Secrets of the Universe

by Paul Murdin

  Despite this attempt to refine the existing Ptolemaic theory, discrepancies between the predicted and observed orbits of the planets continued to build up over time. This was particularly a problem with the planet Mars, which moves unusually quickly through an especially eccentric orbit, and whose orbit seen from Earth is therefore more than usually complex. To predict the movement of Mars accurately, astronomers were obliged to add ever more epicycles and to assign these epicyclical ‘wheels’ a variety of different sizes and spin rates.

  Although the theory of epicycles eventually did allow astronomers to calculate the movements of the planets with relative accuracy – no inconsiderable achievement – the epicycles did not seem to be grounded in real observation. All they did was make the calculations work. Moreover, the theory never seemed to be reaching a stage where it was able to provide a satisfactory unified explanation for the motions of the planets. When further epicycles had to be added, it seemed that there was no end to the arbitrariness of the theory (‘wheels within wheels’ is an expression recognizing the arbitrariness and secrecy of complex social machinery, like the astrologers’ systems of epicycles, and resonating with the biblical expression in Ezekiel 1:16).

  The situation began to change in 1543, when the Polish cleric Nicolaus Copernicus published his De revolutionibus orbium coelestium (‘On the Revolutions of the Heavenly Spheres’), proposing a radical new hypothesis to replace the Ptolemaic theory: all the planets, save the Moon, revolved around the Sun in a series of concentric orbits. Copernicus produced a remarkably accurate diagram of the Solar System that showed the order of the planets from the Sun, which he fixed by making a smooth progression from the Sun outwards of the orbital speed of each planet, with the slowest furthest from the Sun. Copernicus’s theory also simplified the calculations of planetary motions, explaining retrograde motion as a visual effect created by the orbital movement of the Earth, although he still formulated the calculations in terms of a (smaller) number of epicycles. These were astonishing thoughts – too astonishing for many, and Copernicus’s hypothesis remained just that for more than sixty years, until it was given convincing form by the brilliant and eccentric German astronomer Johannes Kepler.

  Kepler based his calculations on observations by his tutor, Tycho Brahe. Brahe was a sixteenth-century Danish astronomer. A strong Protestant, Brahe did not believe that the Earth moved, because that appeared to contradict biblical texts. Brahe had proposed that the Moon and Sun orbited the Earth, but that all the other planets revolved around the Sun. However, the accuracy of Brahe’s observations provided the data that allowed his pupil to complete the simplification of the Copernican theory, which, ironically, was anathema to Brahe’s literalist religious beliefs.

  In his Astronomia Nova (‘New Astronomy’, 1609) Kepler compared the orbit of Mars in the heliocentric Copernican planetary system, the geocentric Ptolemaic system and Tycho’s system (a compromise between the former two) – all using variations on the idea that celestial orbits were circular. On the basis of Tycho’s own data, Kepler showed that it was more accurate to use the Copernican theory to calculate the position of Mars, but only when its orbit was assumed to be an ellipse, not a circle. This was not only a more accurate basis for calculation, it was a simpler, but novel, model of how the planets moved.

  In simplifying Copernicus’s theory, Kepler produced the first recognizably modern plan of the Solar System. He abolished the concept of epicycles altogether, and in their place offered his own description of the motions of the planets. He calculated tables to predict where the planets would be in the future (the Rudolphine Tables, published in 1627), and was able satisfactorily to predict that Mercury would be aligned with the Sun in 1631 so accurately that it would pass in front of the Sun’s disc. This ‘transit of Mercury’ was witnessed by the French astronomer Pierre Gassendi. Eight years later, using Kepler’s tables and theories, the English cleric Jeremiah Horrocks calculated a transit of Venus and, with the merchant William Crabtree, actually saw the transit with his own eyes, just before sunset on 24 November 1639 (plate II). They were both ecstatic with their discovery: Horrocks wrote that, ‘rapt in contemplation, [Crabtree] stood for some time motionless, scarcely trusting his own senses, through excess of joy; for we astronomers are of a womanish disposition and are overjoyed with trifles…’.

