The simplest explanation of a Blue Moon is that it is the second full moon in a calendar month. There was a full moon July 2nd, 2015, and a second one July 31st, 2015.

This is a rare occasion. It only happens every 2 to 3 years. The next Blue Moon won’t be seen again until 2018.

The term ‘Blue Moon’ is not actually indicative of the colour of the moon; the colour doesn’t change because of the frequency of the lunar event. To complicate matters, a moon that does look blue IS possible. Usually, you will see what looks like a blue-coloured moon when there has been a volcanic eruption and there is a higher number of particles in the air. These particles often scatter red light and allow blue light to filter through, which is a pretty rare phenomenon.

“The truth is you are more likely to see a red moon versus a blue moon, and especially at this time of year,” said Global News weather specialist Jodi Hughes.

Forest fires also play with light reflectivity, so when the moon is hanging low it can look red because of the aerosols in the atmosphere. This is the same reason why the sunset can look red. The aerosols from a forest fire are much smaller than those from a volcanic eruption so they tend to scatter blue light, allowing the red to shine through and act like a filter for the sun or moon.