Moon and month: why the Moon was humanity's first clock
Synodic and sidereal months, the kinship of the words moon and month, lunar, lunisolar and solar calendars, the first crescent and days that begin at dusk.
In many languages the word for month and the word for Moon are siblings. In Greek mene was the Moon and men the month, the Latin mensis comes from the same Indo-European root, and the English moon and month, like the German Mond and Monat, clearly preserve the kinship. This linguistic closeness reflects a simple fact: for most of human history the Moon was the most visible and reliable clock for counting periods longer than a day.
The synodic month and the sidereal month
The month we see with our eyes, from one new Moon to the next, is called the synodic month and lasts on average 29 days, 12 hours and 44 minutes, about 29.53 days. It is longer than the sidereal month, of about 27.32 days, which measures the time the Moon takes to return to the same position relative to the stars. The difference arises because meanwhile the Earth moves along its orbit, and the Moon must travel an extra stretch to regain the same alignment with the Sun.
The actual length of each individual synodic month varies by several hours around the average, because the lunar orbit is elliptical and is affected by the pull of the Sun. This is why lunar calendars alternate months of 29 and 30 days, and why calculated calendars must distinguish between a mean moon, convenient for tables, and a true moon, which describes the actual phases observable in the sky.
Three solutions to one problem
Twelve synodic months last about 354.37 days, eleven fewer than the solar year. Civilisations have answered this mismatch in three ways. The first is the purely lunar calendar, which ignores the Sun, like the Islamic one. The second is the lunisolar calendar, which follows the Moon but periodically adds a month to stay in step with the seasons, like the Babylonian, Hebrew, Chinese and Hindu calendars.
The third is the solar calendar, which gives up the Moon and uses conventional months. Pharaonic Egypt adopted it for civil purposes, with twelve months of thirty days and five added days, while keeping, according to many scholars, a lunar calendar for certain cults. The Julian and then the Gregorian calendar belong to this family, even though the date of Easter still depends on the Moon.
The first crescent and the counting of nights
Many ancient calendars began the month with the first crescent visible after sunset, not with the astronomical conjunction, which is invisible. A widespread habit followed: the day began in the evening. This is so in the Hebrew and Islamic calendars, and Caesar attributes to the Gauls the counting of time by nights. The English word fortnight, fourteen nights, also keeps a trace of this way of counting.
The lunar phases then divided the month into natural parts: crescent, first quarter, full moon, last quarter. The seven-day week corresponds roughly to a quarter of the lunar month, although its history is complex and interweaves the biblical tradition of the Sabbath, the Hellenistic astrology of the seven planets and perhaps Mesopotamian customs linked to the days of the month.
Studying lunar calendars therefore means studying how societies turned a phenomenon of the sky into a shared order. Festivals, contracts, harvests, fasts and anniversaries have been tied for millennia to the changing face of the Moon. Each calendar tells of a different choice about how to reconcile that measure with the Sun, and that choice says a great deal about the culture that made it.