Saturn

William Sheehan

• Publisher: Reaktion Books
• Year: 2019
• ISBN:
  9781789141818
• Topic: Biowissenschaften
• Subtopic: Wissenschaft Allgemein

Saturn is the showpiece planet of our solar system. It may not be the largest, nor the smallest, nor even the only planet with rings. But it is among the most stunning objects in the sky and is always breathtaking when seen in a telescope.This is a beautifully illustrated, authoritative overview of the entire history of humankind's fascination with the ringed planet, from the first low-resolution views by Galileo, Huygens, and other early observers with telescopes to the most recent discoveries by the spacecraft Cassini, which studied the planet at close range between 2004 and 2017. Saturn describes the planet from inside out, detailing the complicated system of rings and their interaction with Saturn's bevy of satellites, and it considers how Saturn formed and the role it played in the early history of the solar system. Featuring the latest research and a spectacular array of images, this book will appeal to anyone who has ever gazed with wonder upon the sixth planet from the sun.

ONE

A PALE YELLOW STAR

Saturn appears to the naked eye as a ‘star’ of pronounced yellowish hue. It is a giant planet, second only to Jupiter in our solar system in terms of size, and the most remote of those known to the ancients, wandering leisurely among the background stars and completing each orbit round the Sun in 29½ Earth years.

Though no account of its discovery, historical or traditional, exists, as mentioned, it was already known in ancient times. Though less striking than the other planets (except shy and Sun-hugging Mercury), it must have revealed itself early on as a ‘wanderer’, owing to the fact that it kept to the much-scrutinized region of sky along Earth’s orbital plane, or ecliptic, where the Sun, Moon and planets move. In addition, its occasional appearance near a bright star, or close to the Moon, must have centred attention on it, and helped disclose its slow movements and brightness changes over time. Its light is dull, livid and a pale-yellowish colour – the colour of lead (II) oxide (massicot). There is nothing in its naked-eye appearance to suggest that it is one of the most magnificent objects in the heavens. Instead, it has been regarded as an unlucky planet, baleful and malefic.

Prognosticators

Though the discovery of the five naked-eye planets is lost in prehistory, we know, fortunately, quite a lot about the beginnings of the science of astronomy, which took place some 5,000 years ago in the fertile lands between the Tigris and Euphrates rivers in ancient Mesopotamia – in what is now Iraq. In contrast to the great civilization of Egypt, where the annual inundation of the Nile produced predictably and reliably fertile fields, in Mesopotamia conditions for agriculture were more variable. The annual rainfall is low; the ground becomes dry, hard and unsuitable for the cultivation of crops for eight months of the year, while the sluggish flow of water in the two rivers deposits large quantities of silt and elevates the bed to the point where the waters overflow the banks or change their course. Mastery of this challenging situation was achieved only through the creation of an extensive system of artificial canals. The organization of the irrigation networks required coordinated effort on a hitherto unattempted scale and led to the rise of a powerful ruling class and the invention of a script.

In Mesopotamia, these developments seem to have coincided with the settlement, in the southern part of the country, of the Sumerians in about 3000 BCE. The regulation of the calendar became one of the principal functions of the Sumerian priests, who never adopted the solar calendar, as the Egyptians did, and indeed the lunar calendar has remained important ever since in that part of the world. The priests looked out from their seven-level terraced pyramids (ziggurats) for the first appearance of the thin crescent Moon, which marked the beginning of the new month. (The massive ziggurat of the ancient Sumerian city of Ur, built in the period 2112–2095 BCE, is the most famous.) They were also charged with adding, every two or three years, a thirteenth month so that their calendar remained in synch with the seasons and religious festivals.

Later, as the Sumerians merged with the Semitic population of Akkad to form the Sumer-Akkadian Empire, and still later, with the emergence of the Babylonian Empire, the business of the priest-scribes was greatly expanded. In addition to determining the beginning of the new month, as before, and adding intercalary months, they began paying close attention to eclipses, and to phenomena involving the planets – their heliacal risings and settings (that is, first visibility before or after the Sun). They took note of Mercury and Venus’ shuttling back and forth between the morning and evening skies, and of the retrograde (backwards) motions of Mars, Jupiter and Saturn around the times they appeared opposite to the Sun in the sky (in opposition). The priest-scribes also took note of the planets’ changes in brightness; their conjunctions with the Moon, stars and constellations and with each other; and even their appearances within halos about the Moon, and to meteorological phenomena. They were concerned with everything that happened in the sky.

