An Etymological Dictionary of Astronomy and Astrophysics
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The first sighting of the → New Moon after its → conjunction with the Sun. Although the date and time of each New Moon can be computed exactly, the visibility of the lunar → crescent as a function of the → Moon's age depends upon many factors and cannot be predicted with certainty. The sighting within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the → twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies from one month to another. The visibility depends on sky conditions and the location, experience, and preparation of the observer. Ignoring atmospheric conditions, the size and brightness of the lunar crescent depend on the → elongation which in turn depends on several factors: 1) The Moon's elongation at New Moon (the elongation of the Moon at New Moon is not necessarily 0). 2) The speed of the Moon in its elliptical orbit. 3) The distance of the Moon, and 4) The observer's location (parallax). The combined effect of the first three factors gives geocentric elongation of the Moon from the Sun at an age of one day which can vary between about 10 and 15 degrees. This large range of possible elongations in the one-day-old Moon is critical (US Naval Observatory).

→ crescent; → moon; → visibility.