The Planets

The visits of comets, long interpreted as signs and wonders, have recently sketched the true extent of the Sun’s domain. By tracing the visible parts of comet paths, and extrapolating the rest, astronomers have shown that numerous comets hail from out beyond Pluto’s neighbourhood, from a second comet reservoir, hundreds of times further away. Despite their unimaginable distance, these bodies still belong to the Sun, still heed the Sun’s gravity, still receive some glimmer of the Sun’s light.

Sunlight, which darts through space at the dazzling speed of 186,000 miles per second, takes ages to emerge from the dense interior of the Sun. Light advances only a few miles per year near the Sun’s core, where the crush of matter repeatedly absorbs it and impedes its escape. Radiating this way, light may journey for a million years before reaching the Sun’s convective zone, there to catch a quick ride up and out on roiling eddies of rising gas. As soon as these eddies release their cargoes of light, they sink back down, to soar again later with more.

The light-emitting, visible surface of the Sun – the photosphere – seethes as though boiling from the constant tumult of energy release. Gas bubbles bursting with light give the photosphere a grainy complexion, marred here and there by pairs of dark, irregularly shaped sunspots, with black centres and grey, graded shading around them like the penumbras of shadows. Sunspots designate areas of intense magnetic activity on the Sun, and their darkness bespeaks their relative coolness of about 4000°K, compared to neighbouring areas at nearly 6000.* The level of solar activity rises and falls in cycles averaging eleven years, and sunspots mingle, morph, and multiply according to this same schedule. Their number and distribution vary like famine and plenty, from no spots at ‘solar minimum’, or just a few spots dotting the Sun’s high latitudes, to ‘solar maximum’ five to six years later, when hundreds of them crowd closer to the equator. Although sunspots seem to gather and scud like clouds across the photosphere, really it is the Sun’s rotation that carries them around.

The Sun rotates on its axis approximately once a month, in a continuation of the spinning motion it was born in. Being an enormous ball of gas, the Sun spins complexly, in layers of various speeds. The core and its immediate surroundings turn at one rate, as a solid body. The overlying zone spins faster, and above that, the visible photosphere whirls around at several different rates, more quickly at the Sun’s equator than near its poles. These combined, contrary motions whip the Sun into a fury, with consequences felt clear across the Solar System.

The ‘solar wind’, a hot exhalation of charged particles (reminiscent of the ‘wind from God’), blows out from the turbulent Sun and keeps up a constant barrage on the planets. Were it not for the protective envelope of Earth’s magnetic field, which deflects most of the solar wind, humankind could not withstand the onslaught. From time to time, especially during solar maximum, the steady solar wind is augmented by sudden blasts of higher-energy particles from solar flares on the Sun’s surface, or by gargantuan blobs of ejected solar gas. Such outbursts can disable our communications satellites and disrupt power grids, causing blackouts. In milder doses, particles of solar wind trickle into the upper atmosphere near the North and South Poles, initiating cascades of electrical charge that draw curtains of coloured lights across the sky – the so-called Northern and Southern Lights. Other planets also sprout colourful auroras in response to the solar wind, which billows on past Pluto all the way to the heliopause – the undiscovered boundary where the Sun’s influence ends.

From Earth, we see the Sun as a blazing circle in the sky, brighter but no bigger than the circumference of the full Moon. The ‘two great lights’, as the Sun and Moon are described in Genesis, make a matched pair. For although the Moon measures only one four-hundredth the Sun’s million-mile diameter, it nevertheless lies four hundred times closer to Earth. This uncanny coincidence of size and distance enables the puny Moon to block out the Sun whenever the two bodies converge on their shared path across Earth’s sky.

Approximately once every two years, some narrow swath of Earth – as often as not a godforsaken, all but inaccessible place – is blessed with a total solar eclipse. There, dusk falls and dawn breaks twice on the same day, and the stars come out with the Sun still overhead. Temperatures may drop ten or fifteen degrees at a stroke, allowing even the most jaded observer to sense the bizarre disorientation that birds and animals share as they hasten to their nests or burrows through the sudden midday darkness.

