Secrets of the Universe

by Paul Murdin

  In the space age, Jupiter’s satellites can be seen as individual worlds, each with its own different character (plate III). Io is pockmarked with volcanoes and scoured by lava flows. Europa is completely covered with cracked floes of ice. Ganymede is made of a mixture, half and half, of rock and ice, its surface plastic, mobile and wrinkled under tectonic forces. Callisto is heavily cratered, like Ganymede, and indeed, both look very like our own Moon.

  The Phases of Venus

  Revealing the shape of the Copernican System

  In questions of science the authority of a thousand is not worth the humble reasoning of a single individual.

  Attributed to Galileo by François Arago in his Eulogy to Galileo, 1874

  Having discovered that the ‘Medicean Stars’ orbited Jupiter rather than the Earth, Galileo turned his telescope towards the planet Venus. He discovered that Venus had phases similar to the Moon, which proved that Venus was orbiting the Sun and that Copernicus’s theory of the Solar System was correct. Galileo’s discoveries were at first acclaimed by astronomers, but then perceived as a threat to the biblical worldview, earning him persecution from the Church during his lifetime. History now recognizes Galileo as one of the world’s great scientists.

  In 1610, in Pisa, after discovering the satellites of Jupiter with his new telescopes in Venice, Galileo was finally able to observe the planet Venus – when he had first assembled his telescope, Venus had been too near to the Sun to be examined closely. He saw that Venus had phases like the Moon. When the planet was at its greatest distance from the Sun, it looked like a half-moon, with the bright side facing in the direction of the Sun. As it approached the Sun again, its phase either increased towards full circular illumination, or narrowed to a thin crescent. Galileo reported his discovery using a Latin anagram, which he sent to Kepler. The coded sentence (in not very elegant Latin) was ‘Haec immatura a me iam frustra leguntur o.y.’ It can be translated as ‘Things not ripe for disclosure are read by me’, but its letters can be rearranged to read ‘Cynthiae figuras æmulatur mater amorum’. This means ‘The shapes of Cynthia [the Moon] are emulated by the mother of loves [Venus]’.

  The anagram method of making a coded announcement was a device used in the seventeenth century to establish priority for a discovery – if the announcement was made straightforwardly and was promulgated at the slow pace of communication of the time, someone who got the announcement earlier than anyone else could falsely claim the discovery as his own, and pretend surprise when the announcement became more widely disseminated. The terse nature of the anagram also bought time and room for manoeuvre for the discoverer as a hedge against an incomplete or misinterpreted discovery.

  Galileo’s discovery of the phases of Venus showed immediately that Venus travelled around the Sun in an orbit that lay inside the orbit of the Earth. When Venus was on the far side of the Sun, its face was fully illuminated, like the Full Moon. When Venus was moving between the Earth and Sun, its unilluminated rear partially faced the Earth and it showed only a thin crescent of light. This geometry was exactly the arrangement that Copernicus had hypothesized in his model of the Solar System, and could not be reconciled with the Ptolemaic theory that Venus orbited the Earth in a crystal sphere between the Earth and the Sun.

  Galileo’s observations also confirmed another of Copernicus’s predictions: that the size of Venus would appear to change as the planet came closer to Earth and got further away. Galileo saw that Venus did indeed change size – it appeared smallest when it was at its most distant, beyond the Sun, and showing a full face, and four times larger when it was crescent-shaped and at its closest approach to Earth.

  As Galileo himself realized, this discovery was crucial in confirming Copernicus’s theory of the Solar System. In principle, the phases of Venus were still consistent with Tycho Brahe’s compromise between the Copernican theory and the Ptolemaic theory. Brahe had proposed in 1583 that the planets orbit the Sun, which itself orbits the stationary Earth. However, Galileo dismissed Brahe’s theory (also called the ‘Tychonic theory’) because it assumed that the Sun caused the planets to move, while leaving the Earth stationary and unaffected, even though some planets were evidently bigger than the Earth.

