Your calendar likely says that the first of two July 2015 full moons falls on July 2. The first full moon of July falls on July 2 at 2:20 Universal Time. That’s July 1 at 10:20 p.m. EDT, 9:20 p.m. CDT, 8:20 p.m. MDT pr 7:20 p.m. PDT. So, for us in the U.S., the full moon happens on July 1.
Meanwhile, the second full moon of the month will come on July 31, 2015. By popular acclaim, it’ll be known as a Blue Moon.
How often will we get a Blue Moon in the month of July?
Every 19 years, the phases of the moon recur on or near the same calendar dates. This is the Metonic cycle. That means in 2034 we’ll again have two full moons in July 2034 and another Blue Moon on July 31, 2034.
There are 235 full moons yet only 228 calendar months in the 19-year Metonic cycle. Because the number of full moons outnumber the number of calendar months, that means at least seven of these 228 months have to harbor two full moons (235 – 228 = 7 extra full moons).
However, if a February within this 19-year period has no full moon at all – as is the case in February 2018 – that means this extra full moon must fall within the boundaries of another month, too. Therefore, the year 2018 actually sports two Blue Moons, giving us a total of 8 Blue-Moon months in the upcoming 19-year Metonic cycle:
1. January 31, 2018
2. March 31, 2018
3. October 31, 2020
4. August 31, 2023
5. May 31, 2026
6. December 31, 2028
7. September 30, 2031
8. July 31, 2034
Bottom line: Enjoy the first of two July 2015 full moons. The second July full moon – the Blue Moon – will come on July 31.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.