In the Shadow of Mercury

by Andrew Crumey

Review of The Hunt for Vulcan, by Thomas Levenson. Literary Review, December 2015.

If aliens somehow manage to tap in to our internet then I wonder what they'll make of the Vulcan Language Institute. Its website proclaims, "In addition to language lessons and many, many dictionaries with specialized terminology, we have information on noted Vulcans, Vulcan history etc." The planet is, of course, the fictional home of Star Trek's Mr Spock, but fans have been developing the language since 1980 and it now has a vocabulary of 12,000 words.

Levenson's book is about a fictional planet, but not Spock's, whose pop-culture universe sadly doesn't get a look in. In the nineteenth century there was believed to be a planet inside the orbit of Mercury, and because it was so close to the Sun and presumably very hot, it was named Vulcan after the Roman god of fire. The planet was first of all predicted theoretically, then supposedly observed, and finally killed by Einstein. Those are the three acts of Levenson's narrative.

The story begins at the outermost part of the solar system. Careful measurements of the planet Uranus showed its orbit to be deviating from the one predicted by Newton's law of gravitation. A more distant, unseen planet was tugging it. Two men tackled the mathematical problem independently: Urbain Le Verrier in France, and John Couch Adams in England. It was a formidable task. The mass and distance of the mystery object were unknown, as well as the tilt and shape of its path around the Sun. Both scientists had to make guesses; a clue was "Bode's law", a hypothesis about planetary orbits that has since turned out to be as fictitious as Spock's ears. Even so, both were able to pin down the new planet's position well enough for astronomers to start looking for it. Le Verrier's prediction was closest and his colleagues in Germany were quickest off the mark, discovering Neptune in September 1846. British astronomers continue to fly the flag for Adams, but for the 35-year-old Le Verrier it was the pinnacle of an illustrious career. When the Eiffel Tower was constructed half a century later, the names of seventy-two of France's greatest scientists were engraved on it, and Le Verrier's was among them.

Le Verrier turned his attention to Mercury, whose orbit also showed an irregularity. The planet's elliptical path doesn't bring it back to the same place; instead the orbit slowly swings, tracing a pattern like petals around a flower. The explanation seemed to be the influence of all the other planets beyond Mercury, but when Le Verrier did the maths he found that this could not account for the full effect. In 1859 he announced a very small excess that might be explained by the pull of another planet inside Mercury's orbit.

At the same time, a French village doctor was on the verge of unexpected fame. When not treating patients, Edmond Lescarbault liked to observe the Sun using a specially filtered telescope. He was on the lookout for sunspots - small dark patches now known to be caused by kinks in the Sun's magnetic field - but in March 1859 he saw what appeared to be a tiny dark disc moving at an unusual pace across the Sun's face. Belatedly hearing of Le Verrier's theory, he wrote to the astronomer in December to report it. Le Verrier immediately made the journey from Paris to see him. Lescarbault's methods were crude: his watch had no second hand, and he had missed the precise moment when the object crossed the Sun's edge, so his timings were as unreliable as his estimate of the object's size. But Le Verrier was convinced, and in January 1860 he announced the discovery. Within weeks the new planet was dubbed Vulcan.

After that, many people claimed to see it, while others maintained it was an illusion. A school textbook of the time, Lockyer's Elements Of Astronomy , said Vulcan was "suspected - it is not proved". The best chance for a sighting would be during a total eclipse, when the planet might be found twinkling beside the blackened Sun. In July 1878, ten months after Le Verrier's death, two leading American astronomers grasped the opportunity. Lewis Swift was in Colorado while James Craig Watson set up his telescope at Separation, Wyoming; "a fly-speck of a place" according to Levenson, chosen because it was on a rail route and offered a hope of clear weather. During the two minutes of totality, Swift and Watson both thought they saw something, which they duly announced. It brought a hostile response from rival astronomer Christian Peters, who accused Watson of incompetence and Swift of cheating. Both made dignified replies, but Vulcan's credibility was weakened. Even if it existed, it could not be large enough to explain Mercury's anomalous orbit.

Other ideas were suggested, such as adjusting Newton's laws by a tiny amount to make the numbers fit, but it was Einstein who found the answer in 1915 with his general theory of relativity. Newton's laws are only an approximation; the Sun's bending of spacetime explains Mercury's orbit. Further vindication came four years later, during another eclipse, when starlight grazing the Sun's edge was shown to bend.

Levenson ably tells the three-act story, whose first and third parts are familiar episodes of scientific history. The middle, which best fits the book's title, is intriguing and entertaining. I do still wonder how much the Star Trek creators knew about it.