  Galileo knew that his discovery would not be readily accepted. On New Year’s Day in 1611 he sent to the Tuscan ambassador in Prague the solution to the Venus anagram, explaining that the phases meant that the planet must orbit around the Sun: ‘…something indeed believed by the Pythagoreans, Copernicus, Kepler and myself, but not proved as it is now. Hence Kepler and other Copernicans may glory in their successful theories, although as a result we will be thought to be fools by most bookish philosophers, who will regard us as men of little understanding or common sense.’

  The consequences for Galileo were actually much worse than this. His discoveries were acclaimed by astronomers and even sophisticated churchmen like Cardinal Maffeo Barberini (later Pope Urban VIII), but he was denounced to the Inquisition as a heretic by Tommaso Caccini, a Dominican friar, and cautioned by Cardinal Bellarmine to treat the Copernican theory as a hypothesis only. The telescope was derided as a device capable of making things appear in the sky that were not actually there. (Galileo said that he would pay 10,000 scudi – ten times his annual salary – to anyone who could make a telescope that showed satellites around one planet but not the others.) He was told in 1616 not to advocate or teach Copernican astronomy except as a hypothesis, but nevertheless continued to publish his scientific work, and evidently pushed the boundaries of church tolerance too far. Galileo was put on trial in Rome for teaching, contrary to Scripture, that the Earth moved. In 1633 he was convicted and forced to recant:

  I, Galileo, son of the late Vincenzio Galilei, Florentine, aged seventy years, arraigned personally before this tribunal and kneeling before you…swear that I have always believed, do believe, and by God’s help will in the future believe all that is held, preached, and taught by the holy Catholic and apostolic Church.…I have been pronounced by the holy office to be vehemently suspected of heresy, that is to say, of having held and believed that the Sun is the centre of the world and immovable and that the Earth is not the centre and moves.…I abjure, curse, detest the aforesaid errors and heresies…and I swear that in future I will never again say or assert, verbally or in writing, anything that might furnish occasion for a similar suspicion regarding me.

  He was thus banned from publishing any future scientific discoveries and placed under house arrest until his death in 1642.

  Uranus

  The first new planet

  It has generally been supposed that it was a lucky accident that brought the planet to my view; this is an evident mistake. In the regular manner that I examined every star of the heavens, not only of that brightness but many far inferior, it was that night its turn to be discovered. I had gradually perused the Great volume of the author of Nature, and was now come to the page which contained a seventh Planet. Had business prevented me that evening I must have found it the next, and the goodness of my telescope was such that I perceived its visible planetary disc as soon as I looked at it. And by the application of my micrometer I determined its motion in a few hours.

  William Herschel on the discovery of Uranus

  The only planets that were known to the astronomers of antiquity were those that could easily be seen with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. This changed in the eighteenth century when, armed with a powerful home-made telescope, a music teacher from Bath, England, discovered Uranus, the first new planet identified in recorded history, and the third-largest in the Solar System.

  William Herschel was born in 1738 in Hanover, at a time when it was a British possession. He followed his father into a career as an oboeist in the band of the Hanoverian Guards. They fought as part of the British army at the battle of Hastenbeck in 1757, after which the younger Herschel escaped to England, eventually settling in Bath. He enjoyed great success as a music teacher to society ladies and as a
concert artist. He was eventually joined in Bath by his sister, Caroline, who not only kept house for him and tried to defend him against predatory widows but also accompanied him, singing, in his concerts. William’s father had been interested in astronomy and William also studied the subject, forming an ambition to see the heavens with his own eyes. He made telescopes, casting and grinding the mirrors himself in the basement of his house, designing and forming the tubes from wood and tin, and erecting the telescopes on the garden lawns of his various houses. Herschel then set out systematically to ‘review’ the entire sky with his telescope, inspecting every star and even the spaces between them.

  On 13 March 1781 he discovered a ‘curious either nebulous star or perhaps a comet’. Tracking it over the next hours and days, he found that it moved and could not be a star. Over the next few weeks, the curious object was also observed by astronomers at Greenwich and Oxford, and proved to be in a near-circular orbit outside Saturn’s, rather than the highly eccentric orbit crossing the Solar System that would be expected of a comet. Furthermore, there was no trace of a fuzzy coma or tail, as a comet would have, and the object had a circular disc like a planet. It was, indeed, a planet – the first discovered since antiquity. As a planet of sixth magnitude in brightness, Uranus is marginally visible with the unaided eye and easily seen in modest telescopes. It was mistakenly recorded several times as a star before Herschel’s discovery.

