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.
A black moon by the first meaning can only occur in February, with the next taking place (according to UTC) in 2014. In that year January and March will each have a black moon according to the fourth meaning.
A black moon by the second meaning can only occur in February, with the next taking place in 2018. In that year January and March will each have a second full moon (sometimes called a blue moon).
The next black moon in the third meaning of the term will occur in the northern summer of 2006, with the third new moon of the season on August 23 (19:10 UTC) and the fourth on September 22 (11:45 UTC).
The autumnal equinox occurs on September 23 (04:03 UTC).The next black moon in the fourth meaning of the term, which can occur in any month except February, is set to occur on August 30, 2008 (19:58 UTC).
The last event occurred on December 31, 2005 (03:12 UTC).
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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