A CELESTIAL event in the 5th century BC could be the earliest documented sighting of Halley’s comet – and it marked a turning point in the history of astronomy.
According to ancient authors, from Aristotle onwards, a meteorite the size of a “wagonload” crashed into northern Greece sometime between 466 and 468 BC. The impact shocked the local population and the rock became a tourist attraction for 500 years.
The accounts describe a comet in the sky when the meteorite fell. This has received little attention, but the timing corresponds to an expected pass of Halley’s comet, which is visible from Earth every 75 years or so.
Philosopher Daniel Graham and astronomer Eric Hintz of Brigham Young University in Provo, Utah, modelled the path that Halley’s comet would have taken, and compared this with ancient descriptions of the comet (Journal of Cosmology, vol 9, p 3030). For example, the comet was said to be visible for 75 days, accompanied by winds and shooting stars, and in the western sky when the meteorite fell.
The researchers show that Halley’s comet would have been visible for a maximum of 82 days between 4 June and 25 August 466 BC. From 18 July onwards, a time of year characterised in this region by strong winds, it was in the western sky. At around this time, the Earth was moving under the comet’s tail, so its debris field would have made shooting stars.
None of this proves the comet’s identity, but Graham says such major comet sightings are rare, so Halley must be a “strong contender”. Previously, the earliest known sighting of Halley was made by Chinese astronomers in 240 BC. If Graham and Hintz are correct, the Greeks saw it three orbits and more than two centuries earlier.
The researchers’ analysis reveals this moment to be a crucial turning point in the history of astronomy. Plutarch wrote in the 1st century AD that a young astronomer called Anaxagoras predicted the meteorite’s fall to Earth, which has puzzled historians because such events are essentially random occurrences.
After studying what was said about Anaxagoras, Graham concludes that he should be recognised as “the star of early Greek astronomy”. Rather than predicting a particular meteorite, he reckons Anaxagoras made a general statement that rocks might fall from the sky.
At this time, says Graham, everyone thought that celestial bodies such as the moon and planets were fiery, lighter-than-air objects. But after observing a solar eclipse in 478 BC, Anaxagoras concluded that they were heavy, rocky lumps, held aloft by a centrifugal force. This implied that solar eclipses occurred when the moon blocked the light from the sun. It also meant that if knocked from position, such a rock might crash to Earth.
“When the meteorite fell, no one could deny it,” says Graham. “The headline was ‘Anaxagoras was right’.”
Did Halley’s comet play a role? It is always possible that the comet might have nudged a near-Earth asteroid from its course and sent it hurtling towards northern Greece. From that point on, the idea of rocks in the sky was accepted, and the Greeks had a new understanding of the cosmos.