Without the modern scourge of light pollution, the ancient Greeks and Romans saw more of the heavens than we normally do, and they made stargazing a more central aspect of their culture. Following the number-loving philosopher Pythagoras, Cicero wrote that the intervals of a musical scale echo the intervals between the orbits of our solar system. The Stoics, believers in an interdependent cosmos, looked to the night sky to augur our predestined lives. Of particular interest to all these ancients was the stella erratica, or “errant star,” so called for its shifting location. (Our familiar constellations, by contrast, remain fixed in the firmament.) Romans borrowed a word from the Greeks to denote these celestial strays: planeta, or “planet.”
Derived from the verb “to wander,” the original Greek noun πλάνης was applied to more than just Mars and Saturn—in Euripides’s Bacchae, to take just one example, it refers to a “vagabond” who comes to town. Among the physicians of the ancient world, including Hippocrates himself, πλάνης could also mean “fever,” a pestilence that migrates from person to person. The Romans, of course, had their own words for disease—morbus, pestis—but they adopted this astronomical language in their own medical writings too, using the Latin cognate. In one account, planeta refers to a fever with an “unrestrained onset.” In another, planetae are those illnesses that obey neither finite duration nor predictable prognosis.
According to the lexicographical history of planeta that Fordham’s Matthew McGowan catalogs in the Thesaurus Linguae Latinae, the same ancient word means both a celestial body and a mundane affliction. But with some creative reflection, this paradoxical pairing starts to make a little sense. Both the errant star and the roving illness shatter our attempts at a tidy ordering of the natural world around us and within us. Planets and plagues appear to move according to their own plans, not ours.