The full moon happening on August 22 will be a blue moon. This may surprise many as a blue moon is defined as the second full moon that occurs in a month, but the full moon this weekend is the only full moon this month.

Still, it's a blue moon due to the rule mentioned in Maine Farmer's Almanac. Full moons usually happen 12 times in a year—three times for each season—but sometimes, there will be 13 full moons.

This means that one of the four seasons will have an extra full moon—the third full moon is called a blue moon while the fourth one is called a late moon.

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Going by the rule, the summer of 2021 has four full moons: June 24, July 23, August 22 and September 20. In Hong Kong, the best time to see the blue moon is 8:02 pm when it's at its peak. But don't expect the moon to look blue as moons only appear with a bluish tint under very unusual circumstances such as after the eruption of Krakatoa in August 1883.

The blue moon this weekend will look familiar as any other full moon, perhaps only a little bigger or "fuller".

But the best thing about going blue moon-gazing is that you don't need any special equipment to see them as you might for other celestial phenomenons. It's best to head over to a place where the sky is clear and not obstructed by buildings or pollution. Don't miss your chance to spot this rare phenomenon because it only happens once in a blue moon.

See also: Astronomy 2021: Full Moons, Eclipses And Meteor Showers In Hong Kong