THE ASTRONOMICAL DEFINITION OF “NEW MOON” GIVEN EARLIER IS WHEN THE SUN AND THE MOON HAVE THE SAME RIGHT ASCENSION. THIS CONDITION IS TECHNICALLY KNOWN AS CONJUNCTION. At this instant the Moon is between the Earth and the Sun, its disk is dark and its phase is New (fig.3). However, the Sun, the Moon and the Earth need not be in one line. At this instant the Moon is not completely shadowed as viewed from the Earth. However, the degree of illumination is so low that it is completely invisible. The apparent eastward motion of the Moon produces changes in the shape of the illuminated disk as a result of the changing geometry of the Moon with respect to the Sun and Earth. These are called phases of the Moon. THE ISLAMIC MEANING OF THE “NEW MOON” WHICH IS BASED ON TRADITION, IS THE CRESCENT PHASE AND IN ORDER TO BE VISIBLE WITH THE NAKED EYE A TIME GAP IS NECESSARY FROM THE START OF THE INSTANT OF ASTRONOMICAL “NEW MOON”, i.e. CONJUNCTION.
Unfortunately, this time gap cannot be calculated accurately and therefore a prediction based on calculation can only be very approximate. Both the Sun and the Moon are in apparent motion. The Sun moves about l°/day and the Moon about 12°/day. They separate about 11°/day. The maximum time taken by the Moon to move away from the Sun is 19 minutes. Therefore, 19 minutes after the instant of the conjunction, the Moon can reflect sunlight towards the Earth but such reflected light may not become visible until about 20 to 30 hours after conjunction, due to the very variable conditions in our own atmosphere, as well as the precise geometry of the Moon’s orbit with respect to the Sun and the Earth.
Thus, although, in theory, the crescent phase of the Moon starts immediately after the Moon has moved away from the Sun, the time of the Moon’s visibility can only be estimated very roughly from observational measurements and similarly a purely theoretical calculation will also be very approximate.