Tonight – May 21, 2016 – the Blue Moon will be near red Mars, now exceedingly bright and nearly at its closest to Earth for this two-year period. Wow!
This full Blue Moon and the red planet Mars will shine together all night long, from dusk May 21 until dawn May 22. Of course, the moon is blue in name only. But red Mars really is noticeably red in color, and the full moon and Mars are guaranteed to be spectacular on this date.
Earth will pass between Mars and the sun tomorrow. Astronomers call it an opposition of Mars, because Mars will be opposite the sun, rising at sunset. Likewise a full moon rises at sunset.
And that’s why Mars is near the moon on this full moon night, just one day from Mars’ opposition.
In recent years, the most popular definition of the term Blue Moon has been the second of two full moons in the same calendar month. The May 21 moon is full, but it isn’t the second one in May. Nor is the May 21 moon likely to be blue in color, as that’s caused by exceedingly rare atmospheric conditions.
Rather, the May 21 full moon is the third of four full moons to fall within the same season. To astronomers, a season is 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 March 2016 equinox and June 2016 solstice.
According to skylore, that makes the May 21 full moon a Blue Moon!
This definition preceded the more modern definition of a Blue Moon – the one you might know better – as the second of two full moons to occur in one calendar month. The older definition is likely to enjoy a revival of sorts, in this the age of the internet.
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, yet 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.
The May 21 moon turns full today at 21:14 Universal Time. Although the full moon happens at the same moment worldwide, our clocks reads differently by timezone. The moon turns full in the United States on Saturday, May 21, at 5:14 p.m. EDT, 4:14 p.m. CDT, 3:14 p.m MDT or 2:14 p.m. PDT. Astronomically speaking, the moon is full when it’s most directly opposite the sun for the month.
Bottom line: Celebrate the May 21 Blue Moon – the third of four full moons to take place in between the March 2016 equinox and June 2016 solstice! The bright red light shine near the moon on this date is the planet Mars, now just one day away from its opposition.
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.