The Lunar Month
A lunar month is also known as a lunation, while the astronomical term for this period is a synodic month, from the Greek term synodos, meaning meeting or conjunction.
From New Moon to New Moon
We measure the lunar month from the precise moment of a New Moon, when the Sun and Earth are aligned on opposite sides of the Moon, until the next New Moon.
About 29.5 Days
In the period from 1600 to 2600, an average lunar month lasts 29.530575 days or 29 days, 12 hours, 44 minutes, and 2 seconds. The shortest lunar month ended July 17, 1708 and lasted 29.271819 days (29 days, 6 hours, 31 minutes, and 25 seconds), while the longest was the one that ended on January 14, 1611 and lasted 29.832568 days (29 days, 19 hours, 58 minutes, and 54 seconds).
The exact length varies slightly, due to the elliptical shape of the Moon's orbit.
Since the lunar month is about 29.5 days, it is possible for there to be two Full Moons in one calendar month: this is called a monthly Blue Moon. It also possible for there to be no Full Moon in February: this is one of the definitions of a Black Moon.
Synodic vs. Sidereal Month
The time it takes the Moon to complete one orbit around Earth is called a sidereal month. Sidereal refers to the Latin word for stars and sidereal month means that the Moon returns to the same point under the stars. This takes, on average, 27.3 days.
If Earth stood still, the synodic (lunar) month would be the same as the sidereal month. However, at the same time as the Moon is orbiting Earth, our planet also continues its annual orbit around the Sun in the same direction. So, after completing a sidereal month, the Moon has to move a little further to catch up to the same alignment with the Sun and Earth as at the previous New Moon. This is why a synodic (lunar) month is around 2.2 days longer than a sidereal month.
In astronomy, it is most common to use the Brown lunation number system for lunar months. This system was invented by Professor Ernest W. Brown and presented in the book Planetary Theory from 1933, which Brown co-wrote with Assistant Professor Clarence A. Shook.
Brown starts his count with lunation number 1 at the first New Moon of 1923, which was on January 17. This is why the lunation numbers in the table on our Moon Phase pages are currently in the 1200s. And, for the same reason, lunation numbers listed for years before 1923, are negative.
There are also other lunation cycles in use, such as Herman Goldstine's Lunation Number, Jean Meeus's Lunation Number, and the Hebrew Lunation Number, which counts lunations in the Hebrew calendar.
Primary and Intermediate Moon Phases
In Western Culture, we divide the lunar month into four primary and four intermediate Moon phases.
The primary phases are the New Moon, the First Quarter Moon, the Full Moon, and the Third Quarter Moon. Technically, the primary Moon phases occur at a specific moment in time, and the intermediate Moon phases take up the time in between.
The average time between each primary Moon phase is around 7.38 days (just over 7 days and 9 hours) which is considered the origin for the 7-day week.
Some of the oldest calendars we know of are lunar calendars, such as the Roman calendar, based on the lunar month. The word month is even derived from the word Moon.
Today, most countries use the Gregorian calendar, which is based on Earth’s revolutions around the Sun. However, the current months September, October, November, and December are still named after their original place in the ancient Roman calendar.