A phase of the moon is any of the aspects or appearances presented by the moon as seen from earth, determined by the portion of the moon that is visibly illuminated by the sun. The lunar phases vary cylically as the moon orbits the earth and the earth moon system orbits the sun, according to the relative positions of the earth, the moon and the sun. Since the moon appears bright only due to the sun’s reflected light, only half of the moon facing the sun is illuminated.
With the relative positions of the sun, the earth and the moon changing, the full moon will appear when the sun and the moon are on opposite sides of the earth. At this phase, the moon as seen from the surface of the earth is fully illuminated by the sun, presenting a ‘full’ round disk to viewers.
The new moon is the lunar phase that occurs when the moon, in its monthly orbital motion around earth, lies between earth and the sun, and is therefore in conjunction with the sun as seen from the earth.
At this time, the illuminated half of the moon faces directly towards the sun, and the dark or unilluminated portion of the moon faces directly towards earth, so that the moon is invisible as seen from earth. To base an Islamic calendar upon the birth of the new moon is contrary to the Ahaadith of Nabi [sallallaahu alayhi wasallam] as at this time to sight the moon is impossible.
After the birth of the moon, as it moves away from the sun, sunlight falls on the moon’s surface at the rate of 15.41 km per hour. The moon requires about 275 km of light on its surface for it to be visible from earth. This calculates to about +-18 hours from the birth of the moon. This time is called ‘Imkaan al-Ruyat’ or Visibility Time. Anyone seeing the moon before this is either mistaken or breaching the truth. At this stage, the moon will be visible from the earth and is called the waxing crescent.
The period between the waxing crescent and the new moon as well as the new moon and the waxing crescent is called the dark moon. At this stage, the moon appears so close to the sun in the sky that it cannot be seen even near sunset or sunrise, i.e. there is insufficient light on the moon’s surface for it to be seen from earth. The dark moon may last upto 3.5 days.
The time between 2 full moons (or between successive occurences of any two phases, e.g. ¼ moon to next first quarter moon is about 29.53 days (or 29 days, 12 hours, 44 minutes)
The different phases of the moon have different names. As the moon waxes (the amount of the illuminated surface as seen from the earth is increasing), the moon moves through the new moon, crescent moon, first quarter moon, gibbous moon and full moon phases before returning through the gibbous moon, third quarter moon, crescent moon and old moon phases. The terms new moon and old moon are interchangeable.
Casual observers will not typically notice a waxing crescent moon until about 18 hours after it has passed in conjunction with the sun, i.e. after new moon. But some individuals have crafted a hobby out of attempting to view the moon after a much shorter interval than this. Informal records and their confirmality vary; some have claimed to have seen the moon as little as 12 hours after the moment of conjunction. Three factors increase the likeliheood of spying a very young moon:
Firstly, the angle the moon makes with the ecliptic must be favourable for the applicable side of the earth. The optimum scenario for this would be a new moon that falls in mid March in the Northern hemisphere or in mid September in the Southern hemisphere.
Second: the moon should be at or near pedigree, causing it to appear to move faster (and hence draw away from the sun sooner) and
Third: the new moon must be at or near its maximum separation from the node in a favourable direction based on the hemisphere of the observer. These same principles can be applied to a very old moon just before conjunction.