In the Holy Qur'an, the scripture of the Muslims, there is a verse that begins in translation, "They ask you concerning the moons, the new moons, say that they are just a means of marking time in the affairs of men and for the pilgrimage." There is no superstitious significance to the Moon; it's just an accurate way of marking time. The Islamic calendar is a strict lunar calendar. Twelve months, marked by seeing the new moon, make a year. Thus, the Islamic year is slightly shorter than the solar year that's followed by Christians and used generally by the world as a common calendar. This means that the Muslim holidays will occur about ten or eleven days earlier every year as one goes from one solar year to the next.
The original ancient calendars were all based on the Moon, that is, they had months that were determined by the sighting of each new moon. Among the 12 months, of course, are all the Muslim holidays. There really are only two major holidays in Islam, the Eid-al-Adha, which is the great sacrifice that corresponds to the climax of the pilgrimage, and Eid-al-Fitr, at the end of Ramadan. Ramadan is the ninth month of the Muslim calendar, and Muslims will go out to look for the new moon to determine when the month will begin.It varies from one Muslim country to another. In Pakistan, the people go out en masse and go up to the rooftops and look for the new moon. In Saudi Arabia, they have a prize they award to the first person who sees the new moon, and therefore it's usually seen by a single individual. In the United States, the Islamic Society of North America has a telephone number at which they receive sighting reports. And then they will have experts call back the people who have made the reports to question them about the sighting to see if it is valid or not. The Moon takes twenty nine and a half days to go around the sun; each month is either 29 or 30 days. So on the 29th day, people will go out to look for the new moon. If it's not sighted, the month is completed at 30 days. If it is sighted, then the next day is the first day of the next month.
Science and technology give you the ability to calculate the calendar in advance. There have been algorithms, formulas, equations that have been used to calculate the estimated time that the new moon could be seen, even from ancient times. Greek scientists and Hindu scientists developed methods of estimating when the new moon could be seen. Those methods were refined to an extraordinarily high degree under Muslim civilization, which had extremely advanced science.
In modern times, further advances are being made, not so much to determine exactly when the new moon can be seen as to explain why it's seen at a particular time. Analyses are done to show how the amount of aerosols in the air will affect when it can be seen, how psychological factors in the visual system affect when it can be seen, and so on. There are very sophisticated models, and we can now predict when the new moon can first be seen, give or take a few hours.
Almost all Islamic scholars have determined that it is a requirement to attempt to see the new moon, rather than to go by calculation alone. In the United States, the Islamic Society of North America will use calculation as one of the means to eliminate false early sightings. If someone claims to have seen a new moon before it is scientifically possible to see it, then they say that they would reject that.
Traditionally it's done with the naked eye, although many are claiming that since glasses are allowed for nearsighted or farsighted people to make the observation, then we should also admit binoculars or telescopes. More traditional people are opposed to this on the grounds that a sighting with binoculars or a telescope could be made an hour or two or maybe even three earlier, and therefore would make the modern dates out of sync with the old ones by a couple of hours.
In some parts of the Earth it is cloudy most of the time. The tradition is, if it's cloudy, you go ahead and complete a month of 30 days. However, nowadays, with high-speed communication, a cloudy place can always get a report from a place that isn't cloudy. And modern methods of transportation mean that it is possible to go up in an airplane above the clouds, so there's really no need for clouds to be a complicating factor. I've never heard of any scholar objecting to doing that, but I've also not heard of any Muslims doing it.
There's a strong tendency for immigrants to call up their home countries, and if their home countries have started the new month before it's been announced in America, they'll go ahead and start -- even though, on scientific grounds, those sightings might be questionable.
