The Planets. Dava Sobel
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of eloquence (having given Pandora the gift of language), as well as of cunning, knowledge, luck, roads, travellers, young men in general, and herdsmen in particular. His snake-entwined wand, the caduceus, has invoked fertility or healing or wisdom over the ages.
Mercury and his fellow travellers called attention to themselves by moving among the fixed stars, which earned them the name ‘planetai’, meaning ‘wanderers’ in Greek. The orderliness of their motions brought ‘cosmos’ out of ‘chaos’ in the same language, and inspired an entire lexicon for describing planetary positions. Just as the gods’ names still cling to the planets, Greek terms such as ‘apogee’, ‘perigee’, ‘eccentricity’ and ‘ephemeris’ endure in astronomical discussions. The first observers to coin such words fill a roster of ancient heroes, from Thales of Miletus (624–546 BC), the founding Greek scientist who predicted a solar eclipse and questioned the substance of the universe, to Plato (427–347 BC), who envisioned the planets mounted on seven spheres of invisible crystal, nested one within the other, spinning inside the eighth sphere of the fixed stars, all centred on the solid Earth.* Aristotle (384–322 BC) later raised the number of celestial spheres to fifty-four, the better to account for the planets’ observed deviations from circular paths, and by the time Ptolemy codified astronomy in the second century AD, the major spheres had been augmented further by ingenious smaller circles, called ‘epicycles’ and ‘deferents’, required to offset the admitted complexities of planetary motion.
‘I know that I am mortal by nature, and ephemeral,’ says an epigraph opening Ptolemy’s great astronomical treatise, the Almagest, ‘but when I trace at my pleasure the windings to and fro of the heavenly bodies I no longer touch earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia, food of the gods.’
In Ptolemy’s model, Mercury orbited the stationary Earth just beyond the sphere of the Moon. The impetus for motion came from a divine force exterior to the network of spheres. More than a millennium later, however, when Copernicus rearranged the planets in 1543, he argued that the mighty Sun, ‘as though seated on a royal throne’, actually ‘governs the family of planets’. Without specifying the force by which the Sun ruled, Copernicus ringed the planets round it in order of their speed, and set Mercury closest to the Sun’s hearth because it travelled the fastest.
Indeed, Mercury’s proximity to the Sun dominates every condition of the planet’s existence – not just its tantivy progress through space, which is all that can be easily gleaned from Earth, but also its internal conflict, its heat, heaviness, and the catastrophic history that left it so small (only one-third Earth’s width).
The pull of the nearby Sun rushes Mercury around its orbit at an average velocity of thirty miles per second. At that rate, almost double the Earth’s pace, Mercury takes only eighty-eight Earth-days to complete its orbital journey. The same Procrustean gravity that accelerates Mercury’s revolution, however, brakes the planet’s rotation about its own axis. Because the planet forges ahead so much faster than it spins, any given locale waits half a Mercurian year (about six Earth-weeks) after sunrise for the full light of high noon. Dusk finally descends at year’s end. And once the long night commences, another Mercurian year must pass before the Sun rises again. Thus the years hurry by, while the days drag on for ever.
Mercury most likely spun more rapidly on its axis when the Solar System was young. Then each of its days might have numbered as few as eight hours, and even a quick Mercurian year could have contained hundreds such. But tides raised by the Sun in the planet’s molten middle gradually damped Mercury’s rotation down to its present slow gait.
Day breaks over Mercury in a white heat. The planet has no mitigating atmosphere to bend early morning’s light into the rosy-fingered dawn of Homer’s song. The nearby Sun lurches into the black sky and looms enormous there, nearly triple the diameter of the familiar orb we see from Earth. Absent any aegis of air to spread out and hold in solar heat, some regions of Mercury get hot enough to melt metals in daylight, then chill to hundreds of degrees below freezing at night. Although the planet Venus actually grows hotter overall, because of its thick blanket of atmospheric gases, and Pluto stays altogether colder on account of its distance from the Sun, no greater extremes of temperature coexist anywhere in the Solar System.
The drastic contrasts between day and night make up for the lack of seasonal changes on Mercury. The planet experiences no real seasons, since it stands erect instead of leaning on a tilted axis the way Earth does. Light and heat always hit Mercury’s equator dead on, while the north and south poles, which receive no direct sunlight, remain relatively frigid at all times. In fact, the polar regions probably harbour reservoirs of ice inside craters, where water delivered by comets has been preserved in perpetual shadow.
Mercury usually eludes observation from Earth by hiding in the Sun’s glare. The planet becomes visible to the unaided eye only when its orbit carries it far to the east or west of the Sun in Earth’s skies. During such ‘elongations’, Mercury may hover on the horizon every morning or evening for days or weeks. It remains difficult to see, however, because the sky is relatively bright at those times, and the planet so small and so far away. Even as Mercury draws closest to Earth, fifty million miles still separate it from us, which is quite remote compared to the Moon’s average distance of only a quarter of a million miles. Moreover, the illuminated portion of Mercury thins to a mere crescent as the planet approaches Earth. Only the most diligent observers can spot it, and only with good fortune. Copernicus, caught between the miserable weather in northern Poland and the reclusive nature of Mercury, fared worse than his earliest predecessors. As he grumbled in De Revolutionibus, ‘The ancients had the advantage of a clearer sky; the Nile – so they say – does not exhale such misty vapours as those we get from the Vistula.’
Copernicus further complained of Mercury, ‘The planet has tortured us with its many riddles and with the painstaking labour involved as we explored its wanderings.’ When he aligned the planets in the Sun-centred universe of his imagination, he used observations made by other astronomers, both ancient and contemporary. None of those individuals, however, had sighted Mercury often enough or precisely enough to help Copernicus establish its orbit as he had hoped.
The 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,