Skywatchers across the United States can view a rare celestial alignment on September 29, when Jupiter’s largest moon, Ganymede, transits the face of the gas giant while neighboring moon Io slips into the planet’s shadow. The multi-hour event highlights a busy week for planetary observation as Jupiter shines brightly in the constellation Leo.
Ganymede Transit Visible Across the United States
Jupiter rises around 3 a.m. local daylight time, shining at a bright magnitude of –1.9 and outshining the nearby 1st-magnitude star Regulus. Through a telescope, the gas giant’s disk spans 33 arcseconds. Observers in the eastern half of the U.S. will see Callisto and Ganymede flanking the east side of the planet, with Io and Europa on the west.
Ganymede’s transit across the disk begins at 5:24 a.m. EDT as it closes in on the eastern limb. The event lasts for more than three hours in total, allowing viewers in the western portion of the country to watch Ganymede finally exit the disk at 6:02 a.m. PDT as the morning sky brightens.
Io Enters Jupiter’s Shadow During the Transit
While Ganymede crosses the disk, observers can track a second major moon event. Io approaches Jupiter from the west and passes into the planet’s long, dark shadow around 5:45 a.m. CDT, just before sunrise on the East Coast. Alison Klesman, senior editor of Astronomy magazine, notes that Io winks out of view as it enters the shadow, becoming occulted by the planet about an hour later. Io remains hidden until after sunrise on the West Coast.
Observing Saturn’s Moon Titan This Week
Beyond the Jupiter events, Saturn offers its own satellite pairing. Saturn’s largest moon, Titan, sits just to the southwest of the ringed planet, providing a clear viewing opportunity for backyard astronomers. The following night, Titan shifts to the southeast of Saturn, making it easy to spot both bodies together through standard equipment.
Celestial Event Schedule and Viewing Details
- Jupiter Magnitude: –1.9, outshining Regulus in the constellation Leo
- Ganymede Transit Start: 5:24 a.m. EDT for eastern U.S. observers
- Ganymede Transit End: 6:02 a.m. PDT for western U.S. observers
- Io Shadow Entry: 5:45 a.m. CDT
- Titan Position: Southwest of Saturn, shifting to the southeast the following night