Thursday 19th Sep 201310:14
The Moon will reach full phase – the fourth full moon of summer 2013, traditionally called a blue moon.
The term blue moon does not refer to any change in the Moon’s color, but is traditionally given to any full moon which is the fourth to fall within the same season – defined astronomically to be the periods between the Earth’s solstices and equinoxes.
The term was used in that context in past times because full moons were given traditional names such as the harvest moon and the hunter’s moon, that ran in sequences through the seasons. Since there were only three traditional names given to the full moons which fell within the span of each season, the fourth full moon was left without a name.
There are, on average, 3.11 full moons within each of the Earth’s seasons, since full moons occur on average once every 29.53 days and the Earth’s seasons last on average a quarter of the year. This means that blue moons occur on average once every 2.7 years.
In modern usage, the term blue moon is often used alternatively to refer to any full moon which is the second to fall within a single calendar month. This usage is a twentieth century innovation which originally seems to have stemmed from a misprint in Sky & Telescope magazine in March 1946.