Month. The calendar year is divided into 12 parts, each of which is called a month. But the word month has other meanings as well. Several kinds of months are measured by the moon’s motion. At one point in the moon’s path, it is closest to Earth. This point is called the perigee. The time the moon takes to revolve from one perigee to the next is an anomalistic month. This period averages 27 days, 13 hours, 18 minutes, and 33.1 seconds.
If the moon were viewed from a distant star, it would seem to make a complete revolution around Earth in 27 days, 7 hours, 43 minutes, and 11.5 seconds. This period is a sidereal month. The proper lunar month, which is called the synodical month, is the period between one new moon and the next, an average of 29 days, 12 hours, 44 minutes, and 2.8 seconds.
The synodical month is one of three natural divisions of time. The other two are the rotation of Earth on its axis, or a day, and the revolution of Earth around the sun, or a year. Another astronomical month is the solar month, which is one twelfth of a solar year. The solar month is the time taken by the sun to pass through each of the 12 signs of the zodiac (see Zodiac ).
In the calendar most widely used today, months vary in length from 28 days to 31 days. The lengths of these calendar months have no relation to astronomy. In an early version of the calendar used in ancient Rome, most months had 29 or 31 days. Later, days were added to some months to make the year come out closer to a solar year—the time Earth takes to go once around the sun.
In the Gregorian calendar that we use today, each day of the month is called by its number. June 1 is the “first of June,” and so on. The ancient Greeks divided the month into 3 periods of 10 days, and the French Revolutionary calendar used months of equal length divided into 3 parts of 10 days each. The fifteenth day of the month was called the fifth day of the second decade.
The Roman system was even more complicated. The Roman calendar had three fixed days in each month, the calends, the nones, and the ides. The Romans counted backward from these fixed days. They would say something would happen, for example, three days before the nones. The calends were the first day of the month. The ides were at the middle, either the 13th or 15th of the month. The nones were the ninth day before the ides, counting both days. When the soothsayer told Julius Caesar to “beware the Ides of March,” he meant a definite day.
