RGO ASTRONOMICAL INFORMATION SHEET No. 6
Prepared by HM Nautical Almanac Office, Royal Greenwich Observatory, Madingley Road, Cambridge, CB3 0EZ For the Science & Engineering Research Council
It is not possible to predict accurately the dates on which the new crescent Moon will first be seen each month since there is no collection of reliable, fully documented, observations that can be used to establish the conditions that must normally be satisfied at the time of first visibility. The simplest basis for prediction is that the Moon should be more than a certain age (measured from the time of astronomical new moon) at the time of sunset at the place concerned. It is, however, better to use the true elongation (i.e. the angular separation) of the moon from the sun at this time, rather than the age.
The new crescent is not normally visible until the sun is below the horizon and so it is desirable to take into account the altitude of the moon during twilight. The chances of seeing the new crescent depend slightly on the distance of the moon from the earth, being greatest when the Moon is closest (i.e. at perigee). The local conditions, especially the height of the observer above sea level and the character of the surrounding surface, are important, and even when the sky is free from cloud they can be considerable variations in the clarity of the atmosphere from day to day.
The visual acuity of the observer is also significant. It must be realized too that there are considerable variations in the astronomical conditions with both longitude and latitude on the Earth so that even if the weather conditions were good everywhere, the dates of the first sightings would differ from place to place. Predictions can, therefore, only be valid for restricted areas.