The Planets. Dava SobelЧитать онлайн книгу.
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 over long hours of observation, that only one side of Mercury ever came into view. They thought the Sun had locked the little planet into a pattern that flooded one of its hemispheres with heat and light while leaving the other in permanent darkness. Likewise many of their contemporaries and most of their followers up to the mid-1960s believed that Mercury maintained eternal ‘day’ on one side and ‘night’ on the other. But the Sun constrains the rotation and revolution of Mercury according to a different formula: the planet spins around its axis once every 58.6 days – a rate rhythmically related to its orbital period, so that Mercury completes three turns on its axis for every two journeys around the Sun.
The 3:2 pattern affects observers on Earth by repeatedly offering them the same side of Mercury six or seven apparitions in a row. Schiaparelli and Antoniadi indeed beheld an unchanging face of Mercury throughout their studies, and must be forgiven for reaching the wrong conclusion about its rotation, since the planet’s behaviour indulged them in their error.
Throughout the twentieth and into the twenty-first century, Mercury has continued to be a difficult target. Even the Hubble Space Telescope, orbiting above the Earth’s atmosphere, avoided looking at Mercury, for fear of pointing its delicate optics so dangerously close to the Sun, and only one spacecraft has so far braved the hostile heat and radiation of the near-Mercury environment.
Mariner 10, Earth’s emissary to Mercury, flew by the planet twice in 1974 and once more in 1975. It relayed thousands of pictures and measurements of a landscape riddled with crater holes, from small bowls to giant basins. Light or dark trails of debris marked the places where newer assaults had overturned the rubble of the old. Lava that flowed among the impact scars had smoothed over some of the depressions, but overall poor battered Mercury preserved a clear record of the era, ended nearly four billion years ago, when leftover fragments of the Solar System’s creation menaced the fledgling planets.
The most violent attack on Mercury inflicted a wound eight hundred miles wide, which has been named Caloris Basin (‘the Basin of Heat’). The mile-high mountains on Caloris’s rim must have sprung up in response to the massive impact explosion that excavated the Basin, and all around the mountains, further signs of disturbance lay in ridges and rough ground rippling out for hundreds of miles. The collision at Caloris also sent shock waves clear through Mercury’s dense, metallic interior, to set off quakes that lifted the crust on