Friday, 14 June 2013

Saturn over the rooftops

Once when we pointed out to my Mother-in-Law the presence of Mars in the northern sky she exclaimed. ‘Eee. Well I never knew Mars was up my road!’ I experienced a similar revelation the other week myself when I deployed my telescope to the top patio and aimed it just above the rooftop, which was helpfully hiding the bright moon. As I pointed the scope at the bright object that my handy mobile app' identified as the planet Saturn and twiddled the appropriate knobs to scan the heavens, all at once the planet came unmistakably into view.

I caught my breath. It was beautiful. I had not expected with my puny little 76mm telescope that I would have had such a fine view of Saturn but there it was, rings and all, shining brightly with its distinctive yellow hue. I could even make out one - no - two moons. To be able to look upon that impossibly distant world with my own eyes moved me more than I had known that it would. I still haven't tired of gazing at it as it shines brightly over my rooftop.

When Galileo turned his telescope towards Saturn in July 1610 he found himself greatly puzzled by its appearance. The limitations of his equipment meant that Galileo was not able to observe the rings with clarity but instead beheld what he described as a ‘planet triform’. Galileo supposed that Saturn was flanked by two smaller moons which never altered in their positions relative to the larger planet. Until such time as he could be confident of his findings, in a common practice amongst enlightenment thinkers at the time, Galileo circulated his theory in the form of an anagram. In the event that another astronomer came to the same conclusion and published their findings before him, Galileo could provide the solution to the anagram and reveal that he had been right all along! Two years later however, when he observed Saturn again, the great scholar was perturbed to find that the ‘moons’ had disappeared. Despite his puzzlement Galileo confidently predicted that the moons would return and so in due course they did. Indeed, as more curious observers turned their instruments towards the heavens a bewildering array of manifestations of the planet Saturn were described. What on earth were these strange phenomena?

Danish lens maker and astronomer Christiaan Huygens, from whose 1659 publication Systema Saturnium the above diagram is taken, was the first observer to finally be able to discern the truth of the mystery when he deduced that the planet Saturn was surrounded by a ring. Huygens was also the first to observe the moon Titan in orbit around Saturn. Huygens, like Galileo, at first released his findings in the form of an anagram whilst he continued his observations and firmed up his convictions regarding the planet. The solution to the anagram as he revealed in Systema Saturnium was Annulo cingitur, tenui, plano, nusquam cobaerente, ad eclipticam inclinator: It is encircled by a ring, thin, plane, nowhere attached, inclined to the ecliptic.

Huygens believed the ring to be a solid structure, although he was uncertain as to its composition; merely ascribing its existence to the ‘power and majesty of nature’.

I believe that I should digress here to meet the objection of those who will find it exceedingly strange and possibly unreasonable that I should assign to one of the celestial bodies a figure the like of which has up to this time not been found in any one of them, although, on the other hand, it has been believed as certain, and considered as established by natural law, that the spherical form is the only one adapted to them; and that I should place this solid and permanent ring (for such I consider it) about Saturn, without attaching it by any joints or ties, although imagining that it preserves a uniform distance on every side and revolves in company with Saturn at a very high rate of speed. These men should consider that I do not construct this hypothesis from pure invention and out of my own fancy, as the astronomers do their epicycles, which nowhere appear in the heavens, but that I perceive this ring very plainly with the eyes; with which, obviously, we discern the figures of all other things. And there is, after all, no reason why it should not be possible for some heavenly body to exist having this form, which, if not spherical, is at least round, and is quite as well adapted to the possession of circumcentral motion as the spherical form itself. For it certainly is less surprising that such a body should have assigned to it a shape of this kind than that it should have some absurd and quite unbeautiful shape. Furthermore, since, owing to the great similarity and relationship that exists between Saturn and our Earth, it seems possible to conclude quite conclusively that the former, like the latter, is situated in the middle of its own vortex, and that its centre has a natural tendency to reach toward all that is considered to have weight there, it must also result that the ring in question, pressing with all its parts and with equal force toward the centre, comes by this very fact to a permanent position in such a way that it is equally distant on all sides from that centre. Exactly so some people have imagined that, if it were possible to construct a continuous arch all the way around the Earth, it would sustain itself without any support. Therefore, let them not consider it absurd if a similar thing has happened of itself in the case of Saturn; let them rather regard with awe the power and majesty of Nature, which, by repeatedly bringing to light new specimens of its works, admonishes us that yet more remain.
Christiaan Huygens Systema Saturnium 1659
Christiaan Huygens
Next to turn his telescope towards Saturn was our old friend Gian Domenico Cassini, now overseeing the Paris observatory under the patronage of Louis XIV. Between 1671 and 1684 Cassini discovered four more moons of Saturn; Iapetus, Rhea, Tethys and Dione. He named these moons the Sidera Lodoicea or Louisean Stars in honour of his royal patron. He also discovered in 1675 that Huygens had been incorrect in his assertion that the ring was a single solid structure with his observation of a visible gap between the rings which still goes by the name of the Cassini Division. Cassini correctly deduced that the rings of Saturn were not a solid structure but rather were composed of millions of tiny satellites orbiting the planet.

In 1789, Hanoverian astronomer and composer William Herschel, a favourite of King George III best known for his discovery of Uranus, began observations with his famous Great Forty Foot Telescope. (Pictured above) On the very first night of using the giant instrument, Herschel discovered another moon of Saturn; Mimas. Within a month he had discovered a second; Enceladus. The forty footer was the largest telescope yet created. This great ‘penetrator of the heavens’ as Herschel described it, was a national sensation and challenged the Christian preconceptions of the day which still saw the universe as a cosy firmament which enclosed God’s creation, in which Earth remained of primary importance. Instead, Herschel, himself a firm believer in the possibility of extra-terrestrial life, was revealing a boundless universe filled with countless unknown and distant worlds. This new, bigger vision of the cosmos made many uncomfortable and Herschel’s scientific endeavours were criticised by more romantically inclined contemporaries such as Wordsworth and Blake who dismissed the giant telescope as a sideshow. King George III however thought that it was a marvellous device. ‘What what!’ And commissioned several smaller instruments from Herschel for his own use. With such enthusiastic royal support, a craze for studying the heavens was born in an Eighteenth Century equivalent of the ‘Brian Cox effect’. Of which I am myself a recent victim.

Huygens’ Systema Saturnium

Herschel’s Great Forty Footer

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