Why Saturday's full moon will be "blue"

GREAT RAPIDS, Mich. (WOOD) – Our next full moon, visible on Saturday, will be a blue moon – but maybe not as you originally thought.

Astronomy enthusiasts probably know this definition of the Blue Moon: the second full moon of the calendar month. However, there is another less known definition of the blue moon. Every two or three years, it is possible to see four full moons in a season, the third of which is called a blue moon.

Here's how it works: each year has four seasons and each season lasts about three months. The time between full moons is close to that of a calendar month, but this is not correct. The shift can allow us to see two full moons the same month or four full moons between an equinox and a solstice.

The vernal equinox fell on March 20th of this year and we had the full moon on March 21st. The second full moon of the season was April 19th. The full moon on Saturday will be the third, or the blue moon. Our last moon of the season will be on June 17, just days before the June 21 summer solstice.

Blue moons are not common, but not as rare as the phrase "once in a blue moon" might imply. There were two monthly blue moons in 2018, one in January and one in March. The seasonal blue season Saturday will be the fourth of the last decade. The three precedents were November 21, 2010; August 20-21, 2013; and May 21, 2016. Our next seasonal blue moon will come on August 22, 2021.

It is much less likely to see only two full moons in one season. It was the last time in 1961-1962, when there was a full moon just before the winter solstice and a full moon just past the vernal equinox, leaving only two full moons during all the winter season. The next time this phenomenon occurs will be the winter of 2314-2315.

In addition to the blue moon that we will see this year, there are several other names for the full moon in May. It has been called the moon of flowers, the moon of milk or even the moon of corn plantation.

And despite what his name might suggest, the moon will not take any kind of blue color. It is possible that the moon appears blue, but only when certain atmospheric conditions are present. Volcanic eruptions and forest fires can release enough particles into the atmosphere to disperse the red light. Once the red light is scattered, the blue light can filter and give the moon a blue hue.

The moon will be officially full at 17:11. Saturday.