The most anticipated month of the Islamic year is the holy month of Ramadan. Ramadan marks the time in which the first verses of the Qur’an were revealed. Muslims observe the event by fasting daily from sunrise to sunset. Ramadan is the ninth month of the Islamic calendar, which is lunar and contains 12 months that are based on the motion of the moon. A new month in the lunar calendar is determined by the appearance of a new crescent moon and is not linked to the change of seasons like the widely used Gregorian calendar.
A lunar month spans about 29.5 days, so each month in the Islamic calendar is about 1 to 2 days shorter than a typical month in the Gregorian calendar, which is based on the movements of the Sun. This means that a lunar calendar year, like Islam's, is about 11 days shorter than the standard Gregorian year. As a result, dates of events in the Islamic lunar year “move forward” about 11 days every Gregorian year. For example, in 2006 the first of Ramadan was September 24 and in 2007 it was September 13. Thus, with each passing year, Ramadan can fall in different seasons which affects the length of the daily fast. The fast may be comparatively short if Ramadan falls during the winter months or much longer if it occurs during the summer.
Some Muslims rely on the actual sighting of the new moon to determine the beginning of a new month, while others use astronomical calculations. Although new moons may be precisely calculated, their actual visibility can be difficult to predict. Factors such as weather, properties of the atmosphere, the altitude of the moon at sunset and its closeness, the quality of the eyesight of the observer and location of the observer must be taken into consideration in the sighting of a new moon.