In astronomy, the term black moon is not well known nor frequently used. As a consequence it has no accepted definition, but seems to have occasionally been applied to at least four different situations:
2. the absence of a full moon in a calendar month;
3. either the third or the fourth new moon or dark moon in a season that has four of them (a season normally has only three). This is in analogy to the term blue moon which is the third full moon in a season that has four;
4. the second occurrence of a dark moon or new moon in a calendar month; this in analogy to an improper use of the term blue moon which sometimes is applied to the second full moon in a calendar month.
Dark moon is the period when the Moon appears so close to the Sun in the sky that it cannot be seen even near sunset or sunrise. Depending on how close the Moon passes to the line between Earth and Sun, dark moon may last between 1.5 to 3.5 days. The astronomical new moon occurs in the middle of this period when the Moon and Sun are in conjunction, and is also referred to as dark moon to distinguish it from the traditional new moon, which is the moment when the crescent moon is first seen after conjunction.
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