Have you ever heard the saying once in a blue moon? The full moon coming up this weekend is the reason for the phrase. A blue moon happens about every three years. This is the reason behind the phrase. You may know the definition of a Blue Moon as the 2nd of two full moons in a month but there’s another kind of Blue Moon which is what is coming up on May 18, 2019. In recent years, people have been using the name Blue Moon for another type of moon which is the third of four full moons in a single season.
There are usually only three full moons in each season of the year. However, since full moons occur every 29.53 days, on occasion a season will contain 4 full moons. This extra full moon is referred to as a blue moon.
The astronomical season started with the equinox on March 20, and the first Full Moon was less than four hours later March 21. We had the second Full Moon of the season on April 19, and the third, which will be the Blue Moon, which will occur on May 18th. The fourth and final full moon of the season will occur before the June solstice and will arrive on June 17th.
The Blue Moon will most likely not appear blue. A Moon that looks blue is very rare. The Moon, whether it is full or in any phase can appear blue but depends on what particles are in the atmosphere. When the atmosphere is filled with dust or smoke particles of a specific size, these particles scatter the red light which make the Moon appear blue. According to astronomers this can happen due to dust storms, forest fires, or a volcanic eruption.