Of the four moons Galileo discovered, Ganymede is the largest, followed by Callisto, Io and Europa.The Galilean moons Io, Europa, Ganymede, Callisto in order of increasing distance from Jupiter. Ganymede is the largest and there are more than 50 moons with a diameter of 5km or less. The moons vary dramatically in size from 1km in diameter to over 5,200km.The irregular moons have elliptical orbits, high inclinations and are much further away from Jupiter. The moons with regular orbits travel around Jupiter in an almost circular orbit aligned with the planet’s equatorial plane. The moons of Jupiter are split into two main categories: those with regular orbits and those with irregular orbits.The latest Jovian moon to have been discovered was not long ago in 2017. Galileo is credited with discovering the first four main moons hence them being called the Galilean Moons.However, there are four that are large and easily visible from Earth using a small telescope.

They are wrong! There are in fact 79 recognised moons orbiting in the Jovian System. Most people think that Jupiter has four moons.The Great Red Spot is easy to view in a modern small telescope on a night with good visibility when Jupiter is high in the sky. It was documented in 1899 in the Monthly Notices of the Royal Astronomical Society, but there were even earlier scientific observations from H Schwabe in September 1831, and it may well have been observed before that. The most famous feature of the Jovian atmosphere is the storm known as the Great Red Spot.Altogether there are 15 defined areas in Jupiter’s atmosphere which are mirrored in the northern and southern hemispheres. The darker belts are warmer and thought to be lower in the atmosphere. The lighter zones are clouds at a higher altitude and are seen to be cooler in the infrared spectrum. The storms present themselves in bands whirling around the orbital plane of the planet.Jupiter does not have seasons like the Earth as the orbital inclination of the pole is about three degrees so only varies a little throughout the 11.9 year orbit.Although Jupiter is a long way away it’s clouds are very reflective. However, when Venus is in crescent it is possible that Jupiter will outshine it. The brighter objects are the Moon and Venus. In our night skies, Jupiter is the third brightest object to be seen.Planets closer to the Sun can often be seen as crescents, but planets further from the Sun will always be seen as disks. As Jupiter is always outside the Earth it is always observed as a disk.Ultra HD imagery from the Hubble Space Telescope revealed details never before seen on Jupiter. It is possible that there is no surface and that an object sinking through the atmosphere would just become more and more compressed the lower it went. As it is not possible to see through the clouds the presence of a surface cannot be determined. Little is known about the internal composition of the planet, but it is speculated that it has a small, very dense rock and ice core that is encompassed by dense metallic hydrogen.The storms last very long periods of time as there is no landmass to disperse them. These clouds are active in a very violent storm system. The apparent surface of the planet if formed of gaseous clouds.In this respect, it is very similar to the Sun with a mass proportion of around four hydrogen to one helium. Jupiter is a Gas Giant, with its atmosphere being mainly composed of hydrogen and helium.Jupiter is so big that it contains more matter than all of the other planets combined. It is the largest of the Gas Giants which makes it the biggest planet in the Solar System. Jupiter has an equatorial radius of about 44,000 miles compared to the Earth’s of just under 4,000 miles, making it 11 times larger than the Earth.A Jovian year is 11.9 Earth years, but the Jovian day is just 9 hours 55 minutes.It is 5.2 Astronomical Units away from the Sun, or 5.2 times further away from the Sun than the Earth. Jupiter is the fifth planet in our Solar System counting out from the Sun, with an average orbital distance of 778,340,821 kilometers.Its moons were discovered by Galileo, in 1610, observed through a refracting telescope. Jupiter was reputedly discovered by the Babylonians approximately 2,800 years ago. The Jovian system encompasses Jupiter, its rings and moons. Artist impression of water vapor plumes on Jupiter’s moon Europa.

Io moon diameter in km