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

The August blue supermoon is seen over Los Angeles Thursday, Aug. 31, 2023. (AP Photo/Marcio Jose Sanchez)

The phrase "once in a blue moon" is going to ring true as a super blue moon is set to 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 difference between a supermoon and blue moon

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.

This summer season, there are four full moons. Two passed, one on June 21 and the other on July 21, and after Monday, the remaining full moon is Sept. 17, Sivanandam said.

Ilana MacDonald, public outreach communications strategist for the Dunlap Institute for Astronomy and Astrophysics, told CTV News Toronto a blue moon is when there are two full moons in the same month. According to NASA, the term "blue moon" has been used to describe the monthly phenomenon rather than the seasonal one since the 1940s.

So, this upcoming blue moon is a seasonal one rather than a monthly blue moon.

The next time stargazers can see a super blue moon won't be until 2037, Sivanadam said. Macdonald noted the next blue moon will be visible on May 31, 2026.

How to see it in Toronto

"Anywhere you can see the southern sky," MacDonald said. "Like everything, the moon rises in the east and sets in the west – at least if you're in the northern hemisphere – and it would be going across the southern part of the sky."

For those who want to look at it earlier in the night, MacDonald recommends going somewhere with a view of the eastern sky, and if it's around midnight, the moon will be at the highest point due south.