Yes, it’ll be a Blue Moon that lights up the night sky on August 20-21 from dusk till dawn! But it won’t be a Blue Moon by the most popular definition of the term – the second of two full moons to occur in the same calendar month. Nor is the moon likely to be blue in color, as that’s caused by exceedingly rare atmospheric conditions.
Rather, the August 2013 full moon will present the third of four full moons to fall in the same season. A season is defined as the period of time in between a solstice and an equinox – or vice versa. This full moon is the third of four full moons to take place in between the June 2013 solstice and the September 2013 equinox. That makes the August 2013 full moon a Blue Moon!
The moon turns full on August 21, at 1:45 Universal Time. Although the full moon happens at the same moment worldwide, the clock reads differently by time zone. The moon turns full in the United States on Tuesday, August 20, at 9:45 p.m. EDT, 8:45 p.m. CDT, 7:45 p.m MDT or 6:45 p.m. PDT. Astronomically speaking, the moon is full when it’s most directly opposite the sun for the month.
Many of you may not be familiar with the definition of Blue Moon as the third of four full moons in one season. But this definition actually preceded the more modern definition of a Blue Moon as being the second of two full moons to occur in one calendar month.
In both instances, the Blue Moon is a calendar oddity caused by the 19-year Metonic cycle. There are 235 full moons in 19 calendar years, but only 228 calendar months (or 76 three-month seasons). Therefore, it’s inevitable that 7 out of 19 years will feature two full moons in one calendar month. And it’s also inevitable that 7 out of 19 years will have four full moons in one season.
Celebrate the August 2013 Blue Moon – the third of four full moons to take place in between the June solstice and the September equinox!