There are many governing factors for the moon to be sighted as a crescent from ground level. Some of these are detailed below:

  1. Moon's elongation (angular distance from the sun).
  2. The time difference between sunset and moonset.
  3. The age of the moon after the Conjunction.
  4. Weather conditions.
  5. Aid and assistance of instruments.

Fig4 SunMoonAngles
Figure 4. Sun Moon Angles

After sunset, the moon should be at least 11-12 degrees further away from the sun on the horizon. At the time of sunset, the moon should be at least 8 degrees high on the horizon. By this time there will be sufficient darkness on the horizon for the human eye to differentiate between the natural light and the light of the sun reflecting from the crescent. If during the initial 15 minutes after sunset the moon is only 4-5 degrees high on the horizon, its sighting will be difficult by the naked eye. This is due to the fact that there is still sunlight present on the lower horizon and that only 1/100th of the moon is illumined (for the 1st night crescent).

NATURAL LIGHT ON THE HORIZON: People with normal vision will be able to distinguish between the natural light on the horizon and the light of the crescent if, together with other factors being favourable, the age of the new crescent is about 20 hours. When factors are exceptionally favourable the crescent may be sighted earlier than 20 hours. (e.g. favourable angle and altitude, exceptionally clear sky, weather free from storms and dusty winds etc.).

THE PERSON SIGHTING THE CRESCENT: A keen and an observant eye together with past experience and knowledge of the shape of the crescent in the quest are all factors which effect the sighting. e.g. The moon is in the north of a particular place at sunset and the people are searching for it in the south, thus there will be no chance of a sighting.