Sky Matters: Astronomical events that come along ‘once in a blue moon’

From meteor storms to exploding stars or supernovae, there are a myriad of rare or once-in-a-lifetime events that take place in the universe all around us... but the rarest and most unique event of all is us

The blue moon refers to the second full moon in one calendar month, which occurs approximately once every two or three years.

The term “once in a blue moon” refers to something which happens rarely. In May we have a very specific “blue moon” on 31st. In astronomical terms a blue moon refers to the second full moon in a calendar month. These happen every 2-3 years and they occur because the time for the Moon to orbit the Earth is 29.5 days whilst the lengths of our months is usually 30 or 31 days (with February and leap years being the obvious exceptions). If the first full moon is within the first day or two of a calendar month, that makes it possible for the next full moon – some 29.5 days later – to also appear in that calendar month.

The term “blue moon” does not refer to the colour of the moon. Indeed a full moon close to the horizon can appear blood red, getting progressively more grey-white as its altitude increases. The only time the moon might take on a blueish hue is after certain volcanic eruptions, such as Krakatoa or Mount Saint Helens. This is due to fine dust particles ejected into the upper atmosphere scattering the red light away from us, leaving only the blue light, which is the opposite of what happens normally.