Astronomy Picture of the Day . Tonight: A Blue Moon . ... Credit Copyright: Vic Winter ( ICSTARS ) Explanation: How often does a full moon occur twice in a single month? ... In fact, the modern usage of the term "Blue Moon" refers to the second Full Moon in a single month. ... The above picture was taken not during a full moon but through a morning sky that appeared dark blue. ... Publications with keywords: Moon - blue moon - Jupiter . Publications with words: Moon - blue moon - Jupiter . ...