This year's only Blue Moon is coming up this weekend, but this is not an ordinary blue moon that many know. Typically, Blue Moon is the name given to the second full moon of the month. However, this Sunday, August 22, the Blue Moon is the first and only full moon for the month.
So why is it also called the Blue Moon of 2021?
Blue Moon 2021 Follows the Older Definition
The notion of Blue Moon can be traced back hundreds of years ago. In his article "Once in a Blue Moon," Folklorist Philip Hiscock said that the calendrical term for this rare full moon could be traced back to the 1937 issue of Maine Farmer's Almanac.
However, Sky & Telescope magazine said that the almanac did not use the popular definition of Blue Moon as the "second full moon in a month." So, where did that popular definition came from that people use nowadays?
Although it might not be known, Space.com reported that the definition for August 2021's Blue Moon follows the old definition. The almanac says that a year typically has 12 full moons with three full moons for each season. But there will come a time when a year will have 13 full moons, which means one season will get four full moons.
"As the third full moon in a season that has four full moons, this will be a blue moon by the older definition," Gordon Johnston wrote in NASA's full-moon guide for August and September.
This was considered an unfortunate event; hence the number 13 is considered unlucky, according to Space.com. As there will be a fourth full moon in one season of the year, the third full moon is considered the Blue Moon, so that the fourth and final one is still called the "late moon."
Blue Moon Will Not Be Blue
Despite its name, the full moon this weekend will not be blue. The almanac said that the term Blue Moon denotes it as a rare event.
According to Forbes, the moon will look like it always does, although it will look orange for 15 minutes as it rises in the east. Then it will slowly turn to yellow as it rises higher into the sky before turning into its typical whitish-gray color.
The color-changing event can be explained by Rayleigh's scattering, which works like how the sky looks blue in daylight. The atmosphere of Earth contains oxygen and nitrogen molecules that absorb the wavelength of light.
Shorter wavelengths from the sunlight, particularly the blue color, strike more particles in the atmosphere and are absorbed. They scatter easily, making the sky blue during the day. Meanwhile, orange is a longer wavelength and easily passes through the atmosphere without being scattered.
So, when looking at the moonrise, Rayleigh's scattering is intensified with the orange dominating light that the eyes perceive because they have to look through many atmospheres.
Check out more news and information on Moon in Science Times.