- There are two main calendar types: solar and lunar.
- A solar calendar
A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun.
In a solar calendar with 12 months, the average month length is 365.24 ? 12 = 30.44 days. Months in these types of calendars, as most of us will be familiar with from the Gregorian, are either 30 or 31 days long (not counting February, the freak month).
The first developers of the solar calendar were the Egyptians. As a fixed point the annual sunrise they were using reappearance of the Dog Star—Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River.
They constructed a calendar of 365 days, consisting of 12 months of 30 days each, with 5 days added at the year’s end. The Egyptians’ failure to account for the extra fraction of a day, however, caused their calendar to drift gradually into error.
Ptolemy III Euergetes of Egypt, in the Decree of Canopus (237 B.C.), introduced an extra day every four years to the basic 365-day calendar (this practice also having been introduced in the Seleucid calendar adopted in 312 B.C.).
In the Roman Republic, Julius Caesar in 45 B.C. replaced the confused Roman republican calendar, which probably was based on the lunar calendar of the Greeks, with the Julian calendar. The Julian calendar assigned 30 or 31 days to 11 months but fewer to February; it allowed for a leap year every four years. The Julian calendar, however, made the solar year slightly too long by adding a full quarter of a day annually—the solar year actually runs 365.2422 days.
By mid-16th century the extra time had resulted in an accumulated error of about 10 days. To correct this error, Pope Gregory XIII instituted the Gregorian calendar in 1582, dropping October 5–14 that year and omitting leap years when they fell on centurial years not divisible by 400—e.g., 1700, 1800, 1900.
- A lunar calendar
Lunar calendars use lunations as calendar months
A lunation is the average length of the synodic month, the time between new moons, equal to about 29.53 days. Months in lunar calendars therefore use patterns of months 29 and 30 days in length to maintain this average.
A common purely lunar calendar is the Islamic calendar or Hijri Qamari calendar. It is used mainly for religious purposes, and in Saudi Arabia it is the official calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to 12 days. So there is an 11-day difference between one solar year and one lunar year. This difference is called one epact.
Since the lunar year is roughly 11 days shorter than the solar year, the lunar months were other getting out of whack ("drifting") from the cycle of the seasons: after a few years, they couldn't describe you when it was time to plant your crops or when it be time to harvest. Imagine summer coming within January -- you wouldn't know when to do what. This could be a problem in agriculture, tourism and other industries that revolve around seasons. So people contained by different cultures had to invent adjectives sorts of ways to "re-set" their lunar calendars regularly (e.g., adding an extra (embolismic) month), so that the average year length matches the seasonal cycle. Some years thus have 12 months (lunations) = 354.36 days (avg.), and others have 13 lunations = 383.39 days.
Even afterwards, it wasn't perfect. That's why you never know on exactly what daylight Easter or Ramadan or Rosh Hashanah is going to fall: they're on the lunar calendar.
The Islamic calendar was first introduced by the close companion of the Prophet, 'Umar ibn Al-Khattab. During his leadership of the Muslim community, in approximately 638 A.D., he consulted with his advisors in order to come to a decision regarding the various dating systems used at that time. It was agreed that the most appropriate reference point for the Islamic calendar was the Hijrah, since it was an important turning point for the Muslim community. After the emigration to Madinah (formerly known as Yathrib), the Muslims were able to organize and establish the first real Muslim "community," with social, political, and economic independence. Life in Madinah allowed the Muslim community to mature and strengthen, and the people developed an entire society based on Islamic principles.
- Interesting to know
Since it was Pope Gregory XII who made the final correction to our solar calendar surrounded by 1572, Protestant England and Protestant America refused to adopt his reform and continued going by the antiquated Julian solar calendar. They weren't going to let the Pope describe them how to run their calendars. The American colonies didn't switch to the new calendar until 1752! Until they finally get around to making the change, there was one calendar day contained by America and a different calendar day contained by Europe...
Lunar calendar is calculated plinth on rotation of the moon. The month makes one rotation around the earth is one lunar month. On the 15 of every lunar month, the moon is full. This type of calendar is used by the Chinese for more than five thousand years.
It takes approximately 33 years for the Hijri lunar months to make a complete rotation through the seasons. So over the course of 33 years, there will be a lag of one year between solar and lunar calendars.