Ganymede

Provenance-Backed Scientific Measurements

Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Ganymede (Jupiter III) — Largest Moon Profile at RA 04h 55m 10.00s, Dec +22° 05′ 10.0″ within Taurus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.

Astrophysical Context & Structure

Measuring 5,268.2 kilometers across—8 percent larger in diameter than the planet Mercury and three-quarters the size of Mars—Ganymede (Jupiter III) reigns as the largest and most massive satellite in the Solar System. Orbiting 1.07 million kilometers from Jupiter every 7.155 days, it serves as the outer anchor of the three-body Laplace resonance with Io and Europa. Its low bulk density of 1.936 grams per cubic centimeter reflects an almost equal 50-50 split by mass between silicate rock and water ice, differentiated into one of the lowest moment-of-inertia factors (0.3115) of any solid Solar System body.

During its close flybys in 1996, NASA's Galileo spacecraft made the astonishing discovery that Ganymede possesses its own permanent dipole magnetic field (~720 nanoteslas at the equator), carving a mini-magnetosphere inside Jupiter's vast magnetosphere and generating ultraviolet oxygen auroral ovals around Ganymede's poles. This intrinsic dynamo arises from convection inside a liquid iron-iron sulfide core. Furthermore, Hubble measurements of how those auroral ovals rock back and forth in Jupiter's time-varying magnetic field confirmed the presence of a global, electrically conductive saltwater ocean ~100 kilometers thick sandwiched beneath a 150-kilometer icy crust.

Ganymede's surface is split between ancient, dark, clay-rich cratered regions (such as Galileo Regio) and younger, brighter swaths of tectonically faulted grooved terrain (sulci). In the 2030s, the European Space Agency's Jupiter Icy Moons Explorer (JUICE) will become the first spacecraft to enter orbit around a moon other than Earth's, mapping Ganymede's ocean and magnetosphere in detail.

Practical Visual Observing Guide

• At magnitude V = 4.61 and elongations up to 5.9 arcminutes from Jupiter, Ganymede is bright enough to be glimpsed with the naked eye by sharp-sighted observers when Jupiter is masked behind a building or tree limb.
• Subtends up to 1.8 arcseconds at opposition; 8-inch to 14-inch amateur telescopes under steady seeing resolve its disk and even capture broad albedo patches (Galileo Regio vs. Osiris ray system) via lucky imaging.
• Its large shadow is the easiest Galilean moon shadow to spot crossing Jupiter's cloud belts in small 60 mm to 80 mm refractors.

Catalog Cross-Identifications & Aliases

Common Aliases: Jupiter III · Largest Solar System Moon

Related Objects, Comparisons & Guides

Scientific Sources & Dataset Provenance

• NASA/JPL Solar System Dynamics & Planetary Fact Sheets (DE441 (2021/2026)): Park, R. S., et al. 2021, AJ, 161, 105 (The JPL Planetary and Lunar Ephemerides DE440 and DE441) — NASA Jet Propulsion Laboratory / GSFC