On a crisp winter’s evening a few days after Christmas, my wife Adele and I carefully carried my new telescope outside into the back garden. After a few trips back and forth to the house to fetch hats and gloves and a torch we were ready to take our first tottering steps into the world of astronomy.
After a little trial and error, peering through the star pointer and twiddling the declination and right ascension knobs, we at last experienced a sudden eureka moment as the planet Jupiter all at once came into focus. Big and bright in the centre of the eyepiece there it was, with its dark bands of raging clouds clearly visible. We were very chuffed indeed. A few nights ago (at time of writing) I was even more excited when I once more brought the planet into focus and was able to observe all four of its moons.
It was exciting enough for me. Just imagine therefore how chuffed another keen astronomer was one night in January 1610 when he pointed his new telescope skywards to investigate that same bright spot in the heavens and beheld for the first time through such an instrument the shining disc of Jupiter.
Galileo Galilei had already had some success with his shiny new toy which he had fashioned himself following a recently patented Dutch design rather than purchasing it from the internet as I had done. After carrying out observations of the lunar surface, Galileo turned his attention to Jupiter and was soon intrigued by the fact that the three stars around the planet were changing their positions relative to Jupiter and each other on a nightly basis. Galileo conducted nightly observations of Jupiter over the next eight weeks, during which he sketched the position of these Medician Stars as he had dubbed them in honour of his patron Cosimo di Medici. As he traced the movements of these bodies, which he observed did not twinkle like other stars, Galileo was able to conclude that there were in fact four rather than three of them and that they were orbiting around the planet Jupiter. This was a remarkable revelation, for it leant great weight to the highly inflammatory theory of Copernicus which proposed that the planets circled around the sun rather than the Aristotelian position favoured by the Catholic Church which maintained that the earth itself was the centre of the universe.
Galileo demonstrates a telescope to the Doge of Venice
Galileo published his findings in his treatise Sidereus Nuncius. His work was immediately seized upon with great enthusiasm by one Johannes Kepler, who was at that time Imperial Mathematician at the university of Prague. Kepler was a keen adherant of Copernicus' theory and having obtained a telescope of his own soon published a response to Galileo supporting his findings. All at once the universe had been shown to possess not a single point around which all other heavenly bodies rotated but at least two. The earth-centric Aristotelian viewpoint had been shaken to the core.
Various unsatisfactory naming systems were put forward for the newly discovered satellites of Jupiter but it was Kepler who proposed the pleasing classical monikers that stuck. The moons of Jupiter, drawn irresistibly towards it as they were, Kepler suggested should be named after that notoriously seductive god’s best known sexual conquests: Io, Calisto and Europa. The fourth he proposed should be named Ganymede; after the young shepherd boy who was borne aloft by an eagle to serve as cup bearer to the greatest of the Gods.
Like Galileo, Kepler later suffered for his beliefs. He was hounded from his University position in 1626 as a result of his Protestant faith and his mother was even tried as a witch. One can only imagine how a man at the forefront of the enlightenment felt when faced with such barbarous superstition.
The best known of Jupiter’s landmarks is of course its great red spot. This remarkable surface feature; a giant storm the size of three earths, was first observed by English polymath and Royal Society founding member Robert Hooke in 1664. Hooke noted that the position of the spot changed over the course of several hours, leading him to conclude that the planet was rotating on its axis.
Proceedings of the Royal Society 1664
Also taking an interest in the progress of the red spot was Gian Domenico Cassini in Bologna. By tracking the spot's movements Cassini was able in 1665 to calculate Jupiter’s rotation period at 9hrs and 56 minutes. Cassini also studied the movements of Jupiter’s moons and was puzzled by the refusal of their orbits to conform to his predictions. In 1676 the eminent scholar, now based in Paris, found himself upstaged by his brilliant young pupil Olaus Roemer who proposed that the reason for the apparent variability in the orbit of Jupiter’s moons was due to the varying distance between Earth and Jupiter at different times according to their respective orbits. Because light travels at a finite speed, Roemer suggested, the light from the moons would sometimes take longer to reach the earth. Cassini dismissed the suggestion but Roemer was proved right when he correctly predicted that on a given date the moon Io would emerge from behind Jupiter ten minutes later than Cassini’s model suggested that it should. There was in fact nothing wrong with Cassini’s reckoning but due to the increased distance between the two planets, the moon was not visible at the time that it emerged as the light reflected from it took longer to reach the earth.
Gian Domenico Cassini
So there you have it. From the mere process of gazing up at a distant planet in the heavens and observing the movements of its surface features and moons, far greater minds than mine had deduced that the planets revolved around the sun, that Jupiter revolved upon its axis in a little under ten hours and that light travelled at a finite speed. By contrast I had merely pointed my telescope in the right direction, gawped up at the planet and been very pleased with myself at having observed it at all. Ah well, we can’t all be geniuses can we?
Galileo's observations of Jupiter's moons
The Great Red Spot
Cassini, Roemer and the Speed of Light
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