What is a Blue Moon?
- Written by Paul Fish
A blue moon is the second full moon in a calendar month. In 2001, there were two full moons in November, and the full moon of November 30 was the Blue Moon.
The older definition referred to the blue moon as the 4th full moon in a season. Each of the four seasons of the year has three months, usually containing three full moons. Each of these 12 moons has a name like “Harvest Moon or Hunter’s Moon” and so on. When a season occurs that contains four full moons, there is no name for this occasion, so it was given the name “BLUE MOON”.
How Does the Moon Turn Blue?
The Moon won’t actually
change colour, despite the
name. However, there have been occasions when the Moon has appeared to be blue in hue. This isn’t an astronomical phenomenon.
Instead, dust or smoke high in the Earth’s atmosphere causes it. The dust is thrown up by major volcanic eruptions such as Krakatoa, Mount St. Helen’s or Mount Pinatubo, whilst smoke can come from large forest fires.
Whether it’s dust or smoke,
the tiny particles have a strange effect on the moonlight (or sunlight) passing through them. They scatter the light in every direction, but red light is
scattered more strongly than blue light, so that less red light passes directly through the dust or smoke. Thus, the Moon has a blue tinge.
Because of the unusual
size distribution of the dust particles, this scattering works in the opposite sense to the normal scattering by dust low
on the horizon which makes the Sun appear deep red in colour as it rises or sets.
How often does a Blue Moon occur? Once in a Blue Moon is a common way of saying not very often, but what exactly is a Blue Moon? It is the second Full Moon to occur in a single calendar month.
The average interval between Full Moons is about 29.5 days, whilst the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.
On average, there will be 41 months that have two Full Moons in every century, so you could say, “that once in a Blue Moon actually means once every two-and-a-half years.”
Blue moons are not all that rare. Over the next twenty years, there will be 17 blue moons. No blue moons will occur at all in the years 2014, and 2017.The more recent occurrence of a blue moon happening was November 30 2001 and then not again until July 2004.
Owls are the only birds who can see the colour blue.
