The Planets

by Dava Sobel

  *The ancients recognized seven planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.

  *Gassendi quotes here from Ovid, referring to the Sun god Apollo by his other name, Phoebus.

  BEAUTY

  For a breeze of morning moves,

  And the planet of Love is on high,

  Beginning to faint in the light that she loves

  On a bed of daffodil sky,

  To faint in the light of the sun she loves,

  To faint in his light, and to die.

  —Alfred, Lord Tennyson, “Maud”

  Now “morning star,” now “evening star,” the bright ornament of the planet Venus plays prelude to the rising Sun, or postscript to the sunset.

  For months at a time Venus will vault the eastern horizon before dawn and linger there through daybreak, the last of night’s beacons to fade. She begins these morning apparitions close to the Sun in time and space, so that she arrives in a lightening sky. But as the days and nights go by, she comes up sooner and ventures farther from the Sun, rising while dawn is still a distant idea. At length she reaches the end of her tether, and the Sun calls her back, making her rise a little bit later each night, till she again verges on the day. Then Venus vanishes altogether for the time it takes her to pass behind the Sun.

  After fifty days, on average, she reappears at the Sun’s other hand, in the evening sky, to be hailed as evening star for months to come. Shimmering into view as the Sun goes down, Venus hangs alone in the twilight. The first few sunsets find her bathed in the afterglow colors of the western horizon, but at length Venus comes to light already high overhead, where she dominates night’s onset. Who knows how many childhood wishes are squandered on that planet before the gathering darkness brings out the stars?

  Thou fair-haired angel of the evening,

  Now, whilst the sun rests on the mountains, light

  Thy bright torch of love; thy radiant crown

  Put on, and smile upon our evening bed!

  Smile on our loves, and, while thou drawest the

  Blue curtains of the sky, scatter thy silver dew

  On every flower that shuts its sweet eyes

  In timely sleep. Let thy west wind sleep on

  The lake; speak silence with thy glimmering eyes,

  And wash the dusk with silver.

  —William Blake, “To the Evening Star”

  Hours into the night, Venus still outshines every other light, unless the Moon intrudes to best her. The Moon appears bigger and brighter, by virtue of lying about one hundred times closer to us, though Venus is the larger and fairer by far. Venus’s shroud of yellow-white cloud reflects light much more effectively than the dun-colored, dust-covered surface of the Moon. Virtually 80 percent of the Sunlight lavished on Venus just skitters off her cloud tops and spills back into space, while the Moon beams back a mere 8 percent.

  The remarkable brightness of Venus gains luster from her nearness to Earth. Venus comes within twenty-four million miles of Earth at closest approach—closer than any other planet. (Mars, Earth’s second nearest neighbor, always stays at least thirty-five million miles away.) Even when Venus and Earth recede as far from each other as possible, separated by more than one hundred fifty million miles, Venus retains her superlative brilliance for Earthbound observers. On the scale of “apparent magnitude” astronomers use to compare the relative brightness of heavenly bodies, Venus far exceeds the most luminous stars.*

  What strong allurement draws, what spirit guides,

  Thee, Vesper! brightening still, as if the nearer

  Thou com’st to man’s abode the spot grew dearer

  Night after night?

  —William Wordsworth, “To the Planet Venus”

  The nearer Venus draws to Earth, the brighter she appears, naturally enough. Yet as her glow crescendos, the globe of Venus actually diminishes from full to gibbous through quarter and then crescent phase. Like the Moon, Venus appears to change shape as she moves along her orbit, and by the time she reaches her closest, most vivid aspect in our skies, only about one-sixth of her visible disk remains illuminated. But proximity stretches this little sliver to a great length, allowing the perceived brightness of Venus to increase even as she thins and wanes away.

  Galileo, whose telescope enabled him to discover the phases of Venus, depicted them this way.

  Watching Venus through a telescope or binoculars every evening over a period of months shows how she gains in height and brightness as her disk shrinks, and vice versa. Little else becomes apparent, however, since none of Venus’s surface features can ever be discerned by sight through her cloud deck. Thus the very clouds that account for her blatant visibility also act to veil her.

