Skywatchers, stargazers, and astrophotographers will have an oppor-two-nity to catch stunning full supermoon events in the next month, a supersized version of the rare double-header known as a blue moon.

August will mark the first and only blue supermoon of the 2020s, following the most recent double-supermoon back in 2018. After the coming blue supermoon, it will be another 14 years before two of these oversized full moons grace the sky in the same month, next expected in 2037.

Despite common idiomatic use to describe rarity, blue moons (at least the normal-sized ones) actually happen pretty frequently, depending on your preferred definition of the term.

NASA describes them seasonally, referring to the third full moon in a season, an event occurring on average every two to three years. A less rare colloquial definition that has since been adopted as fact refers to blue moons as the second full moon within a month.

However, August's blue supermoon will be far rarer than either of those definitions.

The first of the two full supermoons will arrive on Tuesday, when the Moon will rise over the southeast horizon, appearing just a bit larger than usual.

The Moon typically averages a distance of 384,400 kilometres from the Earth, however, it will be almost a full 30,000 kilometres closer on Tuesday, at just 357,530 kilometres, making viewing and photography of the celestial body slightly easier.

Viewers will get an even better look on August 30, when the moon's orbit will bring it to a distance of just 357,344 kilometres.

The phenomenon's descriptive name is often mistaken for a literal meaning, and anyone expecting a blue-tinted moon this month might end up disappointed at the ordinary not-at-all-blue colour during the lunar spectacle.

Overcast skies may hamper viewing attempts for the first of the two supermoon events on Tuesday, Aug. 1, though skywatchers will still have a second chance for the next full supermoon a few weeks later, on Aug. 30.