Jupiter's Galilean satellites (from left) Ganymede, Callisto, Io, and Europa. Image credit: NASA/JPL/DLR

May 31, 2015 Four centuries of observing Jupiter's main moons

Sen—Great news this week was that progress has been made with plans for a mission to Europa. Science instruments have been chosen to help learn more about this enigmatic satellite after the spacecraft launches in the 2020s.

Europa is the smallest of the four biggest moons of Jupiter, yet still the sixth largest in the Solar System. These four main satellites of the planet—Io, Europa, Ganymede and Callisto—are known as Galilean moons, because they were observed by the famous Italian astronomer Galileo Galilei in 1610, with an early telescope.

Galileo Galilei’s work with his “optik tube” is sometimes overstated. He did not invent the telescope. Its origin is generally credited to a Dutch spectacle maker called Hans Lippershey in 1608. Neither was Galileo the first to use one in astronomy. Records show that Thomas Harriot, a modest mathematician from Oxford, England, was sketching the Moon four months before Galileo from a park west of London.

Harriot’s observations are considered by experts to have been of better quality than Galileo’s too, perhaps because his optics were better. He had at least eight telescopes made—they were known as “cylinders” to him at the time—and he supplied some to two friends in Wales so that they could also observe the sky.

Harriot had a colourful personality and his mathematical skills were employed in teaching navigation to explorer Sir Walter Raleigh and his ship captains. He kept careful record of his observations, made from Syon Park, at Isleworth, West London. But unlike Galileo, he did not publish them. This might be because he needed to keep a low profile.

Both Raleigh and his new benefactor, Henry Percy the Earl of Northumberland, had been thrown into the Tower of London by King James 1 in the purge that followed the Gunpowder Plot to blow up Parliament in 1605. Harriot spend a brief time in the Tower too before his innocence of any crime was accepted.

When I asked leading science historian Dr Allan Chapman, of Oxford University, about Harriot, he told me: “Harriot was the first person in the world ever to see an astronomical body through a telescope, record it and draw it on July 26th, 1609. He was also certainly the first person we know to make a map of the whole Moon.

“These were two major achievements. Harriot’s moon map is just spectacular. There’s no doubt about that. It is better than anything produced by Galileo.

“But Galileo was a much more ambitious person than Harriot and more interested in recognition and fame. Had Galileo been alive today, he would have had his own press office. He had an incredible genius for grabbing the media.”


A simple 20-second shot with the author's Canon EOS 600D camera and a telephoto lens recorded an over-exposed Jupiter plus its four Gallean moons, from left, Ganymede, Europa, Io and Callisto. Image credit: Paul Sutherland 

A German contemporary of Galileo, Simon Marius, claimed that he spotted Jupiter's moons a few weeks before Galileo did, and he gave them their names. But since Marius failed to publish his observations, it is fair to credit Galileo with their discovery.

The Italian spotted them as soon as he pointed his telescope at Jupiter, mistakenly describing them as “three little stars”, presumably because the fourth was hidden by the planet at the time. But soon he realised that these “stars” were staying close to Jupiter and, furthermore, appeared to move from one side of it to the other. This was a profound moment because it showed for the first time that not everything orbited the Earth.

Galileo recorded his observations of the moons’ changing positions in his book Siderius Nuncius, which was published in Venice in 1610 and distributed rapidly across Europe. Harriot—who, like Galileo supported Copernicus’s model that put the Sun at the centre of the Solar System—obtained his own copy of the book and went on to make his own accurate records of the four satellites’ motions.

If you want to, you can easily follow in the footsteps of Galileo and Harriot and see the Galilean moons for yourself with the smallest of telescopes. Use a low-power eyepiece to focus on the planet, and you should immediately see the moons strung out like beads on either side of Jupiter’s disc. Like Galileo, you might not see all four at once, depending on whether or not the planet itself is hiding one or more.

Even binoculars will show the moons—you’ll need to hold them still, so a tripod is helpful, but otherwise you could lean your arm against the side of a building as you look towards Jupiter to help keep the binoculars steady. The moons’ brightness is actually enough that you would be able to glimpse them with the naked eye alone in a dark enough sky, were it not for the overpowering brightness of mighty Jupiter.

As I write, at the tail-end of May, 2015, Jupiter is at the latter stage of its current apparition, but well placed after dusk for you to look for those mysterious moons. So do see if you can spot them and then ponder on what forthcoming missions to Jupiter will tell us about their nature and their suitabiity for life.