JupiterMoons is your essential guide to observing the king of planets whenever Jupiter reigns over the night sky.

Sky & Telescope’s newest app helps you locate Jupiter’s four largest moons and time transits of the Great Red Spot with an easy-to-use interface. (The moons’ orbits are shown to scale with respect to Jupiter.) Plus, learn more about what makes these fascinating solar system denizens tick.

Observe Jupiter
• Locate Io, Europa, Ganymede, and Callisto at the specified date and time — unlike in Galileo’s day, they’re clearly labeled.
• Zoom in by tapping on the screen to better watch accurately timed Great Red Spot crossings and moon transits and eclipses. Or zoom out for a full view of the moons’ orbits.

Take Command
• Use the time controls to step through the seconds or hours, or speed through days or even years to see where the moons will be down the line.
• Customize your view to match what you see through your telescope.

Focus on the Details
• Click on any item in JupiterMoons’ complete events calendar to see the action. The calendar lists all the Jupiter-related happenings for the day: moon eclipses and transits, Great Red Spot transits, and moon-shadow transits.
• Learn more about Jupiter and its ice-and-fire moons with our encyclopedic descriptions, accompanied by spectacular images.

Please note: this application is designed to work in landscape orientation only so that you may view Jupiter, the Great Red Spot, and the Galilean satellites at their best.