Galileo Found Them
You Can See Them
Jupiters Moons 

A treat for any stargazer is Jupiter's moons. With four of the six largest natural satellites in the Solar System these objects are easy to spot.

With apparent magnitudes ranging from 4.6 to 5.6 during Jupiter's opposition these satellites are big and bright.

If not for the brightness and glare from Jupiter itself these moons could easily be seen with the naked eye.

Small binoculars bring them into focus easily.

Io and Europa would be the toughest to spot because of their closeness to Jupiter.

Ganymede and Callisto at maximum separation from the planet offer the best chance to catch a naked-eye glimpse.

Galileo Galilei discovered them on the nights of January 7th and 8th 1610. A second astronomer named Simon Marius claimed to have found them earlier but he and Galileo were using different calendars at that time. When comparing the Julian calendar that Marius was using to the Gregorian calendar Galileo was using the dates coincide.

Galileo published his findings first and receives credit for the discovery. A Chinese astronomer in 325 mentions Jupiter's moons in his writings as well.

Simon Marius with suggestions from Johannes Kepler named the satellites in 1614. These names were not widely used until the 20th century. Until then they were simply known as Jupiter I-IV.

The four Galilean moons are all over 1,900 miles in diameter and one, Ganymede is larger than the planet Mercury. The fifth of Jupiter's moons, Amalthea was the last moon discovered by direct observation.

E. E. Bernard found it in 1892. All moons since then have been found from telescope pictures and satellites.

This is a picture of Jupiter's moons taken through a 10in(250mm) telescope. This should give you an idea of what looking through a scope is like.

picture credit: Jan Sandberg


Io moon of Jupiter

Apparent Magnitude:5.02

Io is the closest of the four moons to Jupiter.

Orbiting the planet in about 42 hours as seen from Earth. It is the 4th largest moon in the Solar System.

With over 400 active volcanoes, Io is the most geologically active body in the Solar system. Many of the mountains on the surface are taller than Mount Everest.

Io has a thin atmosphere composed primarily of sulfur dioxide.


Europa moon of Jupiter

Apparent Magnitude:5.29

Second from the planet of the large moons is Europa.

The smallest of the four main Jovian satellites, Europa is the 6th largest moon in the Solar System just after the Moon itself.

Orbiting Jupiter in about 3.5 days it takes twice as long as Io to orbit and half the time as Ganymede.

With its oxygen atmosphere and possible liquid ocean under the surface, scientist think that Europa could be a place to find life in outer space if it exists.


ganymede moon of jupiter

Apparent Magnitude:4.61

Third in line is the big daddy of satellites...Ganymede.

Named after the mythological cup bearer to the gods Ganymede is the largest moon in the Solar System. Bigger around than Mercury though not as dense.

Orbiting Jupiter in 7 days, Ganymede is in an orbital sync with Io and Europa. Every orbit of Ganymede is matched by two by Europa and four by Io.

Contains a thin oxygen atmosphere.


callisto moon of jupiter

Apparent Magnitude: 5.65

The fourth natural satellite of the quartet is Callisto.

The 3rd largest moon behind Ganymede and Saturn's Titan.

Farther away from the planet and not locked into an orbital sync with the other three, Callisto orbits in 16 Earth days.

A thin carbon dioxide atmosphere doesn't protect Callisto from impacts and the surface is heavily cratered. Callisto does have an ionosphere.

Also possibly containing a liquid ocean under the surface. Callisto is not affected as much as the other moons by Jupiter's intense radiation and scientist think that it could be used someday as a base for space exploration.

Jupiter's moons to Jupiter

› Jupiter's-moons
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