International Lunar Date Lines
Introduction
Islamic months begin at sunset on the day of visual sighting of the lunar crescent. Even though visual sighting is necessary to determine the start of a month, it is useful to accurately predict when a crescent is likely to be visible in order to produce lunar calendars in advance. Although it is possible to calculate the position of the moon in the sky with high precision, it is often difficult to predict if a crescent will be visible from a particular location. Visibility depends on a large number of factors including weather condition, the altitude of the moon at sunset, the closeness of the moon to the sun at sunset, the interval between sunset and moonset, atmospheric pollution, the quality of the eyesight of the observer, use of optical aids etc. Since ancient times, many civilisations and astronomers have tried to predict the likelihood of visualising the new moon using different ‘minimum visibility criteria’. However, all these criteria are subject to varying degrees of uncertainty.
International Lunar Date Lines
The subject of crescent visibility has been studied in modern times by Prof. Ilyas in Malaysia who has developed several visibility criteria and the concept of the International Lunar Date Line (ILDL). Ilyas’s main criterion depends on the moon’s altitude (in degrees) above the horizon at sunset and the moon’s angular distance from the sun (relative azimuth) at sunset. If a moon’s altitude and relative azimuth are greater than certain threshold values, then the moon is likely to be visible. The ILDL is a curved line on a world map which separates areas (west of the line) where the crescent is likely to be seen at the start of the lunar month from areas (east of the line) where the crescent is unlikely to be seen. The probability of sighting the crescent increases as one travels west of the ILDL and diminishes as one travels east of the ILDL. Unlike the solar date line which has a fixedposition, the position of the ILDL moves from month to month.
Computer Generated World Maps
A computer program called MoonCalc by Dr. Monzur Ahmed was used to search the world for locations which satisfy Ilyas’s altitude/relative azimuth criterion at the start of a lunar month. On the basis of this scan, world maps for the start of each lunar month were drawn showing areas where the crescent is likely to be seen first. The different coloured bands represent the age of the moon at sunset on the day of predicted sighting. MoonCalc has many other features and also supports several other crescent sighting criteria.
Please note :
Conjunction (astronomical new moon) is used to describe the time of the birth of a new moon when the earth, sun and moon are in the same plane. Conjunction is NOT the same as visible new moon. Usually the moon has to be at least 15 hours old before it can be seen from somewhere on earth. Conjunction times are given in Terrestrial Dynamical Time (TD). TD is a uniform time scale based on the atomic clock and is not influenced by the earth’s irregular and unpredictable rotation (unlike GMT and Universal Time). TD tends to be favoured by astronomers for accurate work.
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