Jupiter has at least 79 natural satellites, or moons — many are small objects that have been trapped by the massive planet’s gravity.

The four largest moons were first observed by Galileo Galilei in 1609 using a very modest telescope. By observing the moons nightly over a period of weeks, he discovered that they were orbiting Jupiter — a controversial statement in his day.

Astronomers commonly refer to the big four as the Galilean moons. From closest to farthest from Jupiter, they are named Io, Europa, Ganymede and Callisto. Io is closest to its planet and moves faster than the outer ones, needing only 1.8 days to orbit Jupiter, while distant Callisto takes nearly 17 days.

Jupiter and the four Galilean moons seen through a telescope

Even modern-day binoculars are better than Galileo’s little spyglass, so you can look for the moons yourself.

Unlike the Earth’s axis, which is tilted with respect to the plane of the solar system, Jupiter’s axis is vertical, so the Galilean moons always appear along a straight line that runs parallel to the planet’s equator.

Their differing orbital speeds produce different arrangements of the moons: close together, well separated, arranged symmetrically and sometimes all clumped to the left or right (east or west) side of Jupiter. This makes it fun to check in on them from time to time.

The Jupiter system runs like clockwork, so we can accurately predict events far into the future. This was also very useful for navigators in the past, in fact, Captain Cook used Jupiters moons to work out his longitude. The moons of Jupiter were used right through to the turn of the 20th century by some navigators.

The four moon in more detail photo from NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Jupiters lack of tilt also means that Jupiter does not have seasons. Unlike us, Mars and Saturn – our seasons are 3 months long. On Mars they are 6 months and on Saturn close to 7.5 years.

That makes for a very long winter on Saturn.