Muslims should note that the start of Ramadan and observance of festivals such as Eid should be based on actual sighting of the first visible crescent of the new moon (see previous page). As a general guide this cannot be seen without optical aids when it is less than 12 hours old and it is usually visible to the naked eye when it is between 20-30 hours old.
It is a common practice within larger Muslim communities to appoint a Hilal (crescent) sighting committee for religious observations and fixing the calendar, however some communities also rely upon a single reputable sighting of the first crescent each month in their territory.
Information on the astronomical time for the beginning of each month can be found in a variety of places including the web sites of H.M. Nautical Almanac Office (HMNAO) and the US Naval Observatory. This is usually used for administrative purposes such as printing calendars
The HMNAO also produces a visibility code that gives information on the probability of moon sighting (i.e. varying from easily visible to not visible without a telescope).
There are a number of factors that affect visibility of the moon and hence it is recommended that a number of independent observers should be involved in sighting the moon, if using the naked eye, due to the risk of making an unintentional false sighting.
The factors that typically affect the sighting of the new moon include
the distance of the moon from the Earth
local conditions (especially the height of the observer above sea level)
the character of the surrounding surface and the height of the horizon relative to the observer
variations in the clarity of the atmosphere and wind direction.
the quality of eyesight of the observer
Professor Ilyas in Malaysia has developed the concept of the International Lunar Date Line (ILDL) which uses the moon‘s altitude (in degrees) above the horizon at sunset and the moon‘s angular distance from the sun at sunset. The ILDL forms a curved line on a world map which separates areas (west of the line) where there is a greater likelihood of sighting the moon at the beginning of the lunar month and areas where the crescent is less likely to be seen (east of the line). The position of the ILDL varies over a monthly period.
Dr Monzur Ahmed has developed a computer program, MoonCalc, which uses the ILDL and creates world maps to show where the crescent is likely to be seen first. MoonCalc can be used to find information relating to the position, age, phase, orientation, appearance and visibility of the moon for any given date, time and location on earth.