The Sentience

The Sentience

Wednesday, June 8, 2011

Islamic Calendar - Lunar Crescent Visibility Criteria

The main area of concern regarding the implementation of the Islamic Calendar is the Lunar Crescent Visibility or Hilal. This has been the topic of various debates and there hasn't been a commonly agreed global consensus regarding the accurate sighting of the Hilal across the globe. S.K. Shaukat defined certain criteria for the lunar crescent visibility, which are:
  • Lunar altitude > 4.1 degrees
  • Arc of light, i.e., the elongation of the Moon from the Sun or the apparent angular distance > 10.6 degrees
  • Age of the moon should be between New Moon and First Quarter
This criterion has been used in a lot of astronomical functions and have made calculations approximate at best for many reasons.

MABIMS have defined another criteria for lunar crescent visibility, which is:
  • Lunar altitude > 2 degrees
  • Arc of light > 3 degrees
  • Age of the moon > 8 hours
MUIS further improvised on this criteria for Singapore and modified the values as:
  • Lunar altitude > 6 degrees
  • Arc of light > 7 degrees
  • Age of the moon > 16 hours
I had mentioned about the concept of ILDL in the previous post. However, the computer implementation of the ILDL for calculating the precise lunar crescent visibilty would go a bit off-track considering that the Islamic Calendar is only one of the Astronomical Calendars to be implemented for GSoC 2011. The detailed description of the concept of ILDL can be read in Prof. Mohammad Ilyas' article.
Dr. Monzur Ahmed has worked on the crescent visibility issue through his software MoonCalc. But implementing Dr. Ahmed's work in the KCalendarSystem API would be a daunting task, and it might just become another GSoC/SoK project!

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