  Herschel was invited to London to tell King George III of the new planet. He was asked to erect a telescope at Windsor Castle in order to show astronomical sights to the royal household. He was given a patronage appointment as the King’s Royal Astronomer and given a stipend – his sister Caroline, who assisted him with his observations and discovered the first comet found by a woman, was later given a stipend too, of half as much. In recognition of this royal patronage, William Herschel suggested that the new planet should be named after the era of its discovery ‘in the reign of King George III’ as ‘Georgium Sidus’ – the Georgian star (or planet). This name was used for a time in England, but never in any other country, and it was the suggestion of the mythological name Uranus by the German astronomer Johann Bode that eventually stuck.

  Uranus is the third-largest of the planets in the Solar System, and ranks fourth in mass. It has a characteristic blue-green appearance due to a high layer of clouds of methane ice. It has a ring system, discovered in 1977 when high-speed photometers were used to observe a star that, by chance, was being occulted (passed in front of) by Uranus, with the intention to study how the star’s light faded in the planet’s atmosphere. The starlight was unexpectedly blocked by the unknown rings. The rings were first imaged in 1986 by the Voyager 2 spacecraft.

  Uranus has five bright satellites and more than a dozen fainter ones. The satellites provided the first evidence of the unusual tilt of Uranus. They orbit around Uranus’s equator and show that its pole is tilted by more than 90 degrees. One theory is that Uranus originally had a more normal tilt, but that, late in its formation process, it was struck by an Earth-sized planetesimal that knocked it over. Because its rotational axis stays fixed in direction while Uranus orbits around the Sun, its polar regions are sometimes pointing directly towards the Sun and sometimes at right angles to it. By contrast, the Earth’s polar regions are never presented directly to the Sun, although they do tilt a little because the Earth’s axis is tilted by 23½ degrees to its orbit. This small tilt is the reason for the pattern of the Earth’s seasons. The differences in solar heating during Uranus’s orbital ‘year’ are much more extreme than at Earth, and its seasons are peculiar – for example, during its summer time, when the Sun is always above its horizon, one of its polar regions will be hotter than the equatorial regions ever are.

  Neptune

  The planet discovered by the pen

  The method pursued by M. Le Verrier totally differs from all previous attempts of geometers and astronomers. The latter have sometimes accidentally found a movable point, a planet, in the field of their telescopes. M. Le Verrier perceived the new body without the necessity of casting a single look towards the heavens. He saw it at the end of his pen; he determined by the mere force of calculation the place and approximate magnitude of a body situated far beyond the hitherto known limits of our planetary system, of a body the distance of which from the sun exceeds 2,800 millions of miles, and which, seen in our powerful telescopes, barely exhibits a sensible disc.

  François Arago, Comptes Rendu, 1846

  While Uranus was discovered with a telescope, Neptune was discovered with a pen and paper – twice. Two very different astronomers – the Frenchman Urbain Le Verrier and the Englishman John Couch Adams – independently tried to explain why Uranus was being pulled off course in its orbit. Both sets of calculations pointed to a previously unknown planet, making Neptune the first planet to be located through mathematics rather than by direct observation of the heavens.

  After the discovery of Uranus by William Herschel in 1781, several previous observations of the planet had been found – made and recorded by astronomers who had not recognized it as a planet. By 1830 these records enabled an accurate orbit to be calculated for Uranus. At the same time, it became clear that Uranus was departing from this orbit. The Director of the Paris Observatory, François Arago, suggested to his colleague Urbain Le Verrier that a previously unseen planet was pulling the planet off track. In 1845 Le Verrier calculated the expected position. He used Bode’s Law to assume the distance of Neptune from the Sun, although the accuracy of Bode’s Law actually breaks down after Uranus. Le Verrier sent his prediction to Johann Galle, an astronomer at the Berlin Observatory. Galle, together with his assistant Heindrich D’Arrest, began a search on the same night that they received the letter, 23 September 1846.

