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Jupiter

This is the biggest planet in our Solar system and the fourth brightest object in our sky. It has been known since prehistoric times. Galileo's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean Moons) was the first discovery of a center of motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets. Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition.

Jupiter is a big boy.

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. It also has upwards of 60 moons, see some of them here.

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000km, big enough to hold two Earths. Other smaller but similar spots have been known for decades.

Medicea Sidera

Similar to Cassini and his Sidera Lodoicea, Galileo Galilei (1564-1642) discovered four moons around Jupiter in January 1610:

1. Io
2. Europa
3. Ganymede
4. Callisto

Galileo named this group of moons in honor of Cosimo de' Medici, Grand Duke of Tuscany. He was trying to secure patronage from the Duke and succeeded using the moons as leverage. Cosimo Sidera means Cosimo's Stars. Simon Marius claimed to have discovered the moons at the same time and published a paper giving the moons the names they are known by today. Luckily we all see through that and give Galileo the credit for discovering them and refer to them collectively as the Galilean Moons.

The discovery of these moons caused quite a controversy at the time because the commonly held belief was that all celestial bodies revolved around the Earth (e.g. Ptolemaic world system). These moons obviously revolved around Jupiter.

Another interesting face is that for every time Ganymede revolves around Jupiter, Europa revolves twice and Io revolves four times. Callisto messes it all up with it's one revolution per 9.4 revolutions of Io.

These moons are the four largest around Jupiter and all but Europa are larger than Earth's moon. See all Jupiter's moons.

Gallery

These are pictures of Jupiter and it's moons. It does not include images of Callisto, Europa, Ganymede & Io.

Cassini - Jupiter Cassini - Jupiter & Ganymede Cassini - Jupiter & Io 1 Cassini - Jupiter & Io 2 Galilean Moons Core Cutaway Galileo - Amalthea Galileo - Jupiter Clouds 1 Galileo - Jupiter Clouds 2 Galileo - Metis & Adrastea Galileo - Thebe New Horizons - Io & Europa New Horizons - Jupiter New Horizons - Jupiter, Io & Ganymede Voyager 1 - Jupiter Voyager 1 - Jupiter Clouds 1 Voyager 1 - Jupiter Clouds 2 Voyager 1 - Jupiter Clouds 3 Voyager 1 - Jupiter Clouds 4 Voyager 1 - Jupiter & Io Voyager 2 - Jupiter Voyager 2 - Jupiter Clouds 1 Voyager 2 - Jupiter Clouds 2 Voyager 2 - Jupiter Clouds 3 Voyager 2 - Jupiter & Io