  Those who know just where to look can sometimes pick out the steady white light of Venus against the light blue background of a fully daylit sky. Napoleon spotted Venus that way while giving a noon address from the palace balcony at Luxembourg, and interpreted her daytime venue as the promise (later fulfilled) of victory in Italy.

  On Moonless nights when Venus is nigh, her strong light throws soft, unexpected shadows onto pale walls or patches of ground. The faint silhouette of a Venus shadow, which evades detection by the color-sensitive inquiry of a direct gaze, often answers to sidelong glances that favor the black-and-white acuity of peripheral vision. But no matter how avidly you hunt the elusive Venus shadow with eyes averted and downcast, your search may still prove vain, while overhead, as though to mock you, the planet’s dazzle mimics the landing beam of an oncoming airplane, even triggers police reports of unidentified flying objects.

  I stopped to compliment you on this star

  You get the beauty of from where you are.

  To see it so, the bright and only one

  In sunset light, you’d think it was the sun

  That hadn’t sunk the way it should have sunk,

  But right in heaven was slowly being shrunk

  So small as to be virtually gone,

  Yet there to watch the darkness coming on—

  Like someone dead permitted to exist

  Enough to see if he was greatly missed.

  I didn’t see the sun set. Did it set?

  Will anybody swear that isn’t it?…

  —Robert Frost, “The Literate Farmer and the Planet Venus”

  Ancient legends celebrated the beauty of planet Venus by declaring her not only divine but also womanly—perhaps because her visitations generally lasted a significant nine months. Although Venus orbits the Sun in just 224 Earth-days, the Earth’s own orbital motions help govern Venus’s observed behavior. As seen from the moving Earth, Venus averages 260 days as either morning star or evening star, coinciding with the human gestation period of 255 to 266 days.

  The Chaldeans called the planet Ishtar, the love goddess ascending the heavens, and to the Semitic Sumerians she was Nin-si-anna, “the Lady of the Defenses of Heaven.” Her Persian name, Anahita, associated her with fruitfulness. The dual (dawn and dusk) nature of Venus cast her by turns as virgin or vamp to her worshipers.

  Ishtar metamorphosed into Aphrodite, the Greek incarnation of love and beauty. She became the Venus of the Romans, revered by the historian Pliny for spreading a vital dew to excite the sexuality of earthly creatures. In China, Venus blended male and female genders in a married couple consisting of the husband evening star, Tai-po, and his wife, the morning star, Nu Chien.

  Only the Mayas and the Aztecs of Central America seem to have seen Venus as consistently male, the twin brother of the Sun. The rhythmic association between Venus and the Sun inspired meticulous astronomical observations and complex calendar reckoning in those cultures, as well as blood rituals to recognize the planet’s descent into the underworld and subsequent resurrection.

  In North America, among the Skidi Pawnee, the veneration of Venus involved human sacrifice to ensure her return. The last teenage girl known to have died in such devotions was kidnapped and ceremonially killed on April 22, 1838.

 
As a symbol of loveliness, Venus figures in three paintings by Vincent van Gogh. His Starry Night of June 1889, the best known example, depicts Venus as the bright orb low to the east of the village of Saint-Rémy, during the time the artist’s dementia confined him to an asylum there. Art historians and astronomers have also definitively identified Venus in Road with Cypress and Star, which van Gogh completed in mid-May 1890, the day before he left Saint-Rémy. A few weeks later, in Auvers-sur-Oise, near Paris, where he created eighty works in the two months before his suicide, van Gogh depicted Venus for the last time, inside a scintillating halo, hovering above the west chimney of White House at Night.

  Venus voyages…but my voice falters;

  Rude rime-making wrongs her beauty,

  Whose breasts and brow, and her breath’s sweetness

  Bewitch the worlds.