  At D’Arrest’s suggestion, Galle used the latest star chart of the area, which had only just been produced. Within thirty minutes they had identified a star that was not on the map. They confirmed that it was the new planet on the following night by its motion relative to the other stars. Galle wrote to Le Verrier, saying, ‘Monsieur, the planet of which you indicated the position really exists’. Le Verrier replied, ‘I thank you for the alacrity with which you applied my instructions. We are thereby, thanks to you, definitely in possession of a new world.’

  Meanwhile, in England, the Scottish mathematician Mary Somerville had suggested to a young Cambridge student, John Couch Adams, that an unknown planet was affecting the orbit of Uranus. With a letter of introduction from James Challis, Director of the Cambridge Observatory, who had been impressed by Adams’s preliminary calculations regarding the position of the hypothetical planet, Adams applied to George Airy, the Astronomer Royal, for research assistance. However, due to his youth, humble background and reticent manner – and perhaps also due to Airy’s unapproachable character – Adams twice failed to secure an interview with Airy. Airy dismissed the discovery of a new planet as not the job of the Greenwich Observatory, and told none of his colleagues about Adam’s prediction. He passed the project back to Challis, who began a somewhat half-hearted search for Neptune on the basis of Adams’s calculations. But Challis did not have the same up-to-date charts as Galle, and had to compare successive observations of the same area of the sky to see if there was a star that moved.

  Hobbled by a rigid institutional hierarchy and outdated materials, Adams and Challis had been overtaken by the brisk efficiency of Le Verrier and Galle, although Adams’s calculations, which he shared only with Challis and Airy, had accurately predicted the position of Neptune eight months before Le Verrier’s findings were published. When Airy and Challis were forced to justify their delay in following up Adams’s calculations, Adams is said to have reacted with characteristic modesty, offering his congratulations to Le Verrier without a trace of bitterness – as the president of the Royal Astronomical Society it was he who handed its gold medal to Le Verrier in 1875. Eventually both Adams and Le Verrier would be credited jointly with the discovery of Neptune.
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  After Neptune had been located, William Lassell, a wealthy Liverpool brewer and amateur astronomer, was one of the first to inspect the new planet with a large telescope. According to family lore, Lassell just missed out on being the discoverer of Neptune, a letter communicating Adams’s prediction having been accidentally destroyed by an overzealous maid clearing away and burning rubbish. When the coordinates of ‘Le Verrier’s planet’ were published in The Times, Lassell rushed to examine its position in the sky with his telescope. He saw that Neptune had a distinct disc, confirming its identification as a planet. During the same observation, he also discovered Neptune’s satellite, Triton. Triton orbits Neptune backwards, and at a steep angle, suggesting that it was not formed together with Neptune, but was a passing planet that Neptune captured.

  The naming of Neptune generated as much controversy as its discovery. In keeping with the custom for naming celestial features after Roman mythological characters, Galle suggested that the new planet should be called ‘Janus’, and Challis proposed ‘Oceanus’. Le Verrier, eager to put his own stamp on ‘the planet exterior to Uranus’, first proposed ‘Neptune’ and later ‘Le Verrier’, but in the ensuing dispute over who should be credited with the planet’s discovery, the latter suggestion received fierce opposition outside of France. By the end of the year the more neutral ‘Neptune’, the name of the Roman god of the sea, had become the accepted usage.

  Neptune is the outermost of the four giant planets, forming a pair with Uranus. It rotates quickly and is slightly flat (or ‘oblate’). Curiously it emits more than twice as much energy as it receives from the distant Sun. The excess energy is generated by the cooling of Neptune’s hot interior. Neptune and its half-dozen satellites were explored by the Voyager 2 fly-by in August 1989, which discovered Neptune’s ‘Great Dark Spot’, thought to be a hole in the methane atmosphere large enough to fit the entire Earth. Voyager also obtained images of Triton. They showed a bright south polar cap. North of the polar cap, a rugged terrain, reminiscent of the skin of a cantaloupe melon, is cross-cut by a pattern of intercepting ridges, left from some past tectonic event.