Theoritical Contrast for Hilal Visibility Prediction at Pelabuhan Ratu

摘要

In general contrast in the context of the first lunar crescent visibility or hilal is defined as ratio of two components, the illuminance of the lunar crescent and the brightness of the sky (Hoffman, 2003). We used theoretical model developed by Schaefer (1993) to estimate contrast at Pelabuhan Ratu, a famous place for young lunar crescent observation in west Java. The value of contrast is defined the different between brightness of the crescent and the sky divided by brightness of the sky. Some lunar crescent with Fl (Fraction of illuminated area of the Moon) within 0.9% to 1.1% is assumed as hilal. Those hilal which appear in the same days of conjunction will be considered as threshold visible hilal. We calculate the contrast for the first lunar crescent visibility after conjunction for some hilal of VEG (Vernal Equinox Group) and AEG (Autumnal Equinox Group) (Sopwan, 2008) plotted in figure 1; PG and AG (Perihelion and Aphelion Groups) plotted in figure 2 and SSG (Summer Solstice Group) and WSG (Winter Solstice Group) plotted in figure 3. The three figures show systematically some different value of contrast for the appearance group of hilal. It indicates the different level of difficulties to find hilal, the higher value of contrast means less difficult to find hilal.