is no further report (based on [*al-Haytham ibn $^{\rm c}$Ad*]{}*ī*) known about this particular celestial event.

Let us now discuss in detail the dating of the event.\
In both reports, the date of the event is given as [*in the third last night of Shawwāl*]{}.\
The Muslim calendar is a lunar calendar.[^5] A Muslim month runs from the sunset at the evening with the first sighting of the crescent (new) moon (as confirmed by two independent observers) until the sunset with the next such sighting (one month later), the Muslim day runs from sunset to sunset. See Ilyas (1994) for details about the Lunar Date line. According to the Muslim Hadith tradition, a month in the Muslim calendar (or at least the month of [*Ramaān*]{}) should last either 29 or 30 days.[^6] See, e.g., Spuler & Mayr (1961), Ilyas (1994), de Blois (2000), and Said et al. (1989) for more details about Muslim calendar rules.\
In the theoretical [*calculated*]{} Islamic calendar (not used in practice, at least not in former times), the months have an alternating (but otherwise fixed, except the leap month) length of 29 and 30 days, e.g. [*Shawwāl*]{} would then always have 29 days; then, 11 years 