The Planets (5 page)

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 draftsman 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 behavior 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 earthquakes that lifted the crust on the far side of this world and cut it to pieces.

Mariner 10
photo montages, which captured less than half of Mercury’s surface, revealed a network of scarps and fault lines that indicate the whole planet must have shrunk to its present dimensions from some larger beginning. When Mercury’s interior contracted, the global crust readjusted itself to fit the suddenly smaller world—like some furtive trick of the god Mercury, disguising himself.

After a thirty-year hiatus in exploration, a new mission called
MESSENGER
(an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is now en route to Mercury. Launched in August 2004, but unable to fly as quickly or directly as its namesake, the craft will not reach Mercury’s vicinity until January 2008. At
first sight of the planet,
MESSENGER
will start a detailed mapping effort requiring three flybys of Mercury over the following three years, while the spacecraft orbits the Sun, protected under a sunshade made of ceramic cloth. Then, in March 2011,
MESSENGER
will maneuver into orbit around Mercury itself, for a year-long odyssey (as measured in Earth time) to monitor the planet through two of its long days. Circling Mercury rapidly and repeatedly every twelve hours,
MESSENGER
will function as a new oracle, streaming answers to the questions posed by anxious truth-seekers on Earth.

*
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.

N
ow “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?

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.
*

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.

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.