ISLAMIC Astrology And Astronomy

Like the Sun, the Moon appears to go around the circle of the Zodiac, though its circuit lasts a month (of 27.32 days) rather than a year (see astronomy). The twenty-eight Mansions of the Moon divide the Zodiac in the same way as the twelve Signs of the Zodiac, marking out sectors of the circle of the sky, though, since the number of degrees in a circle, 360°, is not neatly divisible by 28, their span is an awkward 12°51'25.7" (a 364 degree circle would be neater). It is thought that the lunar system of division may predate the solar Zodiac at least as rough sectors, since the stars remain largely visible, and the Moon’s apparent motion against the background is clearly noticeable from night to night. The Encyclopaedia Britannica which Yeats had (1911 edition, YL 629) notes that the lunation cycle (the Moon’s synodic cycle, from the Greek synodos, meeting, conjunction) is the reason for dividing the Sun’s annual cycle into twelve, while the lunar Mansions derive from the Moon’s own motion:

The synodical revolution of the moon laid down the lines of the solar, its sidereal revolution those of the lunar zodiac. The first was a circlet of "full moons"; the second marked the diurnal stages of the lunar progress round the sky, from and back again to any star. The moon was the earliest "measurer" both of time and space; but its services can scarcely have been rendered available until stellar "milestones" were established at suitable points along its path. Such were the Hindu nakshatras, a word originally signifying stars in general, but appropriated to designate certain small stellar groups marking the divisions of the lunar track.
"Zodiac", The Encyclopaedia Brittanica, Vol. 28, 995. 


Since the Moon’s sidereal revolution is 27.32 days, the number of Mansions has been approximated as both 27 and 28: in the most commonly used Indian system, there are 27 nakshatras, while 28 divisions are used in the Arabic and Chinese systems, as well as an older Indian system. (For a comparative table of the stars involved in lunar Mansion systems from Babylon, Arabia, India and China, drawn up by David B. Kelley, click here, and for a consideration of the origins of the Mansions, in India and China, see Philip Yampolsky, "The Origin of the Twenty-eight Lunar Mansions", Osiris IX [Lisse: Swets & Zeitlinger B.V., 1950; 1984] 62-83.)