November 12, 2014 goes down in history. On this Wednesday, an unmanned probe landed on a comet nucleus for the first time ever. © ESA/Rosetta/Philae/CIVA

Dirty snowballs

The sight of a bright comet has fascinated humankind through the ages. But what’s behind such celestial spectacles? It’s only in modern times that researchers have got wise to the phenomenon - by then, the comets already had a long career as harbingers of bad tidings or heavenly messengers.

Text: Helmut Hornung

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Bad omen: The Bayeux Tapestry depicts events during the Battle of Hastings in 1066. Seen here in a detail, Halley’s Comet appears over the throne of King Harald II, who later fell in the battle.

The first observations of comets originate from the third millennium before Christ. In ancient cultures, their sudden appearance was considered to a sign from the gods. And because they disturbed the harmony of the starry sky, they were soon deemed to be a bad omen. In ancient Greece the natural philosophers attempted to find an explanation for the phenomenon. Aristotle (384 to 322 B.C.), whose views were to dominate the astronomical and physical worldview in the West for more than one-and-a-half millennia, believed they were emanations of the Earth’s atmosphere.

In the Middle Ages, the fear of this “heavy hand of God” reached its pinnacle; comets were thought to portend terrible natural phenomena, such as floods or earthquakes. In the 16th and early 17 centuries, they became a favourite subject for broadsheets, the forerunners of newspapers.  A poem from the 15th century provides an impressive description of the nature of comets: “They bring fever, illness, pestilence and death, difficult times, shortages and times of great famine.”

The tailed stars also found their way into art and were depicted, for example, in the famous Bayeux Tapestry, which shows the conquest of England by William of Normandy in 1066. Or in a fresco by the Italian Renaissance painter Giotto di Bondone in the Cappella degli Scrovegni in Padua (1304). Both portrayals were inspired by the appearance of the famous Halley’s Comet, and Giotto’s fresco “Adoration of the Magi” gave rise to the still widespread belief that the Star of Bethlehem was actually a comet.

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The model for Christmas crèches: In his fresco “Adoration of the Magi”, Renaissance painter Giotto di Bondone has apparently immortalised the appearance of Halley’s Comet in 1301 in the Scrovegni Chapel in Padua.

“Comet fever” is still rife today: when Halley’s Comet appeared in the sky in 1910 as expected, many feared that civilization would be poisoned by prussic acid, which had recently been discovered to be a component of the tail. Clever wheeler-dealers sold comet pills to ward off this eventuality. And when the Hale-Bopp comet gave an impressive performance in the terrestrial sky in 1997, 39 members of the Heaven’s Gate sect committed suicide because they believed it would enable them to leave Earth and travel to an alien space ship, which was supposedly accompanying Hale-Bopp.

Astronomical research on comets was slow in starting. Nevertheless, Peter Apian from Ingolstadt (really Peter Bienewitz, 1495 to 1552) was the first astronomer of modern times to observe that the tails of comets always point away from the Sun. Apian describes this in his book Astronomicum Caesareum, which was published in 1540 and dedicated to Emperor Charles V.

A few decades later, the Danish astronomer Tycho Brahe (1546 to 1601) recognised that comets were independent celestial bodies: he measured the parallax of the bright comet of 1577 and thereby determined its distance to be around 230 Earth radii, corresponding to 1.5 million kilometres. This conclusively refuted the teachings of Aristotle that comets were phenomena within the terrestrial atmosphere. Comets even turned out to be translunar objects, meaning they stayed beyond the Moon.

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