I find it fascinating that we have developed a sensationalized vocabulary about naturally occurring events. Perhaps it is the result of reality television becoming so prevalent that everything must be a challenge, a contest, the best, the brightest, the most shocking, etc. Having the recent lunar eclipse labelled with 3 different descriptors, super moon, blue moon, and blood moon, made for some interesting headlines. Are these events really deserving of these labels?
The distance between the moon and the earth does change throughout the month due to the moon’s elliptical orbit. For a viewer on earth, the apparent change in the moon’s diameter between apogee and perigee is about 13%. That does make a full moon around the perigee appear larger thus potentially brighter. But do most of us really notice without being told? Probably not. As the earth has a greatly varying atmosphere, which has a significant influence on light transmission, the amount of light from a full moon is not a reliable indicator. How many of us can recognize that a full moon is larger, or smaller, than the previous time we saw it? Photographs will show a difference but most of us do not have that sort of visual memory. Is a moon that appears to be 13% larger than at its smallest apparent diameter a difference worthy of being called a super moon? I lean toward describing it as a full moon near the perigee.
According to sources I have found, the term blue moon, when used to count full moons in a certain time period, was originally used for seasons. Most seasons have three full moons but every tenth season or so has a fourth full moon. The blue moon was the label given to the third full moon of a season with four full moons. Along the way, blue moon has become an accepted expression for the second full moon in a single calendar month. Is having an extra full moon in a month or a season, both of which are arbitrary, human-designated periods of time, significant? Why not just refer to it as the second full moon of January? By the way, March 2018 also has 2 full moons so we will have a second blue moon this year.
The term blood moon is used to describe the red-coloured moon that we see during a lunar eclipse. During the eclipse, the sun’s light is blocked from reaching the moon’s surface directly. The earth’s atmosphere scatters light, particularly in the blue-violet end of the spectrum. This means that light that has passed through the earth’s atmosphere and travels on toward the moon is primarily at the red end of the spectrum. Some of this light will end up reaching the moon’s surface and the result is a moon illuminated by light that is strongly in the red wavelengths. I’m not sure why we just don’t call it a red moon.
So, although we did not see much of the eclipse here, I hope those who did enjoyed seeing the large, red moon caused by the lunar eclipse during the second full moon of January 2018 when the moon was close to perigee. Ok, maybe saying the super blue blood moon sounds better!