Blue Moon this Sunday won't be blue - but sometimes they really are

A blue moon can refer to two different full-moon timing definitions. A calendar blue moon is the second full moon occurring within a single calendar month. A seasonal blue moon is the third full moon within a season that contains four full moons. On 31 May, a calendar blue moon will occur, with no special color change expected. A moon can appear blue to human observers when volcanic eruptions inject ash into the atmosphere. In 1883, the Krakatoa eruption was extremely violent, and scientists compared its energy to a 100 to 200-megaton nuclear bomb. Volcanic ash particles can scatter red light, shifting transmitted light toward the blue end of the spectrum, making the moon appear blue at night when it is the main light source.

"There are in fact, two types of blue moon: Calendar Blue Moon: The second full moon in a single calendar month. Seasonal Blue Moon: The third full moon in a season that has four full moons. On Sunday 31st May, we will see a calendar blue moon. However, as the moon has no idea that some tiny creatures on the surface of the planet it orbits have calendars and give nice names to things, it won't do anything special."

"But very occasionally, the moon actually turns blue, but only from the point of view of humans looking up. A spacecraft full of aliens would have looked at the very grey moon and wondered what the earthlings are getting so excited about. It's all because of plate tectonics. The slow grinding of the planet's plates occasionally gives the planet an outbreak of zits - although we call them volcanoes, and every so often one of them pops."

"In 1883, the Indonesian volcano Krakatoa erupted so violently that scientists later compared it to a 100 to 200-megaton nuclear bomb. As it happens, in that wonderful way that the god of science plays with us, volcanic ash particles can be just about the correct size to scatter red light in the atmosphere. As a result, any light passing through the atmosphere would be filtered towards the blue end of the spectrum."

"Barely noticeable during the daytime, the effect becomes much more pronounced at night, when the moon is the primary source of illumination. And it would be a moon that is blue. Blue moons, as in the calendar version, don't happen every year, and sadly, with Krakatoa popping its top in August 1883, there wasn't a calendar blue moon until 1885 - so there wasn't a Blue Blue Moon."

Read at ianVisits