Jupiter reaches magnitude -2.7 at opposition and is the most rewarding planet for small telescopes. Even 7x50 binoculars reveal the four Galilean moons — Io, Europa, Ganymede, and Callisto — whose nightly dance of eclipses, transits, and shadow crossings was first observed by Galileo in January 1610. A 100mm telescope shows the North and South Equatorial Belts, the Great Red Spot (a storm larger than Earth raging for at least 350 years), and festoons and barges in the cloud bands. The disk spans 30-50 arcsec, large enough to track atmospheric detail in real time as the planet rotates every 9 hours 55 minutes. Named for the Roman king of the gods. Pioneer 10 and 11 made the first flybys in 1973-74. Voyager 1 and 2 discovered Jupiter's faint ring system and active volcanism on Io in 1979. Galileo orbited from 1995 to 2003, dropping an atmospheric probe and finding evidence of a subsurface ocean on Europa. Juno has orbited since 2016, mapping the deep atmosphere, gravitational field, and polar cyclone clusters. The Europa Clipper mission launched in 2024 to investigate Europa's ocean habitability. Jupiter has 95 known moons; Ganymede is the largest moon in the solar system.