The saying "once in a blue moon" is especially pertinent this week: This Sunday (Aug. 22), the full Sturgeon Moon is expected to impress skygazers, particularly because of its "blue" designation.
Typically, the term "Blue Moon" refers to the second full moon within the same month. The last one rose on Oct. 31, 2020, when an eerie Blue Moon lit up the night sky on Halloween. But there's a lesser-known definition, dating to 1528, which applies to the third full moon in a season with four full moons, according to NASA.
In general, each season has three full moons. But summer 2021, which began June 20 and ends Sept. 22, has four full moons (June 24, July 23, Aug. 22 and Sept. 20). Seasonal Blue Moons are uncommon, occurring about once every two to three years, according to EarthSky. The last one rose on May 18, 2019, and the next one won't shine until Aug. 19, 2024.
This weekend's moon will reach its fullest at 8:02 a.m. EDT on Sunday (1202 GMT), but it will appear full for about three days, from Friday night (Aug. 20) through Monday morning (Aug. 23), making it a "full moon weekend," NASA reported. To find your local moonrise and moonset times, go to timeanddate.com.
August's full moon won't actually look blue, however; unless smoke particles from this summer's raging fires turn it orange-red, the moon will appear its usual ghostly white. However, it is possible for a full moon to appear blue. This can happen when particles high in the atmosphere, such as those from a powerful volcanic eruption, scatter light to make the moon look blue from Earth.
August's full moon is known as the Sturgeon Moon, according to the Maine Farmers' Almanac, which first published the names of the full moons in the 1930s, NASA reported. The Algonquin tribes in North America reportedly called this the Sturgeon Moon because these fish were plentiful in the Great Lakes and other waterways this time of year, the almanac reported. Another Algonquin name for this moon is the Green Corn Moon.
Other events associated with August's full moon include the Hindu festival Raksha Bandhan, which celebrates the bond between brothers and sisters; the Nikini Poya holiday in Sri Lanka, which commemorates the first Buddhist council 2,400 years ago; and the Hungry Ghost Moon Festival in China, when ghosts and spirits, including those of deceased family members, are thought to visit the living, NASA reported.
Skywatchers, especially those with backyard telescopes, will also be able to see Jupiter and Saturn. Both planets will look unusually bright (Saturn was at its closest to Earth on Aug. 2, and Jupiter was at its closest Thursday, Aug. 19) and will appear to move westward in the evening sky, according to NASA. A telescope will help you see Jupiter's four largest moons (Ganymede, Callisto, Europa and Io), as well as Saturn's rings and its largest moon, Titan.
Originally published on Live Science.