More than two thousand years ago, getting loose change was about as easy as it is today. Hand a shopkeeper a silver dollar in today’s world, and you can expect four quarters in change. What isn't the same as today is the design of the coin one might want to get change for. Hand the same shopkeeper a Roman coin from the first century, especially one with a bright comet engraved on its head, and one of two things might happen: either you’d get thrown out of the store, or the shopkeeper would treat you to dinner and then bequeath his children to you.
After all, if the shopkeeper read Shakespeare, he would know that the coin was celebrating Julius Caesar’s Great Comet, the comet that appeared in the northern sky during the games held shortly after the assassination of Julius Caesar on the Ides of March , March 15, 44 BC. In the Shakespearian tragedy Julius Caesar, Calpurnia even predicts the murder, and the comet:
When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.
In Shakespeare’s play, Caesar was assassinated on the Ides of March, 44 B.C. The play mentions neither the games, nor that they were played in celebration of the new emperor, Augustus Caesar. A bright comet was visible in the northern sky during those games. It was widely interpreted as Julius Caesar’s soul on its way to the stars. At the time, comets were omens. Calpurnia was well aware that her husband’s death could be preceded or followed by a bright comet. And decades later, Seneca, in his anxiety to avoid execution by the suspicious Emperor Nero, insisted that the bright comet of A.D. 61 was a favorable omen to Nero. (It didn't work; Nero had Seneca put to death.)
To engrave a comet on a coin may seem strange, but in fact, most people never get to see a bright comet, an apparition in the night with a head and flowing tail, in their entire lives - I have. My nights under the stars have been brightened by the light of more than two hundred comets. Only a few of these comets were visible without the aid of a telescope, and most were only barely seen as specks of slowly moving haze. But even these were magical.
Comets have appeared in literature all over the world, in almost all languages, because writers since time began have seen comets and have become fascinated by them. Writers like Geoffrey Chaucer, like Alfred, Lord Tennyson, like James Joyce, and like me. I caught the comet bug when I was twelve years old. Our teacher in the sixth grade, Mr. Powter, wanted us to give speeches. The topic I chose was comets. I was interested in their appearances in the sky, their appearances in history, in art, and in literature.
What I knew nothing about was their role in the origin of life on Earth. I was far too young to consider the possibility that when comets collided with the Earth, their debris included the CHON particles –Carbon, Hydrogen, Oxygen, and Nitrogen—the alphabet of life. Thirty-four years later, one of the comets I helped discover taught me that lesson as it careened into Jupiter in one of the biggest events in the history of science.
This comet didn't get onto a Roman coin, or even a modern one, but it did find its way onto a German stamp. Not too bad for a tiny comet that wandered through the solar system for eons, gradually got attracted into an orbit about Jupiter, and then, in a series of explosions, reconstructed our understanding of how life could begin on a world.
Editor's Note: Comet 46P/Wirtanen is currently visible through a telescope low in the southern sky around midnight.