Month

A month is a unit of time used within calendars to organize days. The concept is deeply connected to the Moon. In fact, the words "month" and "Moon" are cognates, meaning they share a linguistic origin. This relationship exists because the traditional idea of a month arose from the natural cycle of Moon phases. These cycles are known as lunations, or synodic months. A synodic month is the time between two consecutive occurrences of a specific phase, such as two full moons. This cycle lasts approximately 29.53 days. Because of this length, there are about 12.37 synodic months in one Earth year.

Astronomy identifies several different ways to measure a month. A sidereal month is the Moon's orbital period in a non-rotating frame of reference. This lasts about 27.32166 days. It is the time it takes the Moon to pass a "fixed" star twice. A synodic month is longer than a sidereal month. This happens because the Earth-Moon system is orbiting the Sun in the same direction as the Moon orbits the Earth. The Moon must travel extra distance to reach the same position relative to the Sun.

Other specialized cycles exist to track specific orbital movements. An anomalistic month is the time it takes the Moon to go from perigee to perigee. Perigee is the point where the Moon is closest to Earth. This month averages about 27.55455 days. This cycle is longer than a sidereal month because the perigee moves in the same direction as the Moon's orbit. A draconic month, also called a nodal month, is the period the Moon takes to return to the same node. Nodes are the two points where the Moon's orbit crosses the plane of Earth's orbit. This lasts about 27.21222 days. The draconic month is shorter than a sidereal month because the nodes move in the opposite direction of the Moon's orbit.

Humans have tracked these cycles for a very long time. Researchers have found excavated tally sticks from the Paleolithic age. These suggest that early people counted days in relation to Moon phases. Many modern calendars still use synodic months to divide the year. The Roman calendar system provided the foundation for many current calendars. The internationally used Gregorian calendar is a direct descendant of these Roman systems. The names of our current months were Anglicized from Latin names. These names often referred to important events in ancient Rome. For example, September, October, November, and December were named after the Latin numerals for seven, eight, nine, and ten. This is because they were originally the seventh through tenth months in the Roman calendar.

In the Gregorian calendar, months have specific lengths. Seven months have 31 days: January, March, May, July, August, October, and December. Four months have 30 days: April, June, September, and November. February is the only month with a variable number of days. It usually has 28 days, but it has 29 days during a leap year.

The mean length of a month in this system is 30.436875 days. Any five consecutive months that do not include February will contain exactly 153 days.

Different cultures use different lunar and solar systems. The Islamic calendar is a purely lunar calendar. It defines a year as having exactly 12 lunations. This makes an Islamic year about 11 days shorter than a solar year. Consequently, the Islamic New Year occurs on a different Gregorian date each year. Other systems, like the Hebrew calendar, are lunisolar. They use the Metonic cycle to stay in sync with the seasons. This cycle notes that 235 lunations are approximately 19 tropical years. This allows for 12 years of 12 months and 7 years of 13 months. This method was also used in the Antikythera Mechanism about 21 centuries ago.

It is easy to use physical tools to remember these month lengths. You can use the knuckles on your hand to track them. If you make a fist, the knuckles represent 31-day months. The spaces between the knuckles represent 30-day months.

This pattern also matches the layout of a musical keyboard. The wide white keys correspond to 31-day months. The narrow black keys correspond to 30-day months. This helps people visualize the complex structure of our calendar systems.