Observing Jupiter's moons

The Solar System's largest planet has the most moons, four of which are easy to see.

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Image Credit: 
Pete Lawrence

The Solar System is truly an incredible place, but one world in particular stands out and truly deserves the title King of the Planets: Jupiter. It 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.

It’s the massive gravitational effect of Jupiter that does the trick, attracting more than 100 moons into orbit around the planet at the latest estimate. Many of these satellites are fairly small and can’t be observed from Earth, but the biggest four are easy to spot with just a small pair of binoculars.

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. You can certainly catch glimpses of these Galilean moons (named after Galileo, who first observed them) with hand-held binoculars, but 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. With binoculars though, Jupiter itself will not appear as anything more than a large, slightly oval-shaped disc.

Moonwatch
The next step in viewing Jupiter is to use a small telescope – one with a front lens 3 to 6 inches in diameter. As this gathers more light, it can magnify the view more, so the Moons will appear brighter and fill more of the field of view.

Don’t necessarily expect to see all four, however: 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 in diameter that you really start to see detail on the planet itself: not only the darker belts and lighter zones, but features within the gaseous atmosphere as well. At this level of detail, observers can also see the occasional dark spot caused by the moons casting their shadows onto Jupiter’s atmosphere. The joy of Jupiter is that whatever your level of equipment, there’s always something to see.

Make sure you look at Jupiter this month. It’s a great time to see if you can spot it since it’s at opposition. That’s when it’s directly opposite the Sun in the sky from our point of view, and so it’s really bright.

At the start of the month the planet rises almost due east at 10.30pm, and by the 30th it’s about a fist’s width (10˚) above the horizon by the time it’s properly dark at 8pm. For the best views, wait till Jupiter reaches its highest point in the sky, due south around midnight. It really stands out in the fairly faint area of sky that is Pisces, shining at magnitude -2.5.

Happy observing, and remember – what seems like an easy amateur target today made history at the start of the 17th century. When Galileo first saw Jupiter’s moons, it proved scientifically that the Earth was not unique and wasn’t at the centre of the Universe.

The Galilean Moons

Io
Diameter: 3,650km

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. Physically, Io is the most volcanic place in the entire Solar System. The whole world is covered in sulphurous lava flows and volcanoes erupting in plumes more than 500km high.

Eurpopa
Diameter: 3,140km

The second Galilean moon out from Jupiter, Europa, should theoretically be visible with the naked eye since it shines at magnitude +5.3. But Jupiter’s overwhelming brightness makes it difficult to separate the moon from the planet. Europa’s brightness is due to its surface being smooth and icy. Scientists suspect that underneath is a liquid water ocean, leaving open the possibility that life may lurk in the depths.

Ganymede
Diameter: 5,260km

The third major moon out from the planet is not only Jupiter’s biggest, but it is also the largest moon in the entire Solar System. This is a world with a cold ice surface, a large warm ice (possibly water) mantle, a rocky interior and a liquid iron core. It measures a tremendous 5,260km across, which is bigger than Mercury. Indeed, if Ganymede was released into space, it would be classed as a planet.

Callisto
Diameter: 4,820km

The last of the four giant Galilean satellites is Callisto. It is the third largest of the Solar System, after Titan, the biggest of Saturn’s moons. Callisto ranks as one of the most cratered worlds known – its entire icy, ancient surface is covered with impact craters that date right back to the time of the early Solar System, when the moon formed. Like Europa, it is thought that beneath the surface may lie a watery ocean.

This article appeared in the September 2010 issue of Sky at Night Magazine

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