Synodic and sidereal month: the two measures of lunar time
Why the month of phases lasts 29.53 days and the month of the stars only 27.32: the difference explained, with the calendars and lunar mansions it shaped.
How long is a lunar month? The answer depends on what is taken as the reference. If one measures the Moon's return to the same phase, one obtains one interval; if one measures its return in front of the same stars, one obtains another, shorter by more than two days. This apparently subtle difference guided the construction of calendars and astrological systems that are very different from one another.
The month of the stars
The sidereal month is the time the Moon takes to complete one full circuit around the Earth relative to the fixed stars: on average 27.32 days, that is, about 27 days and almost 8 hours. If one evening the Moon stands beside a bright star such as Regulus or Spica, after one sidereal month we shall find it at that point of the sky again, but showing a different phase from the one of the first observation.
The month of the phases
The synodic month, by contrast, is the interval between two consecutive new moons or full moons, and lasts on average 29.53 days, about 29 days, 12 hours and 44 minutes. The difference depends on the Earth's motion around the Sun. While the Moon completes its circuit, the Earth advances along its own orbit by almost one degree a day; to return to the same alignment with the Sun, the Moon must travel an extra arc, which takes just over two days.
The length of each single lunation varies around the mean value because of the Moon's elliptical orbit and the perturbations of the Sun: it ranges from about 29 days and 4 hours to almost 29 days and 22 hours. The mean value was already known with great precision to Babylonian astronomers, and the one adopted by Hipparchus in the second century BC differs from the modern figure by less than a second.
Calendars and lunar mansions
The synodic month is the basis of lunar and lunisolar calendars: the Islamic calendar, made of twelve months of 29 or 30 days, the Hebrew calendar and the traditional Chinese one. Twelve lunations make about 354 days, eleven fewer than the solar year. It is this gap that forces lunisolar calendars to insert a thirteenth month from time to time in order to stay in step with the seasons.
The sidereal month instead inspired the lunar mansions, divisions of the sky into 27 or 28 sectors, one for each night of the Moon's path among the stars. In India they are the nakshatras, in the Arab world the manazil al-qamar, in China the twenty-eight xiu. In astrological tradition each mansion was given a character and a function, and in the Latin Middle Ages the lunar mansions entered the treatises of astral magic translated from Arabic, such as the Picatrix.
Two measures, two visions
The coexistence of the two months reflects two ways of looking at the sky. The synodic month belongs to everyday experience: it is the rhythm of the night light, of vigils, markets and feasts. The sidereal month instead presupposes careful observation of the stars and the will to measure the Moon's path along the zodiac, a task typical of professional astronomers and astrologers.
Two months, then, for a single body: one marked by light, the other by the stars. The first regulated fasts, harvests and religious festivals; the second drew a map of the night sky that for centuries united astronomy, calendar and symbolism in a single system of thought, from India to the Mediterranean.