The planet parade: for millennia, planets were just mysterious, wandering points of light in the night sky. Now they are destinations

Natural History, May, 2004 by Neil deGrasse Tyson

In the study of the cosmos, it's hard to come up with a better tale than the centuries-long history of attempts to understand the planets-those sky wanderers that make their rounds against the backdrop of stars. Of the eight objects in our solar system that are indisputably planets, five are readily visible to the unaided eye and were known to the ancients, as well as to observant troglodytes. Each of the five--Mercury, Venus, Mars, Jupiter, and Saturn--was endowed with the personality of the god for which it was named. Mercury, which moves the fastest against the background stars, was named for the Roman messenger god--the fellow usually depicted with small and aerodynamically useless wings on his heels or his hat. Mars, the only one of the classic wanderers (the Greek word planets means "wanderer") with a reddish hue, was named for the Roman god of war and bloodshed. Earth, of course, is also visible to the unaided eye. Just look down. But terra firma was not identified as one of the gang of planets until after 1543, when Nicolaus Copernicus advanced his Sun-centered model of the universe.

To the telescopically challenged, the planets were, and are, just points of light in the sky that happen to move. Not until the seventeenth century, with the proliferation of telescopes, did astronomers discover that planets are orbs. Not until the twentieth century were the planets scrutinized at close range, with space probes. And not until later in the present century are people likely to visit them.

Humanity had its first telescopic encounter with the celestial wanderers during the winter of 1609-10, when Galileo, looking through an excellent telescope of his own design and manufacture, saw the planets as spheres--perhaps even other worlds. One of them, brilliant Venus, went through phases just like the Moon's: crescent Venus, gibbous Venus, full Venus. Another planet, Jupiter, had moons all its own, and Galileo discovered the four largest: Ganymede, Callisto, Io, and Europa, all named for assorted characters in the life and times of Jupiter's Greek counterpart, Zeus.

The simplest way to explain the phases of Venus, as well as other features of the planet's motion against the sky, was to assert that the planets revolve around the Sun, not the Earth. Indeed, Galileo's observations strongly supported the universe as envisioned and theorized by Copernicus.

Jupiter's moons took the Copernican universe a step further: though Galileo's twenty-power telescope could not resolve the moons into anything larger than pinpoints of light, no one had ever seen a celestial object revolve around anything other the Earth. It was an honest, simple observation of the cosmos, but the Roman Catholic Church and "common sense" would have none of it. With the aid of his telescope, Galileo discovered a contradiction to the dogma that Earth occupied the central position in the cosmos--the spot around which all objects revolve. In early 1610, in a short but seminal work he titled Sidereus Nuncius (Starry Messenger), Galileo reported his persuasive findings.

Once the Copernican model had become widely accepted, the arrangement of the heavens could legitimately be called a solar system, and Earth could take its proper place as one among six known planets. Nobody imagined there could be more than six. Not even the English astronomer Sir William Herschel, who discovered a seventh in 1781.

Actually, the credit for the first recorded sighting of the seventh planet goes to the English astronomer John Flamsteed. But in 1690, when Flamsteed noted the object, he didn't see it move. He assumed it was just another star in the sky, and named it 34 Tauri. When Herschel saw Flamsteed's "star" drift against the background stars, he announced--operating under the unwitting assumption that planets were not on the list of things one might discover--that he had discovered a comet. Herschel planned to call the newfound object Georgium Sidus ("Star of George"), after his benefactor, King George III of England. If the astronomical community had respected Herschel's wishes, the roster of our solar system would now include Mercury, Venus, Earth, Mars, Jupiter, Saturn, and George. In a blow to sycophancy, however, the object was ultimately called Uranus, in keeping with its classically named brethren--though some French and American astronomers kept calling it "Herschel's planet" until 1850, just several years after the eighth planet, Neptune, was discovered [see "The Rise and Fall of Planet X," by Neil deGrasse Tyson, June 2003].

Over time, telescopes kept getting bigger and sharper, but the planetary details astronomers could discern did not much improve. Because every telescope, no matter the size, viewed the planets through Earth's turbulent atmosphere, the best pictures were still a bit fuzzy. Nevertheless, intrepid observers managed to discover features such as Jupiter's Great Red Spot, Saturn's rings, Mars's polar ice caps, and dozens of planetary moons. Human knowledge of the planets was still meager, though, and where ignorance lurks, so too do the frontiers of discovery and imagination.