Lunar months vary in length depending on the calculation method used, with the synodic month being the most common at approximately 29 days, 12 hours, and 44 minutes. Different religions use lunar months for their calendars, but they must be adjusted periodically to stay in line with the moon’s motion. The Gregorian calendar also requires leap years and leap seconds due to orbit inaccuracies. Dates in both calendar systems are sometimes given for convenience.
A lunar month or synodic month is a period of time measured in relation to the movement of the moon. There are actually several ways to calculate a lunar month, which means it can vary in length depending on which system you use. Different religions use these months to establish their calendars, and the lunar month is sometimes used to mark time in a pregnancy.
A common method used to determine a lunar month is to look at the period between syzygy, when the moon and sun align to form either a new moon or a full moon. In this case, the synodic month, as it is also known, runs from new moon to new moon or from full moon to full moon. This lunar month lasts approximately 29 days, 12 hours and 44 minutes. Other ways of calculating use the orbital period of the moon, in a type of lunar month sometimes called a sidereal month.
Lunar months can also be measured by tracking the moon as it moves from perigee to perigee, when it is closest to the Earth, or when the moon returns to one of the two nodes in its orbit that intersect with Earth’s orbit. These different calculation techniques used to determine the lunar month can produce months that last between 27 and 29 days.
The problem with the lunar month is that if the lunar months are used to calculate time, over a long period of time, the calendar will start to shut down. Eventually, the lunar calendar will not actually map to the phases of the moon. As a result, lunar calendars must be periodically adjusted to bring them back up to speed and keep them in line with the actual motions of the moon. Among faiths that use the lunar months to keep time, special calendars are issued each year so people can keep up with the lunar months.
As it stands, the internationally used Gregorian calendar is also a bit dull, which is why you have to enter leap years to keep up. Leap seconds are also added periodically. This is because orbits are not perfectly accurate and small variations can translate into huge calendar miscalculations over time. In countries where numerous people use the lunar calendar, dates are sometimes given in both calendar systems for convenience, and converters are also readily available.
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