Danish perfectionist Tycho Brahe, born in 1546, just three years after Copernicus’s death, amassed a great number of Mercury observations – at least eighty-five – from his astronomical castle on the island of Hven, where he used instruments of his own design to measure the positions of each planet at accurately noted times. Inheriting this trove of information, Brahe’s German associate Johannes Kepler determined the correct orbits of all the wanderers in 1609 – ‘even Mercury itself.’
It later occurred to Kepler that although Mercury remained hard to see at the horizon, he might catch it high overhead on one of those special occasions, called a ‘transit’, when the planet must cross directly in front of the Sun. Then, by projecting the Sun’s image through a telescope onto a sheet of paper, where he could view it safely, he would track Mercury’s dark form as it travelled from one edge of the Sun’s disk to the other over a period of several hours. In 1629 Kepler predicted such a ‘transit of Mercury’ for November 7, 1631, but he died the year before the event took place. Astronomer Pierre Gassendi in Paris, primed by Kepler’s prediction, prepared to watch the transit, then erupted into an extended metaphor of mythological allusions when the event unfolded more or less on schedule and he alone witnessed it through intermittent clouds.
‘That sly Cyllenius,’ wrote Gassendi, calling Mercury a name derived from the Arcadian mountain Cyllene, where the god was born,
introduced a fog to cover the earth and then appeared sooner and smaller than expected so that he could pass by either undetected or unrecognized. But accustomed to the tricks he played even in his infancy [i.e., Mercury’s early theft of Apollo’s herds], Apollo favoured us and arranged it so that, though he could escape notice in his approach, he could not depart utterly undetected. It was permitted me to restrain a bit his winged sandals even as they fled. […] I am more fortunate than so many of those Hermes-watchers who looked for the transit in vain, and I saw him where no one else has seen him so far, as it were, ‘in Phoebus’ throne, glittering with brilliant emeralds.’*
Gassendi’s surprise at Mercury’s early arrival – around 9 a.m., compared to the published prediction of midday – cast no aspersions on Kepler, who had cautiously advised astronomers to begin searching for the transit the day before, on November 6, in case he had erred in his calculations, and by the same token to continue their vigil on the 8th if nothing happened on the 7th. Gassendi’s comment about the small size of Mercury, however, generated big surprise. His formal report stressed his astonishment at the planet’s smallness, explaining how he at first dismissed the black dot as a sunspot, but presently realized it was moving far too quickly to be anything but the winged messenger himself. Gassendi had expected Mercury’s diameter to be one-fifteenth that of the Sun, as estimated by Ptolemy fifteen hundred years before. Instead, the transit revealed Mercury to be only a fraction of that dimension – less than one-hundredth the Sun’s apparent width. The aid of the telescope, coupled with Gassendi’s sighting Mercury silhouetted against the Sun, had stripped the planet of the blurred, aggrandizing glow it typically wore on the horizon.
Over the next several decades, precise measuring devices mounted on improved telescopes helped astronomers pare Mercury close to its acknowledged current size of 3,050 miles across, or less than one three-hundredth the actual diameter of the Sun.
By the end of the seventeenth century, mystic and magnetic attractions among the Sun and planets had been replaced with the force of gravity, introduced by Sir Isaac Newton in 1687 in his book Principia Mathematica. Newton’s calculus and the universal law of gravitation seemed to give astronomers control over the very heavens. The position of any celestial body could now be computed correctly for any hour of any day, and if observed motions differed from predicted motions, then the heavens might be coerced to yield up a new planet to account for the discrepancy. This is how Neptune came to be ‘discovered’ with paper and pencil in 1845, a full year before anyone located the distant body through a telescope.
The same astronomer who successfully predicted Neptune’s presence at the outer margin of the Solar System later turned his attention inward to Mercury. In September of 1859, Urbain J. J. Leverrier of the Paris Observatory announced with some alarm that the perihelion point of Mercury’s orbit was shifting ever so slightly over time, instead of recurring at the same point in each orbit, as Newtonian mechanics required. Leverrier suspected the cause to be the pull of another planet, or even a swarm of small bodies, interposed between Mercury and the Sun. Returning to mythology for an appropriate name, Leverrier called his unseen world Vulcan, after the god of fire and the forge.
Although the immortal Vulcan had been born lame and ever walked with a limp, Leverrier insisted his Vulcan would hasten around its orbit at quadruple Mercury’s speed, and transit the Sun at least twice a year. But all attempts to observe those predicted transits failed.
Astronomers next sought Vulcan in the darkened daytime skies around the Sun during the total solar eclipse of July 1860, and again at the August 1869 eclipse. Enough scepticism had developed by then, after ten fruitless years of hunting, to make astronomer Christian Peters in America scoff, ‘I will not bother to search for Leverrier’s mythical birds.’
‘Mercury was the god of thieves,’ quipped French observer Camille Flammarion. ‘His companion steals away like an anonymous assassin.’ Nevertheless the quest for Vulcan continued through the turn of the century, and some astronomers were still pondering the whereabouts of Vulcan in 1915, the year Albert Einstein told the Prussian Academy of Sciences that Newton’s mechanics would break down where gravity exerted its greatest power. In the Sun’s immediate vicinity, Einstein explained, space itself was warped by an intense gravitational field, and every time Mercury ventured there, it sped up more than Newton had allowed.
‘Can you imagine my joy,’ Einstein asked a colleague in a letter, ‘that the equations of the perihelion movement of Mercury prove correct? I was speechless for several days with excitement.’
Vulcan fell from the sky like Icarus in the wake of Einstein’s pronouncements, while Mercury gained new fame from the role it had played in furthering cosmic understanding.
Still Mercury frustrated observers who wanted to know what it looked like. One German astronomer postulated a dense cloud layer completely shrouding Mercury’s surface. In Italy, Giovanni Schiaparelli of Milan decided to track the planet overhead in daylight, despite the Sun’s glare, in the hope of getting clearer views of its surface. By pointing his telescope upward into the midday sky, instead of horizontally during dawn or dusk, Schiaparelli avoided the turbulent air on Earth’s horizon, and also succeeded in keeping Mercury in his sights for hours at a time. Beginning in 1881, avoiding coffee and whisky lest they dull his vision, and forswearing tobacco to the same end, he observed the planet on high at its every elongation. But the pallor of Mercury against the daytime sky confounded his efforts to perceive surface features. After eight years at this Herculean task, Schiaparelli could report nothing but ‘extremely faint streaks, which can be made out only with greatest effort and attention’. He sketched these streaks, including one that took the shape of the number five, on a rough map of Mercury he issued in 1889.
A more detailed map followed in 1934, drawn as the culmination of a decade-long study by Eugène Antoniadi at the Meudon Observatory outside Paris. By his own admission, Antoniadi saw little more than Schiaparelli, but, being an excellent draughtsman and having a bigger telescope, he rendered his faint markings with better shading, and named them for Mercury’s classical associations: Cyllene (for the god’s natal mountain), Apollonia (for his half-brother), Caduceata (for his magic wand), and Solitudo Hermae Trismegisti – the Wilderness of Thrice-Great Hermes. Although these suggestions have disappeared from modern maps, two prominent ridges discovered on Mercury by spacecraft imaging are now named ‘Schiaparelli’ and ‘Antoniadi’.
Both Schiaparelli and Antoniadi assumed, given the persistence of the features they discerned