  These observations were convincing proof that Kepler’s calculations of the planets were the most accurate ever produced. How did he do it? Kepler showed that the orbits of the planets about the Sun, particularly the orbit of Mars, were simple ellipses. He also demonstrated that there were basic relationships between the motions of the planets and the sizes of their orbits – his ‘laws’ of planetary motion. Not only did these interrelationships provide a grip on the calculations, they removed a lot of the arbitrariness in the arrangement of the planets. Kepler attempted an explanation of these relationships by proposing that there existed a ‘magnetic’ virtue or force between the Sun and the planets, including the Earth and Moon. In so doing, he prepared the way for the identification of the force of gravity by Isaac Newton.

  The theory of epicycles had seemed arbitrary to astronomers, but the implications of the Copernican system were nothing short of startling. Far from lying stationary at the centre of the Universe, the Earth is moving. Our planet spins on its axis, it orbits around the Sun, and it wobbles as it orbits. Despite the lack of sensation its gyrations produce in us, the Earth’s motions are dizzying as a dance.

  DISCOVERING THE SOLAR SYSTEM

  The Orbits of Comets

  Disasters, sun-grazers and the ‘Lady’s Comet’

  …Now we know

  the sharply veering ways of comets, once

  A source of dread, no longer do we quail

  Beneath the appearance of bearded stars.

  Edmond Halley, Dedication to Newton’s Principia, 1686

  Unlike stars and planets, comets appear without warning and move rapidly and erratically across the sky. In early cultures, comets were regarded as harbingers of doom because of this behaviour, but in the centuries following the Enlightenment, this same unpredictability has become an irresistible attraction for comet-hunters. One of the earliest and most famous comet-hunters was an eighteenth-century English woman, Caroline Herschel, who laboured under extreme conditions to discover fourteen comets and was rewarded with a royal stipend for her pioneering efforts. She described in her memoirs her first discovery and the change of status it earned her:

  On the 1st of August 1786 I found an object very much resembling in colour and brightness the 27th nebula of Messier’s catalogue, with the difference however of being round. I suspected it to be a comet, but, a haziness coming on, it was not possible to entirely satisfy myself as to its motion until the following evening, and thus to confirm it as my first comet discovery. My brother William was on his return commanded to show it to the King, who said that it was very small and had nothing striking in its appearance. Miss Fanny Burney acclaimed my comet as the first ‘lady’s comet’. It gave great pleasure to the ladies of the Court. I heard that Princess Augusta was in particular desirous that the lady guests should view it, calling them from the card-table.

  Partly as a result of this, and in specific consequence of a recommendation made to the King by Sir Joseph Banks, President of the Royal Society, a salary of 50 pounds per year was settled on me as an assistant to my Brother. In October 1787 I received the first quarterly instalment of 12 pounds 10 shillings. It was the first money in all my lifetime that, at the age of 37 years, I ever thought myself at liberty to spend to my own liking.

  Comets are small, dark bodies in the Solar System, which are hard to see when far from the Sun. They have a solid part, the ‘nucleus,’ which is composed of both ices and solid material (plate IX). When a comet comes close to the Sun, its nucleus melts, vaporizes and becomes ‘active’. It develops a bright, dusty atmosphere, the ‘coma’, and ‘tails’ of gas and dust, which reflect more sunlight (plate XII). Coupled with th
e speed of the comet as it approaches the Sun, this means that comets can spring into sight, unexpectedly and suddenly.

  For a long time, comets were interpreted superstitiously because, in an age when the cyclical movements of the stars were thought to control events on Earth, they arrived sporadically and unpredictably. In his 1665 book De Cometis, English astrologer John Gadbury warned that comets were ‘threatening the world with Famine, Plague and War: To Princes, Death! To Kingdoms, many Crosses; To all Estates, thunder, lightning and earthquakes, inevitable Losses! To Herdsmen, Rot; to Plowmen, hapless Seasons; To Sailors, Storms, To Cities, Civil Treasons!’ The word ‘disaster’ to designate events like these is a linguistic fossil left over from this era, meaning literally ‘bad things from the stars’; likewise ‘influenza’ is a disease induced, it was believed, by the influence of comets.

