Heads up — and enjoy. The full moon on Saturday will appear to be unusually big. In fact, it will be a supermoon, the first of three to adorn the sky this year.

Full moons vary in size because of the oval shape of the moon's orbit. The moon follows an elliptical path around Earth with one side ("perigee") about 30,000 miles closer than the other ("apogee").

A supermoon results when there's a full moon as the satellite makes its closest approach to the Earth.

A "perigee moon," as it is technically called, shows the largest apparent size of the lunar disk as seen from Earth.

Two other full moons this summer, on Aug. 10 and Sept. 9, are also supermoons. The one on Aug. 10 will be the biggest of the year, as the moon will be closest to Earth on that day.

It's not all that unusual to have a supermoon. There were three in a row last year. "Generally speaking, full Moons occur near perigee every 13 months and 18 days, so it's not all that unusual," said Geoff Chester of the U.S. Naval Observatory as quoted by NASA on its website

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

The best time to look at the full moon, experts say, is when it's hanging low, near the horizon.

The effects on Earth from a supermoon are minor. According to the most detailed studies, NASA explains, it should not affect the internal energy balance of the Earth since there are lunar tides every day.

The Associated Press contributed to this report.

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