The Islamic calendar is based on lunar months, which begin when the thin crescent Moon is actually sighted in the western sky after sunset within a day or so after New Moon. The ancient Hebrew calendar was also based on actual lunar crescent sightings, although the modern Hebrew calendar is calculated.
The 12 months of the Islamic calendar are: Muharram, Safar, Rabi'a I, Rabi'a II, Jumada I, Jumada II, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Q'adah, Dhu al-Hijjah. Since 12 lunar months are, on average, 11 days shorter than the (Gregorian) civil year, the Islamic year shifts earlier in each civil year by about this amount. The count of years for the Islamic calendar begins in 622 CE; specificaly, 1 Muharram 1 AH corresponds to 16 July 622 CE (Julian calendar).
A tabular Islamic calendar has been established for some non-religious purposes in which the lengths of the months alternate between 29 and 30 days; in leap years an extra day is added to the last month, Dhu al-Hijjah. This calendar consists of a 30-year cycle in which 11 of the 30 years are leap years. Civil dates corresponding to important Islamic dates in this tabular calendar are:
Islamic New Year 1417 - 19 May 1996These civil dates may or may not correspond to the evenings on which the crescent Moon is first visible; for example, the first day of Ramadan in 1418, as determined by an observation of of the Moon, may occur on 30 December 1997 for some observers.
First day of Ramadan - 10 January 1997
Islamic New Year 1418 - 9 May 1997
First day of Ramadan - 31 December 1997
Islamic New Year 1419 - 28 April 1998
First day of Ramadan - 20 December 1998
The visibility of the lunar crescent as a function of the Moon's "age" - the time counted from New Moon - is obviously of great importance to Muslims. The date and time of each New Moon can be computed exactly (see, for example, Phases of the Moon 1996-2000) but the time that the Moon first becomes visible after the New Moon depends on many factors and cannot be predicted with certainty. In the first two days after New Moon, the young crescent Moon appears very low in the western sky after sunset, and must be viewed through bright twilight. It sets shortly after sunset. The sighting of the lunar crescent within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies quite a bit from one month to another. The record for an early sighting of a lunar crescent, with binoculars, is 13.5 hours after New Moon; for naked-eye sightings, the record is 15.4 hours from New Moon. These are exceptional observations and crescent sightings this early in the lunar month should not be expected as the norm. For Islamic calendar purposes, the sighting must be made with the unaided eye.
Obviously, the visibility of the young lunar crescent depends on sky conditions and the location, experience, and preparation of the observer. Generally, low latitude and high altitude observers who know exactly where and when to look will be favored. For observers at mid-northern latitudes, months near the spring equinox are also favored, because the ecliptic makes a relatively steep angle to the western horizon at sunset during these months (tending to make the Moon's altitude greater).
If we ignore local conditions for the moment, and visualize the problem from outside the Earth's atmosphere, the size and brightness of the lunar crescent depend on only one astronomical quantity - the elongation of the Moon from the Sun, which is the apparent angular distance between their centers. For this reason the elongation has also been called the arc of light. If we know the value of the elongation at any instant, we can immediately compute the width of the crescent.
What is the value of the elongation when the Moon's age is one day? It varies, depending on several factors:
(1) The elongation at New Moon. The Moon can pass directly in front of the Sun at New Moon (when a solar eclipse will occur) or can pass as far as five degrees away. That is, the Moon can start the month with an elongation ranging from zero to five degrees. A minor complicating factor involves the definition of New Moon in the almanacs. Astronomical New Moon is defined to occur when the Sun and Moon have the same geocentric ecliptic longitude, which may not occur precisely when the Sun and Moon are closest together in the sky.
(2) The speed of the Moon in its orbit. The Moon's orbit is elliptical, and its speed is greatest when it is near perigee, least near apogee. If perigee occurs near New Moon, the Moon will appear to be moving away from the Sun in the sky at a greater than average rate.
(3) The distance of the Moon. Again, because of its elliptical orbit, the distance of the Moon varies, so even if the Moon moved with a constant speed, its angular motion as viewed from the Earth would be greater when the Moon is near perigee.
(4) The location of the observer. If the observer is located in the tropics such that the one-day-old-Moon is observed just before it sets, its elongation as seen by the observer will be about a degree less than that seen by a fictitious observer at the center of the Earth, which is the basis for most almanac calculations. This decrease in observed elongation is less for observers at middle or high latitudes (although other geometric factors are less favorable for these observers).
Factors (2) and (3) are linked by Kepler's second law, which predicts that the angular speed of the Moon as seen from the Earth will vary by about 22%. If we combine all these factors we find that geocentric elongation of the Moon from the Sun at an age of one day can vary between about 10 and 15 degrees.
This large range of possible elongations in the one-day-old Moon is critical, because at this time the width of the crescent is increasing with the square of the elongation, and the surface brightness of the crescent is also rapidly increasing. Some of the earliest reliable sightings of the crescent occur near elongations of around 10 degrees. Obviously, simply specifying the age of the Moon cannot tell the whole story. Of course, the elongation of the Moon does not tell the full story, either. But, of the two parameters, the elongation is a much more reliable parameter to use as a starting point in assessing the lunar crescent visibility at any given date and time.
The prediction of the first sighting of the early crescent Moon is an interesting problem because it simultaneously involves a number of highly non-linear effects. Stated in less technical language, a lot of things are changing very rapidly. Effects to be considered are the geometry of the Sun, Moon, and horizon; the width and surface brightness of the crescent; the absorption of the Moon's light and the scattering of the Sun's light in the Earth's atmosphere; and the physiology of human vision. The problem has a rich literature. Some modern astronomical references are:
Schaefer, B. E., 1988: "Visibility of the Lunar Crescent", Quarterly Journal of the Royal Astronomical Society, Vol. 29, pp. 511-523.
Schaefer, B. E., Ahmad, I. A., Doggett, L. E., 1993: "Records for Young Moon Sightings", Quarterly Journal of the Royal Astronomical Society, Vol. 34, pp. 53-56.
Ilyas, M., 1994: "Lunar Crescent Visibility Criterion and Islamic Calendar", Quarterly Journal of the Royal Astronomical Society, Vol. 35, pp. 425-461.
Doggett, L. E., Schaefer, B. E., 1994: "Lunar Crescent Visibility", Icarus, Vol. 107, pp. 388-403.
Her Majesty's Nautical Almanac Office at the Royal Greenwich Observatory computes and distributes predictions of lunar crescent visibility - see especially HMNAO's Astronomical Information Sheet No. 75 by B. D. Yallop: "Earliest Visibility of the New Crescent Moon 1996-2000 for Cities around the World". The South African Astronomical Observatory in Cape Town also distributes predictions for South Africa, Mecca, and Cairo (contact persons: Dr. John Caldwell, e-mail firstname.lastname@example.org, or Dr. C. David Laney, e-mail email@example.com). The "Astronomical Calendar" by Guy Ottewell (published annually by Furman University, Greenville, SC) includes good diagrams of the positions of young and old Moons during the year (drawn for the eastern U.S.) and an explanation of the factors affecting their visibility.
- G. H. Kaplan