Blue moons are uncommon to begin with, and the one slated for August 22, 2021, will be extra special. The third full moon of the summer occurs shortly after the planets Jupiter and Saturn reach opposition, and the three celestial bodies will appear in the same region of the night sky. Here's everything you need to know to make the most of the event.
A Blue Sturgeon Moon
The term blue moon has nothing to do with the satellite's color. It's often used to describe the second full moon within a single calendar month, but this meaning comes from an error. Sky and Telescope incorrectly characterized blue moons this way in 1946, and it eventually became an accepted definition.
The original, more obscure meaning of blue moon is the third of four full moons to appear within an astronomical season. The full moon rising on Thursday, August 22, 2021, fits this definition. It will also be a sturgeon moon—the folk name given to the full moon in August.
Saturn and Jupiter at Opposition
August's blue moon closely follows the opposition of Jupiter. A planet reaches opposition when Earth passes directly between that planet and the sun. When this happens, Jupiter will reflect the maximum amount of light from our solar system's star, causing the gas giant to appear especially bright in the night sky. The date of Jupiter's opposition lands on Tuesday, August 19, 2021, but its exceptional brightness will still be apparent on the day of the full blue moon.
Thursday, August 22, is also a great time to spot Saturn. The planet reaches opposition earlier in the month—on August 2, 2021—but it will continue to shine brightly throughout the following weeks. If you want to see Saturn, Jupiter, and the blue moon on the same night, you won't have to look very far.
Saturn comes relatively close the moon on August 20, appearing just 3.8 degrees north of it. On August 22—the date of the actual blue moon—Jupiter will also make a close approach to it. It will come within 4.1 degrees of the moon, also appearing on the top side. All three bodies will look especially bright and close throughout that August week.