After the day, the next measurement of time that early people used was probably the month. They noticed that the Moon changed shape on a regular cycle, with 28 days passing from one full Moon to the next. In fact, the true figure is about 29.5 days. This is a lunar month. The phases of the Moon do not divide exactly into the 365 or 366 days in a year, so over time the months that we use today, known as calendar months, came to have slightly different lengths. April, June, September and November have 30 days each. All the other months have 31 days, except February, which has 28 days, or 29 in a leap year.
The Gregorian calendar is a solar calendar; this sort of calendar gives a date based on the position of the sun in relation to the stars behind it. The seasons are based on the equinoxes and correspond to the declination of the sun.
A lunar calendar is different. The cycles it records are based on the monthly phases of the moon. A month starts when the moon reaches a certain phase in a certain location and a year is defined as 12 months of this kind. The months in the Gregorian calendar are not calculated in this way.
First off, the lunar year is only 354 days, give or takes a few. This means holidays and festivals determined by a lunar calendar, such as Easter, the Chinese New Year, or Rosh Hashanah, will seem to shift by as much as 11 days a year with respect to a solar calendar. Also, since months are determined by the phases the moon is in as seen over a particular place, the length of a month is not standardized and can vary by as much as a day. Months can even start on different days in different places.
This is still a problem for many people, as debate continues over whether the month of Ramadan, which is a lunar month, should start based on local observations of the moon or if the phase observed over Mecca should be used.
Picture Credit : Google