When I decided to add Islamic months to the table of moon timings I did a lot of research into how this worked. What is presented here is my current understanding and may not be entirely correct. Feel free to email me if any of this is madly inaccurate or you disagree in some way.
Islam uses a purely lunar calendar of twelve months. The start of each month is determined by the timing of the New moon. The majority of Muslims seem to wait until the sighting of a thin crescent after the New moon. If the crescent has been seen by just after sunset then the new month starts. If it happens to be cloudy then they simply wait an extra day before starting the new month. Thus no month is ever longer than 30 days. Because the calendar is lunar it moves backwards by about 11 days each solar year.
Since the visibility of the crescent moon depends on a number of factors which are impossible to calculate it is just as impossible to exactly predict when a new month will begin. For civil purposes most Muslim groups and countries either use the ‘normal’ (Gregorian) calendar or a ‘tabular’ Islamic calendar which may differ slightly from the religious calendar.
There are some Muslims groups which argue that it is fine, and indeed correct, to use the predictable astronomical New moon to determine the start of a month and they do just this. For these groups of people the date shown in the New moon column in the moon timings table will indicate the start date of the month provided the time shown is before sunset. If the time is after sunset then the month will start the following day.
There seem to be an almost infinite number of variations on these themes and it has produced much, sometimes heated, debate. Everyone claims their reasoning is backed up by the same verses in the Holy Qu’ran. So far as I can tell no one has actually gone to war over it. Indeed the debate seems to be pretty amicable.
Where in the world to look
If all this isn’t complicated enough already not everyone agrees on just where in the world the crescent moon has to be seen. Some people use visibility in Makkah, others in Jerusalem, others use local sunset while still others use the sunset in the town or country where their family originated if they happen to have moved. Still others say that the month starts when the crescent moon has been seen anywhere in the world.1
All this can lead to different Muslim communities living in the same place starting their months on different days.
My own thoughts
I find all all this very perplexing but as a non-Muslim who has read the relevant bits of the Qu’ran (in English) I’m inclined to agree with the folks who say that Allah (through the Prophet) only suggested using the visible crescent because to do otherwise would have involved too much complication at the time.
The Qu’ran does quite clearly state that the astronomical New moon is entirely predictable but who am I to get involved in the argument? It does rather suggest a reason for just why Islamic countries have produced so many fine mathematicians and astronomers.
So it’s only an estimate
The moon timings in the table are astronomical New and Full moons. The Islamic months are shown next to the New moon on which the start of the month will be based. Also shown in the Islamic month column is an estimated start date for the month. As explained above this can (for most Muslims anyway) only be an estimate but it should be no more than one day out.
9th February 2003 (last updated 28th August 2010)
1 A further complication is the attitude of the Saudi authorities. They seem obsessed with being the first to observe the crescent moon especially at the beginning and end of Ramadan. The obsession goes so far in some years that they claim the moon has been seen when it is clearly impossible to see it. Indeed, they’ve even claimed to have seen it several hours before astronomical new moon. [29th January 2004].
The frequently asked questions at MoonSighting.com include the information that the youngest moon ever seen with the naked eye was 15.4 hours old. This happened on 14 September 1871. The Saudis regularly ‘see’ moons which are younger than this. A few days after I’m writing this they’re going to do it again. The moon marking the start of Ramadan 1425 will be a little over 12 hours old at sunset in Makkah yet I’ve no doubt that the Saudis will claim to have seen it yet again. [11th October 2004]
The system I’m using to estimate the month start date is a simple one: if the New moon occured 17 hours or more before sunset then a crescent is probably visible and the month can start.
Note that although the month actually starts at sunset (Islam starts days at sunset) the date given is the day starting at the midnight following that sunset.
There are more involved systems available for estimating crescent visibility but they’re beyond my capabilities to handle at the current time.
The US Navy has a page explaining the factors involved in predicting the crescent moon.
If you’d like even more information on the problem then try doing a Google search for +"crescent moon" +sighting
For a much more in depth look at this topic why not have a look at the Wikipedia article?