Blue Moon Friday 31 July 2015

What a great way to end one month and enter a new

Dark Sky Diary

This Friday 31 July 2015 there is an event which happens only “once in a blue moon”. Literally. This month there is a Blue Moon.

The occurrence of a Blue Moon doesn’t mean that the Moon will in fact turn blue. Instead a Blue Moon refers to a second Full Moon occurring within a fixed amount of time.

A full moon A Full Moon, definitely not Blue

There are two widely accepted definitions of a Blue Moon: either it is an additional Full Moon within a season, or an additional Full Moon within a calendar month.

Moon Phases

Normally there are twelve Full Moons in a year, with one occurring every month. In fact the word “month” is derived from “Moon”. However the phases of the Moon don’t cooperate and divide the year perfectly into twelve with no left overs.

The Moon orbits the Earth every 27.32166 days, known as a sidereal month. As it…

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