Although they were sighted in the ancient world, at least as early as 1000 B.C., Greek natural philosophers coined the word comet sometime around 500 B.C. It was derived from the Greek word for a head with long hair. The new term, kométés, perfectly described a wandering, bright, starry head followed by a long trail of misty, hair-like light.

Later, Aristotle (384-322 B.C.) demoted comets from star status to dry and warm exhalations from the Earth’s lower atmosphere. It was an erroneous theory that held sway until the late 18th century. 

For much of that period, however, many believed comets to be divine oracles of disaster.

For example, Plutarch recorded a comet that lit up Rome around the time of Julius Caesar’s assassination in 44 B.C. Almost 1700 years later, William Shakespeare repeated this warning sign in his 1623 play about the great Emperor.

Seven years before the birth of Christ, a comet was seen flying over Judea. Soothsayers advising the King of Israel, Herod (74-4, B.C.) told him that it presaged the birth of a boy who would someday outshine the monarch. In response, Herod ordered mass infanticide that came to be known as the "Slaughter of the Innocents."

Warriors from Attila the Hun (406-453 A.D.) to Genghis Khan (1162-1227 A.D.) took these astral apparitions very seriously, as well. In 1066, for example, a comet appeared over England. Many Britons believed it signaled the bloody defeat of King Harold II to William the Conqueror and his band of Normans during the Battle of Hastings.

One of the most famous comets is named for Edmund Halley (1656-1742). A protégé of Sir Isaac Newton, Halley set out to disprove Aristotle by demonstrating that comets traveled—like planets—in closed orbits around the sun. Using mathematical formulas concocted by Newton, in 1705 he showed that many (but certainly not all) of the comet sightings across human history represented a particular astral phenomenon. Based on his calculations, he determined a periodicity of 76 years. It returned in 1758 (exactly 76 years after the previous sighting in 1682), and continues to do so every 75 to 76 years. 

Perhaps the best comet tale concerns the life of Mark Twain. The night he was born, November 30, 1835, a brilliant view of Halley’s comet was visible to the denizens of Florida, Missouri. Twain remained fascinated by Halley’s comet for the rest of his life. Most boldly, in 1909, he bragged to his legion of readers:

"I came in with Halley's Comet in 1835. It is coming again next year (1910), and I expect to go out with it. It will be the greatest disappointment of my life if I don't go out with Halley's comet. The Almighty has said, no doubt: 'Now here are these two unaccountable freaks; they came in together, they must go out together.'"

Like clockwork, Halley’s comet made its closest (and most visible) approach to the sun on April 20, 1910. One day later, on April 21, the creator of Huckleberry Finn died of a massive heart attack. 

Over the past century, thanks to better telescopes, technology, and improved means of computing parabolic orbits and parallax angles, scores of other comets have been discovered.  A growing body of evidence has determined that they are slurries of ice, dust, and gases that orbit the Sun in an elliptical fashion. The future of comet studies looks even brighter as space explorations like the NASA Deep Impact spacecraft gather more data on comets such as Hartley 2 and other "hairy stars" that light up the sky.

In this segment, we'll talk about the origins of the word 'comet' and the history of its meaning.