  —C. S. Lewis, “The Planets”

  If ever two worlds invited comparison, the twin sisters Earth and Venus lay such a claim, for these planets are almost identical in size, and orbit the Sun at similar distances. Early discoveries about Venus from afar—especially the detection of her atmosphere by Russian astronomer and poet Mikhail Lomonosov in 1761—fanned widespread fantasies of a lush abode of Earth-like life.

  Recent research, however, has exposed only the most glaring contrasts between the two planets. Although at an earlier epoch Venus probably possessed many of the same attributes as Earth, including once-abundant seas, her water has all boiled away. Now Venus parches and bakes under an obscuring sky that blocks light but traps heat, and bears down upon her surface with heavy pressure.

  The ten Russian Venera and Vega spacecraft that successfully landed on Venus between 1970 and 1984 barely had time to take a few pictures and measurements, or quickly sample the surroundings, before succumbing to the harsh conditions. Within an hour or so of arrival, each vehicle either melted in the heat or crumpled under atmospheric pressure comparable to that found underwater on Earth, nearly three thousand feet below sea level.

  Discoveries of the drastic differences between Earth and Venus evoked surprise sometimes expressed in moral terms, as though one sister had chosen the right course while the other veered down an errant path. Nevertheless Venus, the wayward sister, preaches an important cautionary tale to careless humans, for her hostile environment proves how even small atmospheric effects can conspire over time to convert an earthly paradise into a hellfire cauldron. Indeed, much current study of Venus aims to save humanity from itself by verifying, for example, the destruction that chlorine compounds wreak in high-altitude clouds.

  And art thou, then, a world like ours,

  Flung from the orb that whirled our own

  A molten pebble from its zone?

  How must the burning sands absorb

  The fire-waves of the blazing orb,

  Thy chain so short, thy path so near

  Thy flame-defying creatures hear

  The maelstroms of the photosphere!

  —Oliver Wendell Holmes, “The Flâneur”*

  Differences between Earth and Venus doubtless began in their youth, with the Sun beating hotter on the closer of the two sisters. The Sun warmed the waters of Venus until they rose in steam, until water vapor and the hot breath of volcanic eruptions enveloped the planet. These gases then did the work of greenhouse glass: They allowed the Sun’s heat to reach the surface of Venus, but refused to let heat escape. Instead of dissipating into space, the heat rebounded back down to ground level and made the surface hundreds of degrees hotter still.

  High over Venus, sunlight split the water vapor into its components, hydrogen and oxygen, and the lighter hydrogen escaped the planet’s hold. Oxygen remained behind; it recombined with the surface rocks on Venus, and with gases vented by volcanoes, to create an atmosphere consisting almost entirely (97 percent) of carbon dioxide, the most efficient and pernicious of all greenhouse gases. Today, although only a trickle of solar energy penetrates Venus’s cloud cover and arrives at the surface, the greenhouse effect keeps temperatures above eight hundred degrees Fahrenheit all around the planet, day side and night side, even at the poles. Ice on Venus? Liquid water? Impossible, although traces of water vapor do lace the sky.

  The abundant carbon dioxide weighs on Venus’s hot terrain with ninety times the pressure of Earth’s atmosphere. On and just above the surface, where the Russian robot explorers conducted their brief surveys, the Venusian air is thick but transparent, enabling the spacecraft’s cameras to see clear to the horizon in the dim available light. All the light was red. Since only the long red wavelengths of light survive the journey down through the cloud canopy, the landscape presents itself as a monochrome in the sepia tones of old photographs. When night takes even this low-level light away, the vista glows in the dark. Its red-hot rocks, cooked halfway to their melting point by the ambient heat and pressure, resemble the embers of a fire.

  Some twenty miles above the surface, the clouds set in, in layers fifteen miles thick, admitting no breaks in their coverage. They bar the Sun from ever showing itself at all during the whole course of the long Venusian day. The planet turns so slowly that a single day takes what would be reckoned as two months on Earth just to get from Sunrise to Sunset. Diffuse signs of the Sun’s light spread slowly from horizon to horizon as the hours pass, but even the brightest hours of the day stay as dimly lit as vespertide. At night, no stars or other planets ever appear through the perpetual overcast.

