The Hijri Calendar: A Brief Introduction
By Zaheer Uddin
The Hijri or Islamic calendar is based on the lunar cycles. The Hijri calendar is usually abbreviated A.H. from the Latinized Anno Hegirae, "in the year of the Hegira". Muharram 1, 1 A.H. corresponds to Friday, July 16, 622 C.E.
In 638 C.E., six years after the death of the Prophet Muhammad (s), the second caliph 'Umar (r) recognized the necessity of a calendar to govern the affairs of the Muslims. This was first of all a practical matter. Correspondence with military and civilian officials in the newly conquered lands had to be dated.
The Hijrah, which chronicles the migration of the Prophet Muhammad (s) from Makkah to Madinah in September 622 C.E., is the central historical event of early Islam. It led to the foundation of the first Muslim city-state, a turning point in Islamic and world history.
In pre-Islamic Arabia, various systems of measuring time had been used. In South Arabia, some calendars apparently were lunar, while others were luni-solar, using months based on the phases of the moon but intercalating days outside the lunar cycle to synchronize the calendar with the seasons.
On the advent of Islam, the Himyarites appear to have used a calendar based on the Julian form, but with an epoch of 110 B.C. In central Arabia, the course of the year was charted by the position of the stars relative to the horizon at sunset or sunrise, dividing the ecliptic into 28 equal parts corresponding to the location of the moon on each successive night of the month.
The names of the months in that calendar have continued in the Islamic calendar to this day and would seem to indicate that, before Islam, some sort of luni-solar calendar was in use, though it is not known to have had an epoch other than memorable local events.
There were two other reasons 'Umar (r) rejected existing solar calendars. The Qur'an, in Chapter 10 Verse 5, states that time should be reckoned by the moon. Not only that, calendars used by the Persians, Syrians and Egyptians were identified with other religions and cultures. He therefore decided to create a calendar specifically for the Muslim community. It would be lunar, and it would have 12 months, each with 29 or 30 days.
Islamic (Hijri) year is shorter than the Gregorian year by about 11 days. The months drift backward over the seasons, returning to their starting points after about 33 lunar years.
Hilal Sighting: Hand in Hand with Calculations
Islamic months begin at sunset on the day of visual sighting of the Hilal (crescent). Even though visual sighting is necessary to determine the start of a month, it is useful to accurately predict when a Hilal is likely to be visible in order to produce lunar calendars in advance and to guide the Hilal watchers when and where to look.
The moon travels around the earth on average every 29 days, 12 hours, 44 minutes, and 2.8 seconds. Hence the length of a lunar month is either 29 days (40 percent of the time) or 30 days (60 percent of the time), and no other length. The number of days is consistent with the saying of the Prophet Muhammad (s):
"The month is like this and this, i.e., sometimes of 29 days and sometimes of 30 days".
The "New Moon" (an astronomical term) is defined as the moment when the moon is almost directly between the earth and the sun. At this moment the moon's dark surface faces the earth and is completely invisible. The exact date and time of this invisible "New Moon" is commonly found in almanacs, diaries and newspapers. After about 18 to 24 hours (on average, one day), the moon moves roughly 12 degrees in its circular motion around the earth, sunlight reflects off its outer edge and, just after sunset and the start of a new day, the new Hilal (crescent moon) becomes visible on earth. With the passage of time, the Hilal grows larger until it is white and full; then in the second half of the month the moon begins to shrink and finally disappears.