The Sky Today on Monday, September 28: Open cluster M39 shines

The bright, young star cluster M39 in Cygnus is a great target for any observer tonight, easy to catch in the eyepiece. | Published: September 28, 2026 Looking for a sky event this week? Check out our full Sky This Week column. September 27: Catch Saturn’s satellites Although the Moon still dominates much of the night, bright deep-sky objects far from our satellite’s location remain well within reach. A few hours after sunset, M39 in Cygnus is some 80° high in the north, perfect for viewing with any optics you’ve got on hand. M39 is an open cluster of young stars located about 9.2° northeast of Deneb, the Swan’s bright alpha star and one of the three points in the famous Summer Triangle asterism. Spanning roughly the size of the Full Moon, M39 is best viewed under lower magnification, such as with small binoculars or a beginner telescope. This is what makes it such a great target, especially for those just learning to use their equipment to find distant objects in the sky. In all, the cluster contains roughly 30 stars and shines around 5th magnitude — capable of being seen with the naked eye, albeit not on a night with the Moon shedding bright light across the sky. This open cluster is quite close to us, with an estimated distance of less than 1,000 light-years. Its members are between 230 million and 300 million years old. Sunrise: 6:53 A.M.Sunset: 6:47 P.M.Moonrise: 7:34 P.M.Moonset: 9:08 A.M.Moon Phase: Waning gibbous (93%) *Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location. Jupiter’s largest moon, Ganymede, transits the gas giant’s disk the morning of the 29th. Some part of the event, which lasts more than three hours in total, is visible for all U.S. observers. Early on September 29, Ganymede is crossing Jupiter’s disk as Io approaches the planet from the west; the smaller moon will pass into the gas giant’s shadow around 5:45 A.M. CDT. Credit: Stellarium/James Hastings-Trew/NASA Cassini/Oleg Pluton Jupiter is currently in Leo, rising around 3 A.M. local daylight time. The bright planet shines at magnitude –1.9, outshining 1st-magnitude Regulus, the Lion’s brightest star, which lies to Jupiter’s east and follows it up into the eastern sky. Through a telescope, the gas giant’s disk spans 33”. For those in the eastern half of the U.S., the planet is flanked by two moons on each side as it rises: Callisto and Ganymede on the east, and Io and Europa on the west. Ganymede is closing in on the eastern limb and its transit begins at 5:24 A.M. EDT. It continues across the disk as the hours pass and the sky brightens, until those in the western portion of the country see Ganymede finally exit the disk at 6:02 A.M. PDT. But Ganymede isn’t the only moon to watch: Not long before Ganymede reaches a halfway point across the disk, Io winks out of view on Jupiter’s western side as it passes into the planet’s long, dark shadow around 5:45 A.M. CDT (just before or around sunrise on the East Coast). It is occulted by the planet about an hour later, and won’t reappear until after sunrise on the West Coast. Alison Klesman is senior editor of Astronomy magazine. She holds a Ph.D. in astronomy and has studied a variety of topics, from minor planets to supermassive black holes.