Tabular Islamic calendar
The Tabular Islamic calendar is a variation of the Islamic calendar. It has the same numbering of years and months, but the months are determined by rules rather than by observing the crescent moon directly. The tabular calendar was developed by early Muslim astronomers of the second hijra century (the 8th century of the Common Era). They wanted to be able to predict the positions of the moon, sun, and planets. It is now used by historians to convert an Islamic date into a Western calendar when no other information (like the day of the week) is available. Its calendar era is the Hijri year and its epoch (year 1) is the date of the Hijra (622 in the Common Era or Gregorian calendar).