Hisab dan rukyat: Perbedaan antara revisi

Namun demikian, tidak selamanya hilal dapat terlihat. Jika selang waktu antara ijtimak dengan terbenamnya matahari terlalu pendek, maka secara ilmiah/teori hilal mustahil terlihat, karena iluminasi cahaya Bulan masih terlalu suram dibandingkan dengan "cahaya langit" sekitarnya. Kriteria Danjon (1932, 1936) menyebutkan bahwa hilal dapat terlihat tanpa alat bantu jika minimal jarak sudut (''arc of light'') antara Bulan-Matahari sebesar 7 derajat. [http://the-moon.wikispaces.com/Danjon+Limit Danjon Limit:]

<ref>The Danjon Limit is French astronomer André Danjon's estimate of the smallest angular separation (center to center) between Sun and Moon at which a lunar crescent can be seen. Danjon set the value at about 7° based on the crescent observations available to him in the early 1930's. Despite the obvious difficulties of accurately interpreting a dim and slender Moon in a bright twilight sky, Danjon felt that the inability to detect crescents at smaller elongations was an intrinsic property of the Moon caused by the roughness of the lunar terrain preventing any direct sunlight striking the Moon's surface from being seen at smaller angles (even under the best of circumstances). More recent observations suggest, contrary to Danjon's conclusion, that this is mostly a perceptual problem, and that the sunlit crescent does not actually vanish (at least not at this angle) [http://the-moon.wikispaces.com/Danjon+Limit Danjon Limit].</ref>