  Anyone who is looking in the right place at the right time can discover a comet, which will usually be named for its discoverer. The most successful comet discoverers are of course those who dedicate lots of time to systematic searches – one highly successful Japanese comet-hunter is said to have dedicated the rest of his lifetime to finding comets after a deathbed vow to his dying father. The usual method is to sweep a telescope or binoculars from side to side to examine a section of the sky, looking for objects that are fuzzy. Nebulae, galaxies and star clusters can be eliminated by reference to a catalogue or atlas. French comet-hunter Charles Messier compiled a list of such objects that might be mistaken for a comet. Published between 1774 and 1781, this became known as the Messier Catalogue, and is still in use. The clinching difference is that a comet moves in the sky, whereas nebulae and galaxies are stationary.

  Armchair astronomers have discovered over 3,000 comets using data from the Solar and Heliospheric Observatory (SOHO) satellite; the satellite was launched late in 1995 and has been gazing constantly at the Sun for more than twenty years. Comets show in SOHO’s camera as they graze the surface of the Sun, some of them approaching the Sun but not seen to leave (i.e. melting). Most of them are members of the Kreutz sungrazing family. In 1888, Heinrich Kreutz discovered that a number of comets have similar orbits passing near the Sun. They come from a single comet that has disrupted into many fragments, which have further broken into small pieces about 10 metres in size – the comets seen in SOHO’s cameras. Armchair astronomers use online archives to scan SOHO pictures day by day for new discoveries of these fragments before they disappear forever.

  The Satellites of Jupiter

  Shattering the crystal spheres

  … behold! afar,

  Four radiant Moons surround th’ imperial Star,

  Large as our boasted World; whole silver Light

  His regions visit in the Gloom of Night;

  Nor this the Fancy of deluded Eyes;

  Mark’d are their Periods thro’ sublimer skies:

  Oft does th’ Astronomer with his Tube display,

  And view ’em in Eclipse with pleas’d Survey;

  To this curious Genius Knowledge owes,

  Of Light’s swift Motion, and its Measure knows.

  Moses Browne, Sunday Thoughts: The Works and Rest of the Creation, 1752

  The belief that the heavens orbited the Earth on a series of crystal spheres had survived for 2,000 years, but shortly after the invention of the telescope it shattered in an instant. A few years after Kepler and Copernicus had used their calculations of planetary movements to develop the theory that the Earth and the planets orbited the Sun, Galileo Galilei saw the irrefutable evidence with his own eyes: mountains on the Moon, and four mysterious stars near Jupiter.

  In 1608 Hans Lippershey (or Lipperhey), a German-born optician living in Middelburg, Zeeland, applied for a patent from the Dutch government for ‘a certain device by means of which all things at a very great distance can be seen as if they were nearby, by looking through glasses which he claims to be a new invention’. One story is that Lippershey discovered the principle of the telescope when two children were playing with lenses in his shop and looked through two at the same time, one held up behind the other, exclaiming with surprise at what they saw.

  In Venice later that year the learned monk Pietro Sarpi, recently retired from high office in the Venetian government, heard about the patent application, and when a few months later Sarpi was visited by a protégé, Galileo Galilei, they discussed Lippershey’s invention. Galileo was professor of mathematics at Padua University in the Venetian republic and he realized the importance of the invention to Venice, a maritime power. He had been seeking a salary increase and in 1609 he made a prototype of a telescope that, through Sarpi, he brought to the attention of the Venetian authorities. Galileo was able to describe ships approaching the port of Venice before they could be seen at all with the unaided eye. The value of the invention was clear and his salary was doubled, although there were strings attached to his promotion: he would never again get a salary increase, and could never move from Padua. This resulted in him looking elsewhere for a new position, a prize that the momentous astronomical discoveries he was about to make with his telescope would eventually win for him.

