Ganymede
What is Ganymede?
Moons· Diameter 5,268 km. The largest moon in the solar system — bigger than Mercury, and the only moon with its own magnetic field. ESA's JUICE arrives in 2031.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
The orbit of Ganymede at this moment: https://nowsky.org/solar/orbit?fw=moon:jupiter:ganymede&sp=0.018 — replayed live, no login.
Orbit and size
| Diameter | 5,268 km |
|---|---|
| Parent | Jupiter |
| Note | largest moon — bigger than Mercury |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Frequently asked
- How big is Ganymede?
- 5,268 km.
- What is notable about Ganymede?
- largest moon — bigger than Mercury.
- Which planet does Ganymede orbit?
- Jupiter.
Named not by Galileo but by Marius
Galileo discovered Ganymede and its three companions, but the names in use today were given by his rival Simon Marius in 1614 at Kepler's suggestion. Galileo called them the "Medicean Stars" after his patrons and told them apart by number. Marius claimed to have found them first and feuded with Galileo for life; his names survived while the credit for discovery went to Galileo.
All four names come from lovers or attendants of Zeus (Jupiter) — Ganymede was the Trojan youth who poured the gods' wine. Astronomers avoided the names into the mid-twentieth century, preferring "Jupiter III", and only as the count of moons grew did the mythological names become standard. Jupiter's 95 moons still follow Zeus's lovers and descendants, with a rule that inner moons end in -a or -o and outer, retrograde moons in -e.
The moon in depth
Ganymede is wasted on moonhood: at 5,268 km it outsizes Mercury, it is the only moon with its own magnetic field, and beneath its icy crust likely lies more water than Earth holds. Orbiting the Sun instead of Jupiter, it would be a respectable planet.
Its surface wears two faces — dark, densely cratered terrain four billion years old, patched with brighter, grooved tracts torn open as the crust stretched: evidence this moon was once geologically alive.
In December 2034 ESA's JUICE enters orbit around it — the first spacecraft ever to orbit another planet's moon. Zoom to Jupiter in the Orbit view and find Ganymede on its seven-day lap.
Ganymede's magnetic field is what betrayed its hidden ocean: Hubble saw its auroras rocking less than Jupiter's field should force them, and the best explanation was a conducting salty sea damping the sway — likely holding more water than all Earth's oceans. When JUICE settles into orbit in 2034, the real survey begins.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.