This particular full moon is a blue moon because it is the third of four full moons in a season, as defined from solstice to equinox or vice versa.
This week, the giant planet Jupiter reaches opposition, just days before a blue moon cruise past it.
Opposition is when an outer planet lies 180 degrees away from the Sun, placing it directly opposite in the sky. This puts Earth directly in between, bringing our world as close as it can go to the outer planet.
On 20 August, Jupiter finds itself at opposition. This is not rare, it happens roughly every 13 months as Earth laps Jupiter. Earlier in the month, on 3 August, Saturn was at opposition.
Because Saturn is further from the Sun and travels more slowly in its orbit than Jupiter, Earth laps it roughly every 12.5 months. This year, these oppositions take place in the same month because the planets happen to be in the same part of the sky, having come together for the great conjunction of December 2020.
The chart shows the view looking south at 01:00 BST on 22 August, when the two bright planets will be joined by a full moon. This particular full moon is a blue moon because it is the third of four full moons in a season, as defined from solstice to equinox or vice versa. This summer’s full moons on 24 June, 24 July, 22 August and 21 September. From Sydney, Australia, the conjunction takes place high in the north-north-west.
Originally published at The guardian