Their interest was not scientific in any modern sense, but astrological. They did not believe the planets were gods, but ‘interpreters’, and the behaviour of the planets contained ‘omens’, signals to the kings in which they expressed their pleasure or displeasure. They saw a correspondence between what happened in the heavens and what happened on the Earth (the whole principle of astrology): they believed that the occurrence of phenomena in the sky preceding events of importance – such as the deposition of a king, an uprising, a famine or a war – meant the two must be connected, and that if the same sky phenomenon recurred, the event was certain to follow. Thus they were led to keep careful records on cuneiform tablets of observations of the planets’ irregular motions among the stars and the almost endless variety of their phenomena, together with terrestrial events presumed to have foreshadowed them.

Many of the early observations were very imprecise, and there was no clear distinction made yet between astronomical and meteorological phenomena. Clouds and halos stood on an equal footing with eclipses, and there are many records like this: ‘Last night a halo surrounded the Moon; Saturn stood within it near the Moon.’¹ It is unlikely that such observations as these would have directly led ancient observers to discover the regularities that underlie the development of mathematical astronomy. Rather, it is likely that, as historian of astronomy Antonie Pannekoek (1873–1960) supposed, the regularities in the planetary phenomena gradually ‘imposed themselves’ upon the observers, arousing expectations, which developed into astronomical predictions.²

Saturn glistens above the great ziggurat at Ur.

The periodicities thus discovered make up the backbone of Babylonian mathematical astronomy. They include the discovery that Venus takes five complete journeys around the zodiac (the synodic period) in almost exactly eight years, so that after eight years it is in the same position relative to the Earth and Sun; and that Mars returns to the same relative position after 79 years, Jupiter after 71 years and Saturn after 59 years. It is probably no coincidence that the oldest Babylonian observations of Saturn, dating from the sixth and seventh centuries BCE, are 59 years apart, and so provide the oldest known material from which the Babylonians could have derived the periodicity of Saturn. (Note that none of these periods is quite precise; in the case of Venus, for instance, the periodicity is actually eight years, minus 22/10 days, and for Saturn, 59 days, plus 3 days.)

Cycle and Epicycle

The data of Babylonian astronomy was expressed entirely as these arithmetical regularities, which allowed the prediction of omens. The Babylonians had no interest in developing underlying descriptive models of the actual motions of the bodies in the heavens. That was to be the accomplishment of the Greeks,

who began to take possession of Babylonian astronomical data following Alexander the Great’s conquest of Babylonia in 331 BCE.

In the hands of the Greek geometers, Babylonian data was interpreted geometrically – that is, geometric constructions were produced that would simulate the apparent paths of the Sun, Moon and planets, as projected onto the apparently flat surface of the sky. They made two assumptions: that the Earth was the centre of the system, and that the planets moved in circular paths. Some ingenious schemes were proposed – notably, that of the fourth-century BCE mathematician Eudoxus of Cnidus, who attempted to represent the retrograde movements of the outer planets (including Saturn) by supposing each one to be moving on a set of interested spheres, rather like a compass in a gimbal. However, though Eudoxus’ model was, to a certain degree, successful in representing the qualitative form of the motions, it completely failed to explain the planets’ variation in brightness. Saturn, for instance, may appear as bright as magnitude –0.4 (rivalling Sirius, the brightest of the fixed stars) or as faint as +1.5 (a little brighter than a star in the Big Dipper). But why, in an Earth-centred scheme, should a planet’s brightness vary at all?

