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Muslim Calendar


Mohamed Nabeel Tarabishy[1]





This paper examines the benefits of the Higri calendar and ways to achieve it by choosing suitable criteria for deciding the beginning of the lunar month.  The criteria should be simple, consistent with visual sighting of the Moon, and predictable.  The resulting calendar would unite Muslims in their religious duties in Fasting and Pilgrimage, and be useful for use in everyday life as a complementary to the solar calendar.



The customary calendar is the Gregorian calendar which is solar based.  It uses the motion of the earth around the sun or the apparent Sun motion in the sky to decide the time so that the seasons come at the same time every year.  It has 12 months, each has 30 or 31 days except for February that has 28 days and it has 29 days on a leap year to make the calendar in step with the seasons-the tropical (actual) year is 365.24220 days whereas the calendar year is 365.2425 days on average.

The Moon motion inspired the lunar calendar.  For example, the concept of the month originated from the phase cycle of the moon.  The Muslim calendar is an example of a  purely lunar calendar and that makes it very unique among other calendars.  It has 12 months where each is 29 or 30 days long (on average it is 29.5 days per month) The lunar year is shorter than the Gregorian year by 11 days which makes the lunar months float with respect to the seasons in a 33 years cycle.

Another type of calendar is the Luni-solar calendar like the Jewish and Chinese calendars.

The benefits of the lunar calendar are:


1- It enables the Muslims to perform their religious duties like Fasting Ramadan (9th lunar month), and performing Pilgrimage (Haj) in Zil-higa (12th lunar month), as well as other voluntary worships at other specific times.  In addition it gives Muslims a better connection with their history.


2- It is beneficial for people who need to know the timings of the tides for fishing or marine transportation.


3- The possible effect of the moon on people -research results in this area are mixed. However, it is useful to be aware of this natural rhythm.


Therefore it is really important to have a practical and reliable lunar calendar.




Purpose of Religion:

At this point it is important to remind the Muslims of the importance of unity and state one of the most important strategic objectives of the faith.

If we examine the rituals of Islam we find that it tries to gather people in one place to do things in unison.

Take for example the daily prayer, it is strongly recommended that people gather in one place, choose a leader and follow him worshiping together in harmony.  Similarly, the Friday prayer is geared toward a larger number of people.  The Eid occasion seeks an even bigger crowd.  The ultimate gathering of Muslims is during the Haj where people from around the globe trek to Mecca at the same time, wearing the same cloth, and doing the same rituals, and being part of this great event.

Similarly, Fasting should be along these lines where people (everybody and everywhere) are immersed together in this endeavor without being divided.

It is clear that the intent of the faith is to gather the people as much as possible and to let them do the noble task of worshiping God jointly so that a sense of community and belonging is created, and strong relations between the individuals are cemented, creating a force of good for the individuals, their community, and the world at large.



To choose a criterion for use in a unifying calendar, it should have the following conditions:

1- It should be simple.

2- It should not contradict the visual sighting.

3- It should be predictable.


Simplicity and the ease of understanding are very important for adoption of the chosen criteria.  At the time of the Prophet Mohamed (P), the simplest and most reliable criteria deciding the new month was the visual sighting of the new moon.  We should be able to keep the spirit of the tradition and incorporate some of the new science we have learned.


As with any criteria, it will not be perfect, and therefore great care should be paid to the possible errors.  There are two types of errors:

a- False positive: To indicate the beginning of the month when it is not the case (impossible to see the new Moon).  This type of error should be minimized below an acceptable limit.

b- False negative: To state that it is not the beginning of the month when in fact the new moon had been sighted somewhere in the world.  Such an error should be eliminated because it causes confusion and the loss of credibility.


Predictability is important for the use of the calendar in daily activities, transactions, and planning.  Otherwise the calendar will not have any practical value outside the religious use.  This condition necessitates that the calendar be based on calculations.



Some of the criteria that are used these days are:

Um-Alkura criterion:

The new moon (geocentric conjunction) occurs before sunset at Mecca, and it sets after the sun.

This criterion is vulnerable to false negative error.


ISNA (Islamic Society of North America), ECFR (European Council for Fatwa and Research) criteria:

Initially, ISNA used the condition that the moon is born before 12:00 Noon GMT , later it adopted the ECRF criterion which is the same as Um-Alkura.


Other criteria that can be used is the time elapsed after the new moon is born where the record times for observing the crescent after the new moon birth is 15.5 hours with the naked eye, and 11.67 hours for aided sighting.[2]

These limits could be used for the criteria, and while it is possible that those records might be broken, it is highly unlikely.


Another indicator that can be used as criteria is the solar elongation (arc of light) which is the angle between the lines of sight from Earth to the Sun and from Earth to the Moon (angular separation).  It is largely accepted that moon sighting is possible only for elongations greater than 7 degrees (Danjon’s limit).

This criterion is more difficult to implement.


More elaborate criteria have been used like Yallop’s criteria which uses the angular separation between the sun and the moon, the best time (sunset + 4/9 moonset lag), and the width of the crescent to generate the “q” parameter that describes the possibility and ease of observation.  Variants of this criterion has been used by Monzur Ahmed and Khalid Shaukat.


Of course we advocate the use of simple criteria that can be easily understood and implemented by non specialist.



We have argued for a practical and unifying calendar that unite the Muslims in a great worship like fasting, and be of practical value so that it can be used for daily activities and as a complement of the solar based calendar in use (It would be a nice contribution of Muslims to the world).  In making the calendar, it is crucial to choose the criteria for the beginning of the lunar month carefully to meet the three stated conditions: simplicity, consistency with the visual observation, and predictability, and we talked about the possible errors.

In the end, if we don’t resolve this problem, the alternative would be chaos, confusion, and divisiveness.




Lieber, Al.; "Homicides and the lunar cycle”, Journal of Clinical Psychiatry, 39(5):385-92, May, 1978.

Xin, et al.; “Lunar Visibility and the Islamic Calendar”, Dept. of Math. National University of Singapore, 2001

US Naval Observatory; web site,

Islamic Society of North America; web site,

Khalid Shaukat; web site,

Monzur Ahmed, web site,



[1] The author was a Visiting Assistant Professor at the Mechanical, Materials & Aerospace Engineering @ Illinois Institute of Technology (1994-2000)

[2] The author has chosen the unaided eye, minimum time criteria in his own software   

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