Skywatchers will be treated to the first of the three summer supermoons this Saturday, according to NASA.

The "perigee moon," its scientific name, occurs because the moon differs in size based on where it's located in its orbit around Earth.

One side of the orbit, the "perigee," is approximately 50,000 kilometres closer to Earth than the other side, which is known as the "apogee." Therefore, on the perigee side, it appears much larger.

This phenomenon will occur July 12, August 10 and September 9. The August moon may appear particularly big, as it will become full in the same hour as perigee, according to NASA.

The supermoon isn't nearly as rare as it is spectacular, Geoff Chester of the U.S. Naval Observatory said in a news release.

"Generally speaking, full moons occur near perigee every 13 months and 18 days, so it's not all that unusual," he said.

"In fact, just last year there were three perigee moons in a row, but only one was widely reported."

Saturday's supermoon should be most visible in Vancouver, Calgary and Halifax, where the Weather Network projects sunny weather with clear skies.

It's uncertain how visible it will be in Toronto and Montreal, as the forecast calls for some clouds on Saturday and rain on Sunday in both cities.

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