What exactly is a 'super blue moon'? Here's when to see it in Toronto

The phrase 'once in a blue moon' is going to ring true as a super blue moon is set illuminate Toronto's night skies starting Monday. The super blue moon will be full for about three days, according to NASA, but what exactly is it?

The phrase "once in a blue moon" is going to ring true as a super blue moon is set illuminate Toronto's night skies starting Monday.

The super blue moon will be full for about three days, according to NASA, but what exactly is it?

Suresh Sivanandam, associate professor at the University of Toronto and director of the Dunlap Institute for Astronomy and Astrophysics, explained to CTV News Toronto that the term supermoon highlights the closeness of the moon to the Earth.

"Everyone thinks the moon's orbit is circular and it's always the same distance away from the Earth, but it's actually slightly elliptical and sometimes the moon is closer to the Earth and sometimes it's a little bit further away," Sivanandam said. "The supermoon happens when the moon is close to the Earth and it also happens to be a full moon."

This makes the full moon look about five to 10 per cent bigger and about 20 per cent brighter than normal, Sivanandam adds.

A blue moon does not mean it will have a blueish hue, but rather how many full moons there are in a month or season.

"You should have one full moon a month and in a season, you have three months, you should have three full moons," Sivanandam said. Since the lunar cycle is 29.5 days, he adds, occasionally, there can be four full moons in a season, but the third one is known as the blue moon.