For the majority of human history, astronomers and scholars and even astrologers (yes, definitely astrologers) have only been aware of what are called the “Classical planets.” These were the objects in the night sky (named from the Greek planetes, “wanderer”) which moved across the heavens, unlike the fixed stars, which rotated together in stately procession. Astronomers from many great cultures of the ancient world, from China to India to Persia to Greece noted these strange objects in the heavens, objects which appeared to follow regular, predictable, and sometimes lengthy paths around the sky, and they tried to explain them in a variety of ways. These five planets ultimately took the names of different gods of the Roman pantheon, names which have survived to the present day: Mercury, Venus, Mars, Jupiter, and Saturn.
To human eyes, the heavens remained largely unchanged and unexplored until the invention of the telescope, which was cunningly turned on the heavens by Galileo Galilei in the early 17th century. The opening of new observational frontiers followed, so that by the latter quarter of the 18th century, the Englishman William Herschel, when not giving music lessons, was contriving and constructing telescopes, and observing the heavens with them, in the company of his sister, Caroline. It was Herschel who, in 1781, announced the discovery of Georgium Sidus, “George’s Star,” in reference to reigning British monarch, King George III. For several reasons, however, the German astronomer Bode suggested the name which we now use for the seventh planet: Uranus (in mythology, Uranus was the father of Saturn and the grandfather of Jupiter; additionally, Urania was one of the nine Classical Muses, and she was the Muse of, among other things, astronomy).
Tom Standage’s excellent book takes up the story with the discovery of Uranus, and the work to calculate its orbital path (a remarkably difficult mathematical task in the pre-computing, pre-calculating machine era). Complex tables, giving a planet’s position in the heavens, had been known in various forms since Almagest of Claudius Ptolemy in the 2nd century CE. The point of describing the orbit of a planet is so that one knows where to look with a sextant, or later to point one’s telescope, to observe it in the night sky. It was when these predictive calculations did not match the observed position of Uranus that some astronomers began to realise that something was wrong, either with the maths, or with the planet Uranus. If the maths are correct, then only one thing can cause a giant planet at the edge of the solar system to be in the wrong place: another giant planet, still further out, and previously unobserved.
At first, some pundits suggested that comet might have struck Uranus in the second half of the 18th century, thus altering the planet’s orbit. Other suggested that the maths simply weren’t there, and that by failing to account for the influences of Jupiter and Saturn led to an error in the predicted orbit of Uranus. But some astronomers of the early 19th century were content to concern themselves with other problems. And unfortunately (or fortunately, depending on your point of view), one of these comparatively unconcerned astronomers was the British Astronomer Royal, who had charge of the Royal Observatory at Greenwich, a man called George Airy. By some accounts, Airy’s gaffe would lead to one of the more significant upsets in modern astronomy.
Standage skillfully recounts this story, with suprising immediacy, and there’s little need for me to retell the entire tale here. Cambridge astronomer John Couch Adams became interested in the problem of Uranus, and began working on the solution in earnest in 1844, at which time he implored a colleague to write to Airy at Greenwich for all of the observations which might provide the raw data Adams needed to calculate a correct orbit for Uranus. Airy replied with the data, and later expressed an interest in seeing Adams’ results. In 1845, Adams had come to an astounding result: the unexpected motion of Uranus was in fact due to a previously unknown planet. Better still, Adams had worked out its position.
In one of those perverse turns of history, however, Adams, who could be described as more a theoretician than an observational astronomer, could not persuade even his colleague, Challis, to point a large enough telescope at the right part of the sky. Even in the 19th century, it seems, professional astronomy was bound up in schedules and red tape. As Airy had also expressed an interest, Adams decided – somewhat in defiance of the protocol of the time, perhaps – to call personally on Airy at Greenwich and present him with his results. The events of that fateful 21 October 1845 make the reader wince: Adams arrived without having written first to say he was coming. Airy was at supper, and the footman would not admit Adams without an appointment. Adams sent his research notes in, rather than insisting on admission, and went on his way, hoping that Airy would read his results, make observations, and get back in touch.
It never happened.
Instead, credit for the discovery of an eighth planet, subsequently called Neptune, went to the French astronomer Urbain Le Verrier, who had been working along the same lines as Adams, and was able to convince astronomers in Berlin to look to the correct part of the sky, where they resolved the disk of Neptune for the first time.
As coda, Standage takes up the question of the modern search for planets around other stars. At time of writing just prior to the year 2000, these still numbered only a handful. Fifteen years later, the number would be nearing 2,000 known extra-solar planets. Indeed, Standage’s book represents one of the few times in reading about science that the predictions made (in this case, for more observation and discovery, including a satellite observatory hunting planets) have come out to be fundamentally correct. As Standage himself says near the end of the book:
“We live at the beginning of a golden age of planet hunting. Hardly a month goes by without the announcement that yet another new planet has been found wheeling around a distant star. Within a decade, it will be possible to draw maps of several alien solar systems, showing the orbits and characteristics of their planets, both large and small. None of these planets, however, will have been seen by human eyes. Instead, they will have been detected indirectly, through the mathematical analysis of gravitational perturbations, just as the existence of Neptune was deduced by John Couch Adams and Urbain Jean-Joseph Le Verrier in the 1840s. The discovery of Neptune is essentially the prototype of today’s discoveries of planets around other stars…”
— p. 210-11
Standage writes an excellent, compelling book, and his two works on Victorian science and technology (The Neptune File and The Victorian Internet), not to mention his book on a chess-playing automaton (The Turk), are all well worth reading, fascinatingly told, and well-documented and researched. Read this fascinating tale of how humanity went from a solar system of seven planets, to one in which there were eight. Five stars. Highly recommended.
Reviewed 16 October 2015.
Find your copy of The Neptune File at AbeBooks.com.