Among the Babylonians, as we are informed by Mr. Sayce, the year was divided into twelve lunar months and 360 days, an intercalary month being added whenever a certain star, called the "star of stars," or Icu, also called Dilgan, by the ancient Accadians, meaning the "messenger of light," and what is now called Aldebaran, which was just in advance of the sun when it crossed the vernal equinox, was not parallel with the moon until the third of Nisan, that is, two days after the equinox. They also added shorter months of a few days each when this system became insufficient to keep their calendar correct.
They divided their year into four quarters of three months each; the spring quarter not commencing with the beginning of the year when the sun entered the spring equinox, proving that the arrangement of seasons was subsequent to the settling of the calendar. The names of their months were given them from the corresponding signs of the zodiac; which was the same as our own, though the zodiac began with Aries and the year with Nisan.
They too had cycles, but they arose from a very different cause; not from errors of reckoning in the civil year or the revolution of the earth, but from the variations of the weather. Every twelve solar years they expected to have the same weather repeated. When we connect this with their observations on the varying brightness of the sun, especially at the commencement of the year on the first of Nisan, which they record at one time as "bright yellow" and at another as "spotted," and remember that modern researches have shown that weather is certainly in some way dependent on the solar spots, which have a period now of about eleven years, we cannot help fancying that they were very near to making these discoveries.
The year of the ancient Persians consisted of 365 days. The extra quarter of a day was not noticed for 120 years, at the end of which they intercalated a month—in the first instance, at the end of the first month, which was thus doubled. At the end of another 120 years they inserted an intercalary month after the second month, and so on through all their twelve months. So that after 1440 years the series began again. This period they called the intercalary cycle.
The calendar among the Greeks was more involved, but more useful. It was luni-solar, that is to say, they regulated it at the same time by the revolutions of the moon and the motion about the sun, in the following manner:—
The year commenced with the new moon nearest to the 20th or 21st of June, the time of the summer solstice; it was composed in general of twelve months, each of which commenced on the day of the new moon, and which had alternately twenty-nine and thirty days.
This arrangement, conformable to the lunar year, only gave 354 days to the civil year, and as this was too short by ten days, twenty-one hours, this difference, by accumulation, produced nearly eighty-seven days at the end of eight years, or three months of twenty-nine days each. To bring the lunar years into accordance with the solstices, it was necessary to add three intercalary months every eight years.
The phases of the moon being thus brought into comparison with the rotation of the earth, a cycle was discovered by Meton, now known as the Metonic cycle, useful also in predicting eclipses, which comprised nineteen years, during which time 235 lunations will have very nearly occurred, and the full moons will return to the same dates. In fact, the year and the lunation are to one another very nearly in the proportion of 235 to 19. By observing for nineteen years the positions and phases of the moon, they will be found to return again in the same order at the same times, and they can therefore be predicted. This lunar cycle was adopted in the year 433 B.C. to regulate the luni-solar calendar, and it was engraved in letters of gold on the walls of the temple of Minerva, from whence comes the name golden number, which is given to the number that marks the place of the given year in this period of nineteen.
Caliphus made a larger and more exact cycle by multiplying by four and taking away one day. Thus he made of 27,759 days 76 Julian years, during which there were 940 lunations.
The Roman calendar was even more complicated than the Greek, and not so good. Romulus is said to have given to his subjects a strange arrangement that we can no longer understand. More of a warrior than a philosopher, this founder of Rome made the year to consist of ten months, some being of twenty days and others of fifty-five. These unequal lengths were probably regulated by the agricultural works to be done, and by the prevailing religious ideas. After the conclusion of these days they began counting again in the same order; so that the year had only 304 days.