First of two July full moons on July 1

The first of two July 2015 full moons comes tonight. The second July full moon – aka a Blue Moon – will come on July 31.

July 2015 has two full moons. That’s somewhat unusual. Most months only have one. But in cycles of 19 years, or 228 calendar months, seven to eight calendar months will always have two full moons. In other words, there’s a month with two full moons every two to three years. When it happens, the second one is popularly called a Blue Moon.

To learn the history of Blue Moons, click here

The Blue Moon of July 31 and the 19-year Metonic cycle

Image on top by EarthSky Facebook friend VegaStar Carpentier in Paris.

Best photos: Venus and Jupiter, west after sunset

The first full moon of July falls on July 2 at 2:20 Universal Time (July 1 at 10:20 p.m. EDT, 9:20 p.m. CDT, 8:20 p.m. MDT pr 7:20 p.m. PDT). Although the full moon occurs at the same instant worldwide, our clocks read differently according to our local time zones.

The second July full moon will fall on July 31 at 10:43 Universal Time (5:43 a.m. CDT in the central U.S.). This second full moon is the Blue Moon.

By recent popular acclaim, the second of two full moons in a single calendar month goes by the name of Blue Moon. According to folklore, there are other definitions for Blue Moon. A Blue Moon can also be the third of four full moons in a season. But the second-full-moon-in-a-month definition is the easier to remember, and it’s probably what most people think of when they hear Blue Moon. Once again, the Blue Moon – the second full moon of July 2015 – will come on July 31 at 10:43 Universal Time.

Day and night sides of Earth at instant of first July full moon

Day and night sides of Earth at the instant of the first July 2015 full moon (2015 July 2 at 2:20 Universal Time). Image credit: Earth and Moon Viewer

Day and night sides of Earth at the instant of the first July 2015 full moon (2015 July 2 at 2:20 Universal Time). Image credit: Earth and Moon Viewer

After July 2015, we will see two full moons in a single calendar month again in January 2018. There will be no full moon in February 2018, and then two full moons in March 2018.

If we exclude February, each calendar month is either 30 or 31 days long. On the other hand, the time period between full moons varies from about 29.3 to 29.8 days. So if the full moon comes very early in the month – as it does in July 2015 – that leaves enough time for another full moon to beat out the clock before the month’s end.

February is the only month that doesn’t have enough room for two full moons, and it’s the only month where it’s possible to have no full moon at all. In years where February has no full moon – such as in 2018, and 19 years later in 2037 – January and March both count as Blue-Moon months.

In other words, 2018 and 2037 will feature years of double Blue Moons.

Bottom line: July 2015 will have two full moons. The first one takes place on July 2 at 2:20 Universal Time – that’s on July 1, at 10:20 p.m. EDT, 9:20 p.m. CDT, 8:20 p.m. MDT or 7:20 p.m. PDT according to U.S. clocks. The second full moon of July – on July 31, 2015 – will be called a Blue Moon. Watch for the first of the month’s two full moons tonight. This lovely July full moon will light up the nighttime from dusk until dawn!

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Bruce McClure

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