  In the winter of 1609–10 Galileo observed the Moon and saw bright spots in the shadowed part of its surface that gradually grew in size and merged with the illuminated area as the month progressed. He correctly interpreted the bright spots as mountaintops that had caught the first rays of the Sun, moving into full sunlight as the Moon rotated. He measured the heights of these lunar mountains using their shadows, calculating one at 6 kilometres high. He saw that some mountains were arranged in straight mountain ranges and others in circles, surrounding craters (plate VIII). He discovered that the Moon was not a smooth, perfect sphere, as taught by Aristotle and Ptolemy; rather, its surface was ‘rough and uneven, and just like the surface of the Earth itself’.

  In England, a mathematician and cartographer, Thomas Harriott, used a telescope to observe the Moon and drew it in August 1609, several months before Galileo. He also discovered sunspots, but he never published his astronomical work and had no influence on the development of science whatsoever. He thus remains largely unknown for this achievement. He is, however, remembered as one of the founders of the Virginia colonies, having visited Roanoke Island in 1585–86 and written an influential report about its agricultural and mineralogical potential, and its Algonquian inhabitants.

  On 7 January 1610 Galileo drafted a letter to an unknown recipient describing a further momentous discovery that he had made the previous night: ‘And besides my observations of the Moon, I have observed the following in other stars. First that many fixed stars are seen with the telescope, which are not otherwise discerned; and only this evening, I have seen Jupiter accompanied by three fixed stars, totally invisible by their smallness….’

  At first Galileo did not think there was anything remarkable about these stars: a triplet arranged in a straight line through Jupiter, two on one side and one on the other. But, according to his observation journal, when he came to look at Jupiter again on the 8th of January the three stars were all on the other side of Jupiter. Presumably Jupiter had moved from its previous position. The 9th was cloudy, and on the 10th and 11th there were two stars only, on one side of Jupiter, with the third conjoined with Jupiter (or so Galileo speculated). On the 12th the three stars were again arranged differently: two on one side of Jupiter, one on the other. ‘It appears that around Jupiter there are three moving stars invisible to everyone up to this time.’ On the 13th, Galileo realized that there were in fact four little stars; he saw them all again on the 15th. Galileo seems originally to have thought that the stars were moving back and forth in a straight line. But if this was the case, how did they pass one another? Suddenly Galileo realized that the four ‘stars’ were actually in orbit around Jupiter. In an instant, the four tiny stars had disproved the 2,000-year-old Ptolemaic theory that every celestial body orbited around the Earth.

  The orbital motion of the satellites around Jupiter was very like th
e orbital motion of the planets around the Sun as expressed in the Copernican theory, and also very like the motion of the Moon around the Earth. It became clear that the Earth was just like the other celestial bodies, and that it was not the centre of the Universe.

  Galileo wrote up his discoveries in January and February 1610 in a book called Sidereus Nuncius (‘Starry Messenger’). He dedicated the book to Cosimo II de’ Medici. The Medici family was a powerful and wealthy family that ruled Florence from the thirteenth to the seventeenth centuries. Galileo had tutored Cosimo in mathematics, and at the age of nineteen, in 1609, the young man succeeded to the title of Grand Duke of Tuscany on the death of his father, Ferdinand. To flatter the family, Galileo christened the satellites of Jupiter ‘the Medicean stars’, although this term did not stick. As a reward and as a mark of esteem, in 1610 Cosimo appointed Galileo as his Philosopher and Mathematician for his lifetime, and Chief Mathematician at the University of Pisa.

  The moons and their shadows pass in front of one another and provide a spectacular series of occultations and eclipses. Sometimes the eclipses appear to run ahead and then behind schedule. The English Reverend Moses Browne drew on the Danish astronomer Ole Rømer’s interpretion of this phenomenon in his poem, at the head of this chapter. Rømer realized that the variations had to do with the speed of the light coming from Jupiter to Earth over a distance that changed as the planets revolved in their orbits. His measurements of these intervals enabled him to make the first calculation of the speed of light in 1676. The fact that light travelled with a velocity was a discovery with far-reaching consequences, leading eventually to Einstein’s theory of relativity.