Seasonal Blue Moon on May 18, 2019

These next several nights – May 17, 18 and 19, 2019 – the moon will probably look plenty full to your eye, and some people will call this full moon a Blue Moon. It is being called a Blue Moon because it’s the third of four full moons to occur in one season. We’ll elaborate on this type of Blue Moon later in our post.

The star Antares and the planet Jupiter follow the moon into the sky by early to mid evening. They should be fairly easy see, despite the lunar glare.

For the fun of it, we show the dwarf planet Ceres which is not visible to the naked eye. It helps to have a moonless night, a good sky chart and binoculars to see this world, which is a good two times fainter than the faintest star visible to the eye alone on a dark night. Less than one day after the moon turns full, the moon will occult this world but you’d have to be in Antarctica to witness it.

To astronomers, the moon is truly full at a precisely defined instant – when the moon is exactly 180 degrees opposite the sun in ecliptic longitude. That full moon instant falls on May 18 at 21:11 Universal Time.

Worldwide map of the day and night sides of Earth at full moon

The day and night sides of Earth at the instant of full moon (2019 May 18 at 21:11 Universal Time). The shadow line at left (running through South America) depicts sunset (moonrise) May 18 and the the shadow line at right (running through eastern Asia and Australia) depicts sunrise (moonset) May 19. Worldwide map via EarthView.

At Canadian and US time zones, that means the moon turns full on May 18 at 6:11 p.m. Atlantic Time, 5:11 p.m. Eastern Time, 4:11 p.m. Central Time, 3:11 p.m. Mountain Time, 2:11 p.m. Pacific Time, 1:11 p.m. Alaskan Time and 11:11 a.m. Hawaiian Time. The full moon happens at the same moment for all of us worldwide, yet the hour varies by time zone. In North America, the moon turns full during the daylight hours on May 18, when the moon is still below the horizon.

For all of us around the world, the moon stays more or less opposite the sun for the next couple of nights. Look for the moon to shine in the east at dusk, climb highest up for the night around midnight and to adorn the western sky at dawn. In other words, look for a bright full-looking moon to light up the nighttime for the next few days.

What makes the May 18th full moon a Blue Moon?

Some of you may be familiar with the more popular monthly definition of the Blue Moon as the second of two full moons in a single calendar month. Believe it or not, the seasonal definition of Blue Moon as the third of four full moons in one season is older than the monthly definition of the term. A season is the period of time between an equinox and a solstice – or vice versa.

Some people prefer the seasonal definition of Blue Moon because it’s based on natural phenomena (equinoxes and solstices) instead of an artificial constraint (calendar month). Normally, a season only has three full moons, but the present season actually has four.

Notice the dates below. You can see that the first full moon of the season came soon enough after the March 2019 equinox to allow for a fourth full moon to take place before the June 2019 solstice:

Equinox: March 20, 2019

Full moon: Mar 21, 2019
Full moon: Apr 19, 2019
Full Moon: May 18, 2019 (Blue Moon)
Full moon: Jun 17, 2019

Solstice: June 21, 2019

Possible to have only two full moons in a single season?

In a period of 19 years, there are 235 full moons but only 76 seasons. If each season has exactly three full moons, then 76 seasons would harbor 228 full moons (76 x 3 = 228 full moons). Yet there are an extra 7 full moons (235 – 228 = 7 extra full moons) in this 19-year period, each of which has to fall within the framework of another season. In other words, it’s inevitable that 7 out of the next 76 seasons should harbor four full moons.

The 7 seasonal Blue Moons in the next 19-year lunar cycle:

1) August 22, 2021
2) August 19, 2024
3) May 20, 2027
4) August 24, 2029
5) August 21, 2032
6) May 22, 2035
7) May 18, 2038

In North America, we often call the May full moon the Flower Moon, Milk Moon or Planting Moon. But his year, in 2019, it’ll also be a seasonal Blue Moon – the third of four full moons to occur after the March 2019 equinox and before the June 2019 solstice.

Bruce McClure