Implications of Twilight Sky Brightness Measurements on Fajr Prayer and Young Crescent Observation

The morning and evening twilight brightness are important to probe the properties of the upper atmosphere.We conducted a short survey of broad band twilight sky brightness measurements by using a portable photometer with 5 second resolution, in five places with different elevations.The darkest zenith sky brightness reached 22.9 MPASS.We found the complex twilight sky brightness variation and proposed 17 degree solar dip for Fajr prayer.The observable young crescent moon which has less than 24 hours in age has the sky brightness at the range of 8-16 magnitude/arcsec 2 .These correspond to telescope visual limiting magnitude of 2.9 to 9.3.The excellent quality of telescope is needed for this purpose to cope with very low contrast of young crescent moon.