how long between blue moons
If a Blue Moon is defined as the second Full Moon in a calendar month, we can easily calculate the mean period for such events because: 1  The synodic period between Full Moons is 29.53 days. 2  There are 12 calendar months in a year of 365.25 days. 3  A mean calendar month is [math]\frac{365.25}{12}\approx30.44[/math] days.