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The two huge gas giants are home to a staggering number of natural satellites
Jupiter's Moon callisto Cast Shadow Onto Jupiter's Cloud Top by Kim Mitchell
Jupiter is grandiose in all respects. Not only is it the largest of the planets — it would take 1,321 Earths to fill the volume of Jupiter — it's also more than likely that it keeps the largest entourage of moons. There are 67 that we know of, and though many of these are fairly small and can't be observed from Earth, the biggest four are easy to spot with just a small pair of binoculars.
These are Io, Europa, Ganymede and Callisto: the Galilean moons, so named because they were spotted by Galileo in the early 17th century.
A minimum size pair for spotting these four moons would be 7x50s, which magnify what your eyes see seven times and have front lenses that are 50mm in diameter. Your view will be much improved by resting the binoculars on a wall or fence, or even attaching them to a tripod with an inexpensive bracket. Through a 3- to 6-inch telescope the moons will appear brighter and fill more of the field of view. Don't worry if you don't see all four: as the moons travel around the planet they may be behind or in front of Jupiter when you're looking.
It's by using a larger scope with a front lens over 6 inches that you start to see detail on the planet itself, and this includes the occasional shadow cast by the Galilean moons.
Moon with a View
Fellow gas giant Saturn has 62 known moons, but only seven are visible. Due to its sheer size, the easiest of Saturn's satellites to see is Titan. This moon has a diameter of 5,150km, which makes it bigger than the planet Mercury. In the moon rankings, it's the second largest in the Solar System, only beaten by Jupiter's Ganymede. It's also the only moon with a substantial atmosphere. When you're gazing at it through your scope, you're not actually looking at Titan's surface but at its nitrogen-rich cloud tops. In terms of brightness, Titan can reach mag. +8.4, putting it well within the reach of binoculars, while with a small telescope you'll have no trouble seeing it.
The remaining six moons are all within the grasp of a 6-inch scope. In order of brightness, after Titan comes Rhea, which shines at mag. +9.7, Tethys at mag. +10.3, Dione at mag. +10.4, Enceladus at mag. +11.8 and then quirky Iapetus.
The unusual nature of this last moon quickly became apparent to its discoverer in 1671, the Italian astronomer Giovanni Cassini. He first saw the moon on the western side of Saturn but found it missing on a later search, when it should have been on the eastern side.
It wasn't until 34 years later, when telescopes had improved, that Cassini finally saw Iapetus to the east, because when it's here it's almost two magnitudes fainter. This is why it had been impossible to see it before. Cassini deduced, correctly, that this was because the moon has one very bright hemisphere and one very dark one, and is also tidally locked to Saturn.
This means, like our Moon, it always shows the same face to its planet. It follows that we see a different part of Iapetus from our Earthly viewpoint when it is to the east or west of Saturn. As a result, Iapetus varies between mag. +10.1 and mag. +11.9. However, the faintness trophy goes to Mimas, which at mag. +12.9, needs perfect viewing conditions without any light pollution to see comfortably.
Jupiter's famous Galilean moons
IO
Diameter: 3,640 km
The tremendous gravitational pull of Jupiter on this innermost of the four Galilean moons, together with its closeness to the planet, means Io whizzes round Jupiter in just 1.75 Earth days. This fast orbital speed is easily seen in a small telescope: it visibly shifts position in just a few hours.
Europa
Diameter: 3,140km
The second Galilean moon out from Jupiter, Europa should theoretically be visible with the naked eye as it shines at mag. +5.3. But Jupiter's overwhelming brightness means it's difficult to separate moon from planet. Europa's brightness is due to its smooth, icy surface, with perhaps an ocean underneath.
Ganymede
Diameter: 5,260 km
The third major moon out from the planet is not only Jupiter's biggest, it's also the largest moon in the entire Solar System — but only by a whisker. This is a world with a cold ice surface, a large warm ice (possibly water) mantle, a rocky interior and a liquid iron core.
Callisto
Diameter: 4,820 km
The last of the four giant Galilean satellites is Callisto. It is the third largest moon in the Solar System, after Titan, the biggest of Saturn's moons. Callisto's entire icy, ancient surface is covered with impact craters that date right back to the time of the early Solar System.
Saturn's best moons to observe
Titan
Diameter: 5,152 km
The largest of Saturn's moons has a 16-day orbit. At its farthest, you'll find it about five of Saturn's ring diameters from the planet, mag. +8.4 at its brightest, which makes it visible in good binoculars. Titan makes up over 96 per cent of the mass of everything orbiting the planet.
Rhea
Diameter: 1,528 km
The second largest moon of Saturn, ninth largest in the Solar System, and currently the 20th catalogued in distance out from the planet. It makes an orbit in 4.5 days, reaching just under two ring diameters from Saturn. It is mag. +9.7, making Rhea an easy target for a 3-inch refractor telescope.
Iapetus
Diameter: 1,469 km
This is the third largest and most distant of the main moons of Saturn. Its 79-day orbit, which is the most inclined of the inner satellites, takes it out to 12 ring diameters from the planet. The visual magnitude ranges from +10.1 to +11.9, so Iapetus needs about a 6-inch scope to see it at its darkest.
Dione
Diameter: 1,123 km
This moon orbits up to 1.5 ring diameters from Saturn over 2.7 days. Its visual magnitude of +10.4 makes it visible on dark nights with a 3-inch refractor. This is the densest of the moons, meaning it may have a large rocky core. Helene and Polydeuces, two smaller moons, share its orbit.
Tethys
Diameter: 1,060 km
This moon orbits about one ring diameter away from the planet and takes 1.9 days to do so. It has a magnitude of +10.3 and so can be seen in a 3-inch refractor. Tethys has a great canyon that stretches three-quarters of the way round the moon, and two co-orbital moons, Telesto and Calypso.
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