by Geoff Chester, USNO Public Affairs | 13 August 2024 Summer Moonrise, Alexandria, Virginia The Moon climbs northward from her extreme southern declination this week, waxing through her gibbous phases as she passes through the dim autumnal constellations. Full Moon occurs on the 19th at 2:26 pm Eastern Daylight Time. August’s Full moon is popularly known as the Sturgeon Moon due to the abundance of lake sturgeon fish congregating in the Great Lakes. These largest species of fresh water fish native to the lakes was an important food source for Native American tribes that lived along the lakeshores. In other cultures it was known as the Grain Moon, Corn Moon, and Lightning Moon. This year’s Sturgeon Moon is also an example of a “seasonal Blue Moon”. During the course of a year, most seasons will have three full Moons, but every couple of years a season will have four. This upset the Christian calendar’s schedule for the Lenten Moon and the Paschal Moon, which were fundamental for determining the date of Easter. The third full Moon in such a season was thus called a Blue Moon. This year, summer has four full Moons, so August’s Sturgeon Moon is also a Blue Moon. The next seasonal Blue Moon will occur on May 20th, 2027. The other type of Blue Moon is the second full Moon to occur in a calendar month. The next one of these will occur on May 31st, 2026. The waxing Moon begins to wash out the subtle glow of the Milky Way, but there are still a number of bright stars to view as Luna increases her brightness. On the evening of the 13th, look just above and to the left of the Moon for the ruddy glow of Antares, the “heart” of Scorpius, the Scorpion. Antares is a red supergiant star about 550 light years away that is approaching the end of its life. It has consumed most of its primordial hydrogen and helium through nuclear fusion, and it is now fusing heavier elements in shells around its core, which is gradually accumulating atoms of iron. These shells have caused the star’s girth to expand to huge proportions; if it were in the Sun’s place in our solar system, the planet Mars would orbit inside of its outer layers. Its end will come in a spectacular supernova explosion sometime in the next few hundred thousand years. High overhead is a bright, pale blue star that’s very different from Antares. This is Vega, the brightest star in the Summer Triangle asterism and fifth brightest star in the night sky. Vega is bright because it is only 25 light years from us, and it is a star that is evolving comfortably through middle age. In 1850 it became the first star to be photographed through a telescope, and in 1983 it was one of the first stars to have a proto-planetary disc surrounding it. It also has a very fast rotation rate, spinning once every 16.3 hours. If we could see it up close, its equatorial diameter would appear almost 20 percent larger than its polar diameter due to extreme centrifugal forces. A similar star to Vega may be found in Altair, the southernmost star in the Summer Triangle. Like Vega, it is a so-called “main sequence” star, in the middle phase of its evolution. It lies just under 17 light years from us, and, also like Vega, rotates very rapidly. It spins once in just 7.7 hours; by contrast the Sun spins once every 27 days! Saturn is steadily making his way into the evening sky. This week the ringed planet rises shortly after 9:00 pm, and by the late evening his yellow glow may be found in the southeastern sky. The planet’s famous rings are currently tipped just over three degrees to our line of sight, so they appear as spikes on either side of the planet through the telescope. They will gradually open to a maximum of just over six degrees in mid-November, then close up to near invisibility when Saturn reaches its equinox on May 6 next year. Mars and Jupiter rise together in the wee hours of the morning of the 14th. The two planets will be very close to each other, just over one quarter of a degree apart, before sunrise that morning. Mars will have passed Old Jove by the following morning, and by the end of the week he’ll be some three degrees east of the giant planet. This will be one of the closest planetary appulses for the year, so you might want to set an alarm.