Galilean+Moons

The Galilean Moons are the four largest moons orbiting around Jupiter. they are Io, Europa, Ganymede and Callisto.

The term Galilean Moons acknowledges their discoverer, Galileo Galilei, whilst the actual moons are named after Zeus's (the greek god of the sky) lovers.

Galileo Galilei was the inventor of the optical telescope and the first to theorise that the Earth was not the centre of the universe due to seeing the moons of Jupiter revolving around Jupiter and not the Earth.

The problem with this theory was the strange movements of the other planets, Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. This is a picture of Mars (highlighted) moving through the sky as time passes, it is called Retrograde Motion.

The Catholics (who believed the Earth was the centre of the universe) explained this by saying that the planets were on a separate plate that rotated whilst they also orbited around the Earth.



The real reason is because the Earth orbits the Sun faster than Mars so when the Earth passes Mars it seems to slow down, move backwards and then move forward again.

Galileo started this theory when he observed Jupiter's moons in a different position each night.



The circles represent Jupiter and the asterisks represent Jupiter's moons. This proved that some objects didn't revolve around the Earth and instead revovled around other objects. This then sparked the idea that the Earth was itself orbiting around The Sun and all the planets orbited around The Sun too.

References

See Moons Of Jupiter