  Venusian clouds comprise large and small droplets of real vitriol—sulfuric acid along with caustic compounds of chlorine and fluorine. They precipitate a constant acid rain, called virga, that evaporates in Venus’s hot, arid air before it has a chance to strike the ground.

  Scientists suspect that every several hundred million years the clouds may be remade by a fresh injection of sulfur from global tectonic upheaval on Venus, but failing that, they probably never part.

  At their topmost layer, the Venusian clouds display dark swirls when imaged in ultraviolet light. These markings change rapidly, revealing the high velocity at which the clouds roll by—about 220 miles per hour—circling Venus every four Earth-days on fierce winds. Lower down in the atmosphere the winds slacken gradually until they reach the surface, where they don’t so much blow as creep across the planet at two to four miles an hour.

  Fast or slow, the winds head ever westerly, the same way Venus turns. In contrast to all the other planets, Venus rotates to the west, even as she revolves eastward with them around the Sun. If you could see the Sun rise on Venus, it would come up in the west and set in the east. Astronomers attribute the backward spin to some violent collision that overturned Venus early in her history. The same presumed impact could explain Venus’s very slow rotation rate, or perhaps it is the Sun that impedes the planet’s spin by raising tides in the vast ocean of Venusian air.

  Deep within that

  libidinous albedo

  temperatures are hot enough

  to boil lead,

  pressures

  90 times more unyielding

  than Earth’s.

  And though layered cloud-decks

  and haze strata

  seem to breathe

  like a giant bellows,

  heaving and sighing

  every 4 days,

  the Venerean cocoon

  is no cheery chrysalis

  brewing a damselfly

  or coaxing life

  into a reticent grub,

  but a sniffling atmosphere

  40 miles thick

  of sulphuric, hydrochloric,

  and hydrofluoric acids

  all sweating

  like a global terrarium,

  cutthroat, tart, and self-absorbed.

  —Diane Ackerman, “Venus”

  After hiding for an eternity beneath her seething atmosphere, Venus’s surface has surrendered to radar examination by Earth-based telescopes and a series of orbiting spacecraft. The finest of these envoys, Magellan, circumnavigated Venus eight time
s a day for four years beginning in 1990.* Magellan resolved the planet’s vague face into distinct features, most of which turned out to be volcanoes of every variety on plains paved with lava.

  Magellan’s sudden identification of millions of land forms fomented a crisis in nomenclature. The International Astronomical Union responded with an all-female naming scheme that evoked a goddess or giantess from every heritage and era, along with heroines real or invented. Thus the Venusian highlands, the counterparts to Earth’s continents, took the names of love goddesses—Aphrodite Terra, Ishtar Terra, Lada Terra, with hundreds of their hills and dales christened for fertility goddesses and sea goddesses. Large craters commemorate notable women (including American astronomer Maria Mitchell, who photographed the 1882 transit of Venus from the Vassar College Observatory), while small craters bear common first names for girls. Venus’s scarps hail seven goddesses of the hearth, small hills the goddesses of the sea, ridges the goddesses of the sky, and so on across low plains named from myth and legend for the likes of Helen and Guinevere, down canyons called after Moon goddesses and huntresses.

  The only male name on the map of Venus—the great mountain range Maxwell Montes—belongs to Scottish physicist James Clerk Maxwell, who performed pioneering work on electromagnetic radiation during the nineteenth century. When the five-mile-high peaks were detected in the 1960s via Earth-based radar studies made possible by Maxwell’s insights, it seemed fitting to attach his name to them. For several decades after discovery, Maxwell Montes stood as the sole eponymous feature on the planet, while the low regions on either side of the mountains were designated simply as Alpha Regio and Beta Regio (“A” region and “B” region). When Magellan arrived thirty years later, and its revelations gave rise to names derived from women’s history, no one wished to evict Maxwell from his rightful place on Venus.

  Yes, the faces in the crowd,