However, unlike the 12 month Gregorian calendar, the Islamic year is not counted by the number of revolutions of planet Earth around the sun, but by the revolutions of the moon and the moon around planet Earth. Confused yet?
The Islamic 12 month calendar is based on lunar months. A lunar month is the time that it takes the moon to pass through each of its phases – new, half, full. The moon rotates once around its axis in approximately the same time that it takes for the moon to orbit once around planet Earth. (NASA says that it takes the moon around 27.3 days to complete a revolution but 29.5 days to move through the synodic phases). Every person on Earth sees the same phases of the moon although depending on where you are, the moon could appear ‘upside down’ compared to the other side of earth.
Earth takes approximately 365 and 1/4 days (one year) to make a complete revolution around the sun. A lunar month (or lunation) takes approximately 29.5 days (or between 29 and 30 days) to complete one revolution, so that a lunar year comprises approximately 354 days.
The solar calendar is an annual cycle around the sun (I have yet to look at why the months are of varying lengths) while the lunar calendar is the monthly cycle of the moon’s phases (synodic months).
So, the time difference (or epact, a term describing this specific time difference) is usually 11 days. Each year, the Islamic months start 11 days earlier than the year before (when we compare it to the solar calendar). Over an elapse of 33 years, there will be a lag of one year between solar and lunar calendars as a result of consecutive epacts.