No total eclipse can last much longer than seven minutes, because of Earth’s persistent turning on its axis and the Moon’s unwavering march along its orbit. But totality of the briefest duration affords sufficient reason for scientific expeditions and curious individuals to travel halfway around the world, even if they have seen one or more eclipses before.

At totality, when the Moon is a pool of soot hiding the bright solar sphere, and the sky deepens to a crepuscular blue, the Sun’s magnificent corona, normally invisible, flashes into view. Pearl and platinum-coloured streamers of coronal gas surround the vanished Sun like a jagged halo. Long red ribbons of electrified hydrogen leap from behind the black Moon and dance in the shimmering corona. All these rare, incredible sights offer themselves to the naked eye, as totality provides the only safe time to gaze at the omnipotent Sun without fear of requital in blindness.

Moments later, the shadow of the Moon passes and the natural world order is restored by the ordinary grace of the Sun’s familiar light. But visions of the eclipse persist among viewers, as though a miracle had been witnessed. Is it an accident that the Solar System’s lone inhabited planet possesses the only satellite precisely sized to create the spectacle of a total solar eclipse? Or is this startling manifestation of the Sun’s hidden splendour part of a divine design?

* Discarded comet dust litters interplanetary space, and when the Earth trundles into a patch of it, the particles that fall through the atmosphere are incinerated, appearing as isolated ‘shooting stars’ or whole showers of meteors.

* Degrees K, for Kelvin, are the same size as degrees Celsius (or centigrade) – almost double the value of Fahrenheit degrees. However the Kelvin scale starts lower, at −273 °C, or ‘absolute zero’, the point at which all motion ceases, and has no upper limit, which makes it useful for describing the temperatures of stars.

3 MYTHOLOGY

The planets speak an ancient dialect of myth. Their names recall all that happened before history, before science, when Prometheus hung shackled to that cliff in the Caucasus for stealing fire from the sky, and Europe was not yet a continent but still a girl, beloved by a god, who beguiled her disguised as a bull.

In those days Hermes – or Mercury, as the Romans renamed the Greek messenger god – flew fleet as thought on divine errands that earned him more mentions in the annals of mythology than any other Olympian: after the goddess of the harvest lost her only daughter to the god of the underworld, Mercury was sent to negotiate the victim’s rescue, and drove her home in a golden cart pulled by black horses. When Cupid got his wish, making Psyche immortal and therefore fit to marry him, it was Mercury who led the bride into the palace of the gods.

The planet Mercury appeared to the ancients, as it appears to the naked eye today, only on the horizon, where it coursed the twilight limbo between day and night. Swift Mercury either heralded the Sun at dawn, or chased after it through dusk. Other planets – Mars, Jupiter, Saturn – could be seen shining high in the sky all night for months on end. But Mercury always fled the darkness for the light, or vice versa, and hastened from view within an hour’s time. Likewise the god Mercury served as a go-between, traversing the realms of the living and the dead, conducting the souls of the deceased down to their final abode in Hades.

Myth may have conferred the god’s name on the planet, because it mirrored his attributes, or perhaps the observed behaviour of the planet gave rise to legends of the god. Either way, the union of planet Mercury with divine Mercury – and with Hermes, and the Babylonian deity Nabû the Wise before him – was sealed by the fifth century BC.

The persistent image of Mercury, lean and hell-bent as a marathon runner, personifies dispatch. Wings on his sandals urge him on, spurred faster by the wings on his cap, and the magic powers of his winged wand. Although speed tops the panoply of his powers, Mercury also gained fame as a giant-killer (after he slew thousand-eyed Argus) and as the god of music (because he invented the lyre, and his son, Pan, fashioned the shepherd’s pipe of reeds), god of commerce and protector of traders (for which he is remembered in words like ‘merchant’ and ‘mercantile’), of cheats and thieves (since he stole herds from his half-brother Apollo on the very first day of his life), 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, 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’.