Briefly, in about 250 BCE, the brilliant mathematician Aristarchus of Samos experimented with removing the Earth from the centre of the scheme and replacing it with the Sun – thus introducing the first full-fledged heliocentric system – but the idea won little acceptance. In any case, later geometers preferred to keep the Earth at the centre, and in order to account for the retrograde movements and brightness variations, they claimed that the planets moved on circles whose centres were slightly offset from the Earth (eccentric circles). Geometrically equivalent to this, and much more convenient, was the device of the epicycle: a small circle round which the planet moves while pivoting around a larger circle (known as the deferent) centred upon the Earth. It was already in use in the time of Apollonius of Perga (c. 262–c. 200 BCE), best remembered today for his work on the conic sections; it is even possible that Apollonius himself was responsible for introducing it. However, it came to its fullest elaboration at the hands of Claudius Ptolemy ( c.100–c. 170 CE), a Greek who spent his entire career at Alexandria in Egypt and whose book, usually known by the Latin translation of its Arab name, Almagest, represents the culmination of Greek – and hence also Babylonian – astronomy.

The Ptolemaic system of epicycles and deferents centred on the Earth has been ridiculed as clumsy and artificial since at least the time of Alfonso X of Castile, nicknamed ‘the Wise’ (1221–1284), who is reputed to have said, ‘If the Lord God almighty had consulted me before embarking upon the Creation, I should have recommended something simpler.’³ Although the Ptolemaic system is rather complicated, remember that Ptolemy’s purpose was not to describe the actual paths of the planets in space but to provide a calculating machine. As Harvard historian of astronomy Owen Gingerich notes,

Basically, for the first time in history (so far as we know) an astronomer has shown how to convert specific numerical data into the parameters of planetary models, and from the models has constructed a . . . set of tables which allow the positions of the planets to be calculated often to within ten minutes of arc, or to well within the limits of accuracy of the measurements possible at the time.⁴

Ptolemaic system. Saturn moves around a small circle, known as the epicycle, which in turn pivots around a larger circle centred on the Earth.

This, in itself, was a gigantic achievement – and, incidentally, a great boon to astrologers, who then, as now, were among the chief users of planetary theory.

We know from another of his books, the Tetrabiblos, or ‘Four Books’, that Ptolemy, like others of his time, took divination by means of the stars quite seriously, and apparently regarded astrology as a branch of applied mathematics. His views about epilepsy are typical: ‘epilepsy generally attaches to all persons born while Saturn and Mars may be in angles’ with the eastern horizon (when Saturn rules the day and Mars rules the night); but if the converse happens (especially if these planets are in Cancer, Virgo or Pisces), ‘the persons born will become insane. And they will become demoniac, and afflicted with moisture, of the brain.’⁵ With this passage, we remind ourselves that, with Ptolemy, we come to the end of the classical period of learning, and enter upon the dark ages, at least in Europe.

Francesco Goya, Saturn, 1820–23, mixed method on mural transferred to canvas.

During all this time, Saturn’s dull orb, travelling with nearly imperceptible motion, marked the penultimate frontier of the universe, nested between the sphere of Jupiter and that of the fixed stars. It also continued to retain its traditional character as a bad omen. Camille Flammarion (1842–1925) said of Saturn,

The slowness of its motion and the tint of its light made it for the ancients an unlucky planet. Saturn was, indeed, considered as the gravest and slowest of stars, a god dethroned and banished into a sort of exile.⁶

This recalls an ancient myth: Saturn was the upstart who murdered and castrated his own father, Uranus, and then attempted to safeguard his throne by eating his own children. He was ultimately dethroned, along with the Titans, by Jupiter and the Olympians.

Old, baseless ideas never die; at best, they fade away. Even as late as the nineteenth century, according to Flammarion in his Popular Astronomy, the French novelist Victor Hugo (1802–1885) stated, ‘in his opinion, Saturn could only be a prison, or a hell’.⁷ Perhaps Hugo never looked at Saturn through a teles...

Table of contents

• Front Cover
• Half Title
• Title Page
• Copyright
• Contents
• PREFACE
• 1 A PALE YELLOW STAR
• 2 A STRANGE RINGED WORLD
• 3 SATURN IN DEPTH
• 4 AESTHETIC RUBBLE (THE RINGS)
• 5 CASSINI’S EPIC MISSION
• 6 SATURN BY MOONLIGHT
• 7 OBSERVING SATURN
• APPENDIX I: SATURN DATA
• APPENDIX II: SATURN RING DATA
• APPENDIX III: SATURN MOON DATA
• REFERENCES
• BIBLIOGRAPHY
• ACKNOWLEDGEMENTS
• PHOTO ACKNOWLEDGEMENTS
• INDEX