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Moon Sighting

Visibility of new moon or new moon sighting in Islamic world is always a controversial issue when it comes down to moon sighting for the start of the month of Ramdan, Haj or Eid. Scientifically, birth of new moon does not mean moon is visible as well; it may not be visible on the same day.

For instance, the new moon borns (real transit) on 14 October 2004 at 04:48 (Austrian local time), but it sets at 18:21pm whereas sun set occurs at 18:27pm (6 minutes later), clearly on 14 Oct moon is not visible (US Naval Observatory).

The astronomical definition of the lunar calendar is based on real transit of the moon; a moon is considered new moon when it transit or bypasses the prime meridian. However, moon is not normally visible on the same day. In the above example, the first visibility of the new moon is on 16th of Oct. 2004.

It is believed that first Islamic month starts on the evening of the solar day when new moon is sighted first time; the following day is considered as the first day of islamic month. However, lunar calendar of most of the Arab countries is based on the true transit of the moon instead of first visibility of the moon.

I discussed this issue with Prof. Stefan Kimeswenger (Astro Physics – Univ. Innsbruck) and he describes the issue in the following words:

“The first visibility is typically a day later than the real transit. During the real bypass no visibility is possible. The first rim of the moon is visible when it moves about 8-15 degrees from the sun; the moon moves 12.86 degrees/day that means the first visibility is typically one day after the transit date.”

Another logic fallacy is about location-based moon sighting; a new moon could be sighted in one country that might not be visible in other countries on the same gregorian calendar day. Actually new moon sighting is independent from the location on earth; normally a lunar calendar starts on the same solar day all over the world as Prof. Kimeswenger describes:

“the visibility of the moon (the illuminated part seen from earth) is (nearly) independent from the position at earth. The radius of the earth is ~6000 km – the distance of the moon nearly 400000. Thus the maximum angle difference at worse case from 2 positions at earth is 0.8 degrees. The moon changes its position angle resp. illumination angle by 12.8 degrees per day thus the position on earth is negligible.”


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