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Determining future dates for fasting and Eid

After conjunction with the sun, the moon can be first seen as young crescent only after sunset in the western horizon for a very brief period. If on a new moon date moon sets earlier than sunset at a place of observation, the new crescent cannot be visible there on that date. Similarly, if a particular conjunction takes place after local sunset at any place, question of moon's visibility there on that date does not arise.
Prof. Dr. Anwar-ul Karim
From any location on the earth, the moon appears to be a circular disk which, at any specific time, is illuminated to some degree by direct sunlight. Like the earth, the moon is a sphere which is always half illuminated by the sun. but as the moon orbits the earth we get to see more or less of the illuminated half. During each lunar orbit (a lunar month), we see the moon’s appearance changes from not visibly illuminated through partially illuminated to fully illuminated, then back through partially illuminated to not illuminated again. Although this cycle is a continuous process, there are eight distinct, traditionally recognised stages, called phases. These phases designate both the degree to which the moon is illuminated and the geometric appearance of the illustrated part.
These phases of the moon, in sequence of their appearance, are: 1. New Moon. 2. Waxing Crescent, 3. First Quarter, 4. Waxing Gibbous, 5. Full Moon, 6. Waning Gibbous, 7. Last or Third Quarter, and 8. Waning Crescent Following waning crescent is New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. In Fig-1 all these eight moon phases in a month can be seen for March 2008.
Although full moon occurs each month at a specific date and time, the moon's disk may appear to be full for several nights in a row if it is clear. This is because the percentage of the moon's disk that appears illuminated changes very slowly around the time of full moon. The moon may appear 100 per cent illuminated only on the night closest to the time of exact full
moon but on the night before and night after will appear 97-99 per cent illuminated; most people would not notice the difference. Even two days from full moon the moon's disk is 93-97 per cent illuminated. During the lunar phase of new moon, the moon's unilluminated side is facing the earth; the moon is not visible (excepting during a solar eclipse). The moon may appear 100 per cent unilluminated only on the night closest to the time of exact new moon, but the moon’s disk may appear fully dark for more nights because the illumination changes very slbwly around the time of new moon.
The phases of the moon are related to (actually, caused by) the relative positions of the moon and sun in the sky. For example, new moon occurs when the sun and moon are quite close together in the sky. Full moon occurs when the sun and moon are at nearly opposite positions in the sky -- which is why a full moon rises about the time of sunset, and sets about the time of sunrise, for most places on earth. First and last quarters occur when the sun and moon are about 90 degrees apart in the sky. The relationship of the moon's phases to its angular distance in the sky from the sun allows us to establish very exact definitions of when the primary phases occur, independent of how they appear. Technically, the phases new moon, first quarter,
full moon, and last quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the moon over that of the sun is 0,90,180 and 270 degrees respectively.
The first time that the thin waxing crescent is visible after new moon, low in the evening sky just after sunset, marks the beginning of a month in the Islamic calender.
The earliest astronomical criterion for ascertaining the crescent's first visibility goes back to Babylonian era. The simple criterion based on the observational data was used by early astronomers. The Babylonian criterion is more a rule of thumb which may be stated as follows: 1. At local sunset, moon's age must be more than 24 hours ie. from the time of conjunction to the time of evening of observation. 2. Moonset should take place at least about 48 minutes after sunset.
Sightings of moon younger than 20 hours are rare and sightings of moon older than 24 hours are not uncommon although the visibility may at times require it to be more than 30 hours old.
The moonset lag criterion is simple enough and is perhaps more meaningful to a layman who can now easily understand that the local moonset should follow considerably after the local sunset and never before (i.e. conjunction must take place before the local sunset) and can easily estimate the chance
of visibility on a local evening reasonable accurately.
The moon has a powerful allure — it is full of beauty, legend, myth and romance. The phases of the moon are caused by the relative positions of the earth, sun and moon. The moon goes round the earth in 27.3 days, or 27days 7 hours 43 minutes, on average. This measurement is relative to the stars and is called the sidereal period or orbital period. However, because of the earth's motion around the sun, a complete moon cycle (New Moon to New Moon) appears to earthbound observers to take a couple of days longer: 29.5305882 days to be exact. This number is called the synodic period or "lunation," and is relative to the sun.
The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes through the earth's shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon's illuminated surface appears to grow (wax) to full, then decreases (wanes) to the next new moon.
The specific dates and time of future moon phases are already known or can be calculated as shown in Table-1. According to internet claim, future moon phases
to the year 4999 are already available.
The dates of moon sighting in Bangladesh for the first three lunar months of the current 1429 Hijri were declared by the government. With corresponding new moon data from Table-1, moon's age at local sunset from the time of conjunction and moonset time lag which is the time difference between moonset and sunset at Dhaka have been calculated following the procedure as shown in our previous article: Moon sighting for Eid, The Daily Star, December 8. 2007. All three above moon sightings satisfied the Babylonian criteria of moon's age and moonset timelag.
Round the year the moon, as it orbits the earth which orbits the sun, comes into conjunction with the sun at new moon at different specific instants of 24 hours of a day as shown in Table-1, at noon or afternoon, before or after sunset, at night or around midnight, at dawn or in the morning, etc. But after conjunction with the sun, the moon can be first seen as young crescent only after sunset in the western horizon for a very brief period. If on a new moon date as in Table-1, moon sets earlier than sunset at a place of observation, the new crescent cannot be visible there on that date. Similarly, if a particular conjunction takes place after local sunset at any place, ques-
tion ofmoon's visibility there on that date does not arise. Any claim of moon sighting on such evenings by any person anywhere on earth, from the viewpoint of astrophysics, is incompatible. Hence Eidul Fitr celebration of 1429 Hijri on September 30, 2008 or Eidul Azha celebration of 1429 Hijri on December 7, 2008 at any place on the earth would be astronomically incorrect.
New moon for Ramadan 1429, from Table-1: August 30,2008,1958 GMT which corresponds to August 31, 2008, 01 58 Bangladesh Time. New moon for Shawwal 1429, from Table-1: September 29, 2008, 8 12 GMT, Le. 14 12 Dhaka time; using the moonset and sunset time, one finds that the moon sets 12 minutes before sunset at Dhaka on that day. Using the two Babylonian criteria as mentioned earlier, Ramadan fasting is likely to begin on September 2, 2008 TUesday and Eidul Fitr is likely to be celebrated on October 2, 2008 Thursday in Bangladesh.
Reports indicate that fasting begin and Eids are celebrated in USA, Australia, Europe, as well as in some other parts of the world on different days by different communities. Every month of the Islamic calendar begins with the first crescent moon, low in the evening sky just after sunset, which is a visible beginning unlike the solar month. Each month starts with its own new visible crescent after the corresponding astronomical new moon and all the Muslims are obliged to accept the first visible crescent as the definitive announcement from the Almighty Allah of any new month. The writer is former professor of King Abdul Aziz University. Jeddah, Saudi Arabia