Month. This, the earliest of the natural cycles to be observed, was at first reckoned from new moon to new moon. That period is now called a lunation by astronomers to distinguish from sidereal month, the time in which the moon passes round the ecliptic to the same star, and from the tropical month, reckoned from the moon's passing the equinox till she again reaches it. Those three periods are also called, respectively, synodic month = 29.5306 days, stellar month = 27.3217 days, and periodic month = 27.3216 days. The first, or 'lunar month' proper, consists of 29 days, 12 hours, 44 minutes, 3 seconds. The 'solar month' is the time which the sun takes to pass through 30° (see CALENDAR, CHRONOLOGY). From the month, by subdivision, was obtained the week, used by the Chaldeans, Indians, Egyptians, and others from prehistoric times; and as soon as the year became an object of measurement there were numberless attempts to reconcile the solar computation of time with the lunar. The Attic year was of twelve months, alternately 29 and 30 days long, each month being divided into three decades.
The Jews, Arabians, and Turks still reckon by the lunar months of 29 and 30 days, and are therefore compelled, like the ancient Greeks, to insert an intercalary or 'embolismic' month. The French republicans in 1793 divided the year into twelve months of 30 days, with five odd days (six in leap year) to be utilised as national festivals, each month being subdivided into three decades of 10 days each, as with the ancient Greeks. Another distribution of the months has since been suggested, should such opportunity again occur—viz.: 1st, 3d, 5th, 7th, 9th, 11th months, each 30 days; 2d, 4th, 6th, 8th, 10th months, each 31 days; and the remaining month 30 days in the ordinary year and 31 in leap year. The existing 'calendar' or 'civil' months are as irregular in length as they were left by the Romans; the 4th, 6th, 9th, and 11th having 30 days, the second 28 days (or 29 in leap year), and the seven others 31 days. To complicate this disorder, a month in English law is 'a lunar month or 28 days unless otherwise expressed'; 'a lease for twelve months is only for 48 weeks' (Blackstone, ii. 141). Besides the archaic division of the month into four, as already mentioned, the early Greeks of Homer's time and previously seem to have had only two parts, the earlier half and the 'waning half'; and a trace of that probably remained in the Roman Ides, the middle or dividing day of each month.