Month

The month is a unit of time, used with calendars, which is approximately as long as some natural period related to the motion of the Moon (i.e. "Moonth"). The traditional concept arose with the cycle of moon phases; such months are synodic months and last ~29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the paleolithic age. Synodic months are still the basis of many calendars.

Astronomical Background

The motion of the Moon in its orbit is very complicated and its period is not constant. Moreover, many cultures (most notably those using the Hebrew (Jewish) and Islamic calendars) start a month with the first appearance of the thin crescent of the new moon after sunset over the western horizon. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore the beginning and lengths of months in these calendars can not be accurately predicted. Most Jews currently follow a precalculated calendar, but the Karaites rely on actual moon observations.

sidereal month

The actual period of the Moon's orbit as measured in a fixed frame of reference is known as a sidereal month, because it is the time it takes the Moon to return to the same position on the celestial sphere among the fixed stars (Latin: sidus): about 27 1/3 days on average. This type of "month" has appeared among cultures in the Middle East, India, and China in the following way: they divided the sky in 28 lunar stations, characterized by asterisms (groups of stars), one for each day that the Moon follows its track among the stars.

synodic month

The cause of the moon phases is that from the Earth we see the part of the Moon that is illuminated by the Sun from different angles as the Moon traverses its orbit. So the appearance depends on the position of the Moon with respect to the Sun (as seen from the Earth). Because the Earth orbits the Sun, it takes the Moon extra time (after completing a sidereal month, i.e. a full circle) to catch up and return to the same position with respect to the Sun. This longer period is called synodic month. Because of the perturbations of the orbits of the Earth and Moon, the actual time between lunations may range from about 29.27 to about 29.83 days.

Months in the Julian and Gregorian calendars

The Gregorian calendar, like the Julian calendar before it, has twelve months:

• January, with 31 days;
• February, with 28 days or 29 in leap years;
• March, with 31 days;
• April, with 30 days;
• May, with 31 days;
• June, with 30 days;
• July, with 31 days;
• August, with 31 days;
• September, with 30 days;
• October, with 31 days;
• November, with 30 days;
• December, with 31 days.

One mnemonic for remembering the lengths of the months is to hold up your two fists with the index knuckle of your left hand against the index knuckle of your right hand. Then, starting with January from the little knuckle of your left hand, count knuckle, space, knuckle, space through the months. A knuckle represents a month of 31 days, and a space represents a short month.

Another one is:

Thirty days hath September,

April, June, and November;

All the rest have thirty-one,

Excepting February alone,

Which hath but twenty-eight days clear,

And twenty-nine in each leap year.

This article is licensed under the GNU Free Documentation License. It uses material from  Wikipedia and from ShiningRise.com

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