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
http://www.dummies.com/how-to/content/trying-to-measure-the-speed-of-light.html
You may also enjoy - Saturn over the rooftops
http://slingsandarrowsblog.blogspot.co.uk/2013/06/saturn-over-rooftops.html
You may also enjoy - Saturn over the rooftops
http://slingsandarrowsblog.blogspot.co.uk/2013/06/saturn-over-rooftops.html
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