* The Islamic calendar is a lunar calendar of twelve months with durations of 29 and 30 days. The mean astronomical lunation duration is 29 days, 12 hours, and 44 minutes; therefore, the mean lunar astronomical year, composed of twelve mean lunations, has a duration of 354 days, 8 hours, and 49 minutes.
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* The lunar calendar year can have 354 or 355 days, the former being more frequent than the latter.
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*To regulate the Islamic calendar, medieval Muslim astronomers devised computational or arithmetic calendars composed of 30-year cycles, of which 19 have 354 days and 11 years have 355-day. These calendars allow us to compare a date on the Islamic calendar with another on the Julian or Gregorian calendar. Moreover, they allow knowing in advance the calendar of any year in the future or the past. Furthermore, these calendars deviate very little from the astronomical movement of the Moon.
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* Since the emergence of the Islamic calendar, observation has been used to regulate it. On the 29th of each month, the western horizon is observed shortly after sunset; if the lunar crescent is seen the new month begins. If the observation is unsuccessful, the next day is the 30th, followed by the new month's first day.
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* The observational method is a good regulator of the lunar calendar, but it has the disadvantage that it is not possible to know in advance when the new month will begin.
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* Since Babylon's time, numerous rules of thumb have been devised to anticipate when the new lunar month will begin. Since the second half of the last century, physical techniques have been developed to know when the first lunar crescent will be seen shortly after the new Moon. 
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* At present, there is no definitive method that allows knowing with complete confidence the beginning of each lunar month. Furthermore, the problem is scientifically unsolvable since unpredictable atmospheric conditions affect theories that anticipate the beginning of each lunar month by physical and astronomical means.
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* To the scientific problems mentioned, religious and even political considerations are added, which cause the calendar chaos suffered by Muslims around the world.
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* Therefore, the Islamic calendar is a matter of the most significant interest, and it is not wrong to say that it is one of the oldest scientific problems and one of the most important because it affects more than one billion people around the world.
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* My research on the Islamic calendar from an astronomical and physical perspective on this website sheds light on this intricate problem.
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