The detailed tail photograph of Halley's Comet was obtained by Jet Propulsion Laboratory astronomer Eleanor Helin with the 48-inch Schmidt telescope at Caltech's Palomar Observatory on Dec. 13, 1985.
The ancient Greeks probably saw Halley's comet streak past in 466 B.C., pushing the earliest documented observation of the comet back by more than 200 years, a new study has found.
And a meteorite struck northern Greece while the comet was burning in the sky, shaking up the ancients' understanding of the cosmos, the research shows.
"It looks like a promising account of the comet," said Daniel Graham, a professor of philosophy at Brigham Young University, who was lead author of the study. "The evidence is all very consistent with Halley's."
Charting Halley's future and its past
Halley has been a regular visitor to Earth's skies for thousands of years. When the comet travels near our planet every 74 to 79 years, it's bright enough to be seen by the naked eye. The last close pass was in 1986, and the next is due in 2061. [Photo of Halley's comet.]
In 1705, English astronomer Edmond Halley suggested a comet seen in 1682 was the same one that lit up the sky in 1531 and 1607. He further predicted it would be back in 1758. When this came to pass, the comet was given his name.
Scientists have been forecasting the appearance of Halley ever since.
They can also cast backward in time, making "retrodictions" to calculate when and where Halley's comet would have appeared in the past. Many of these can be confirmed with historical records. Babylonian and Chinese documents, for example, confirm the calculations that Halley passed Earth in 87 B.C., 164 B.C. and 240 B.C., scientists say.
"The predictions for 240 B.C. are amazing," said co-researcher Eric Hintz, an astronomer at BYU in Provo, Utah. "They really pinned it down to the right part of the sky."
Pushing the record back
The 240 B.C. observation made by Chinese astronomers had been the earliest confirmed sighting of Halley's comet. But ancient Greek documents describe a comet that could be Halley, visiting in 466 B.C.
The Greek writings mainly describe another dramatic astronomical event of that year: a meteorite the size of a "wagon-load" that fell in northern Greece. Aristotle wrote about the meteorite 100 years later; Pliny, writing five centuries after the event, did as well.
Some of these accounts, including Aristotle's, mention that a comet was lighting up the sky when the meteorite hit. This information is consistent with the mathematical models, which suggest Halley flew by Earth in 466 B.C.
To determine if this comet was actually Halley, Graham and Hintz extended existing astronomical models to include not just information on Halley's orbit, but details about its visibility.
"We took their orbital-element calculations and tried to extrapolate where Halley's comet would've been visible from Earth," Hintz told SPACE.com. "We wanted to know, could the Greek observers have seen it?"
Their answer: Yes.
Not only did the model suggest Halley would have been visible to the Greeks during that pass, it correctly projected that the light show would have lasted about 75 days an unusually long window in line with observations from the ancient Greek writer Daimachus.
"It turned out to be a really strange path," Hintz said. "Halley may indeed have been visible for 70 or 75 days."
The researchers also calculated that Halley's tail would have been very large, creating many shooting stars as debris from the tail flew through Earth's atmosphere. This detail, too, is recorded by Daimachus. (But there is no evidence that the wagon-load meteorite came from Halley, or that the comet caused the strike.)
Still, the evidence that the ancient Greeks recorded Halley's pass is not conclusive, the researchers say. To be entirely sure, researchers would need more details, such as which constellations the comet appeared in, and when.
That's the kind of thing the meticulous Babylonians and Chinese noted in their records for Halley's passes a few centuries later. But the Greeks weren't that detail-oriented, so this information is unlikely to exist for the 466 B.C. comet.
"The Greeks had great physical models of the heavens, but they were lousy at making observations," Graham said. "Their empirical skills were not that great."
Toward a better understanding of the cosmos
Even if they did witness Halley's pass, the Greeks probably didn't understand what the comet actually was. Many Greek thinkers of the day believed comets were optical illusions or the result of strange weather, Graham said. Aristotle, for example, associated comets with windy conditions.
This lack of understanding lasted thousands of years. Throughout much of human history, comets were viewed as supernatural objects that portended doom.
When Halley appeared in A.D. 1066, for example, the English steeled themselves for misfortune. Their fears were borne out when William the Conqueror's Normans defeated them at the Battle of Hastings later that year, killing King Harold II in the process. The comet can be seen in the Bayeux Tapestry, a medieval artwork that chronicles the Norman invasion.
But the events of 466 B.C. did help the Greeks develop a better understanding of the universe, the researchers said. The meteorite strike, especially, had a lasting impact on Greek and astronomical thought.
The space rock's remains on the ground became a tourist attraction for the next 500 years.
"Before that, there's no evidence the Greeks even knew that there were meteors," Graham said. The Greeks had observed shooting stars but regarded them as odd manifestations of the weather, he added.
The meteorite also helped the Greeks understand that heavenly bodies are weighty and massive not ethereal and cloudlike, as many people had believed, Graham said. The heavy-bodies theory had been advanced before the meteorite strike by the great thinker Anaxagoras, who is known for proposing that the seeds of life exist throughout the cosmos.
When people saw the space rock, they tended to turn to Anaxagoras' scientific explanations, and not to angry gods and goddesses.
"I find it really interesting that the meteor was not associated with any mythological stories," Graham said. "The headlines at the time were all about the scientific explanation."
The researchers reported their findings in the July issue of the Journal of Cosmology.
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