list in the Chinese Wikipedia. Read 1 November 2026.
- English Wikipedia, ‘Twenty-Eight Mansions’ (revision of 1 October 2026): the second list of determinative stars, which cites Sun Xiaochun (1997), in Helaine Selin (ed.), Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures, p. 517, and the International Dunhuang Project. Read 1 November 2026.
- Joseph Needham, Science and Civilisation in China, vol. 3 (Cambridge University Press, 1959), Table 24, pp. 234–237: the xiu and their determinative stars.
- NASA NSSDC, Moon Fact Sheet: the sidereal orbital period of the Moon, 27.3217 days.
- Baidu Baike, ‘星宿’: the mansion of the day in Chinese almanacs (a reference work; not yet checked against a printed source).
- Astronomy Engine: the position of the Moon.
- SIMBAD, CDS, Strasbourg (Wenger et al. 2000, A&AS 143, 9): the positions of the determinative stars. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.