III. Visibility

"Blessed is the one who placed the constellations in heaven and placed therein a lamp and moon giving light". Al-Qur'an, Sura 25, verse 61

Because at conjuction the moon sets with the sun, is not only avails itself to be sighted for a brief period but also at a time when it is already very low in the sky. This causes problems that are associated with observing a celestial object which is near the horizon.

The layer of atmosphere in the line of sight to a celestial object, say 6° above the horizon, is 10 times thicker than at zenith. Consequently, the amount of light that reached the observer is approximately two and half time weaker at this altitude than when the object is at zenith, for a perfectly clean atmosphere. Water vapour, oxygen, nitrogen dioxide and ozone, which absorb visible light at specific wavelengths and, in particular, aerosols, which scatter visible light very afficently at all wavelenghts, all contribute to the further reduction of the intensity of light reaching the observer. The amount of scattering that occurs will determine the brightness of the sky near the horizon.

The problem is compounded by the fact that the radiance of the new moon is very small indeed. The full moon is 2 × 10^-6 times as bright as the sun and two days after full moon the intensity is reduced by a factor of two (the rapid drop is due to the peculiar reflecting properties of the moon's surface). Shadows of mountains on the moon in the part that is visible will further reduce the brightness of the new moon.

Given that the sky near the horizon can be bright because of scattering due to the atmosphere and given that the irradiance from the new moon is very small, whether or not sighting can be done will depend on the ability of the human eye to contrast the new moon against the brigth sky. The haze which prevailed in September 1982, as an extreme example, would have completely obscured the new moon, even on an absolutely cloudless day.