Super Moons & Blue Moons

There appears to be quite a bit of fascination by the general public with the types of Moons mentioned in our title. So it seems appropriate to examine what “super” and “blue” actually mean in describing our lovely “Queen of the Night.” And it’s not quite what is implied in either of these cases!

A “Super Moon” is defined as when our lovely satellite is at either its Full or New phases and closest to us. Of course, when the Moon is new it isn’t visible in the sky, so discussions of a super Moon revolve about its appearance when Full. But it isn’t a simple matter of the Moon being closer to the Earth every month at Full Moon due to the fact that its orbit about us is not circular but rather elliptical. Its average distance of about 239,000 miles varies from its furthest point (known as apogee) of 252,000 miles to some 226,000 miles when closest (known as perigee). When at the latter point in its orbit, it appears 14% bigger in angular size than when it’s furthest from us and as a result does look much bigger to the eye. When this happens, we have a Super Moon! Looking ahead, the next ones occur on Monday, March 9th, and then again on Wednesday, April 8th. Following these, there will be no more until October and November.

However, these will occur when the Moon is new and not visible. So the next two months are your last opportunities see a super-sized Moon this year!

And now to the much-misunderstood subject of a “Blue Moon.” By definition, a blue Moon is the occurrence of two Full Moons in a given month—technically it’s the second Full Moon that’s considered “blue.” For this to happen the first Full Moon has to occur right at the beginning of the month and the month has to be long enough to accommodate the roughly 29 day period between these phases. This can be quite rare: there are actually years when there are no Blue Moons at all. (Thus the expression “once in a blue Moon.”) The next (and only) opportunity this year is on October 31st.

Can the Moon actually look blue in the sky? We know that it displays a variety of red and orange and copper colors during a lunar eclipse. And there are some very rare reports of it actually appearing greenish (thus the well-known joke about the Moon being made of “green cheese) and also bluish. These have occurred during volcanic outbursts like Krakotao 1n 1883, Mt. St. Helens in 1980, and Mt. Pinatubo in 1991. And forest fires have also been involved in some of these reports. In any case, these have certainly been “once in a Blue Moon” events!

— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on
stargazing. His latest, Celebrating the Universe!, is available from