A rare ancient coin may feature an early record of Halley's comet, researchers say.

The coin features the head of the Armenian king Tigranes II the Great, who reigned from 95 to 55 BC. A symbol on his crown that features a star with a curved tail may represent the passage of Halley's comet in 87 BC, say the Armenian and Italian researchers.

Their research will be published in Astronomy & Geophysics, a journal of the Royal Astronomical Society.

Halley's comet, which was last visible in 1986, has cropped up periodically in the Earth's history, with regular observations in 1531, 1607 and 1682.

This led Edmond Halley to declare in 1705 that this was the same comet, with an orbit taking it past the Earth about every 76 years. He predicted successfully it would return in 1758, and the comet was named after him.

Now researchers have found further evidence that the comet was significant thousands of years before Halley was born.

Tigranes could have seen Halley's comet when it passed closest to the Sun on 6 August in 87 BC, according to the researchers, who said the comet would have been a "most recordable event".

The appearance of the comet in Armenia, which borders Turkey and Iran, could be useful to date the coin accurately. While the coin dates back to before 83 BC, when Tigranes conquered the ancient city of Antioch, the capital city of Syria at the time, researchers do not know its precise date.

Halley's comet is a ball of dirty snow and ice about 15 kilometres long. Like other comets that periodically pass the Earth, it has a highly eccentric orbit that changes as the larger planets pull at its orbit.

Astronomer Vince Ford from the Research School of Astronomy and Astrophysics at Canberra's Australian National University said the comet would have been bigger and brighter 2000 years ago.

"As comets come round the Sun they lose a lot of material, up to 10%," he said.

Although Halley's comet wasn't losing that much, it would still get smaller over time as the Sun burnt away icy dust and gas.

Like other comets that return within 200 years, Halley's comet is thought to come from the Kuiper belt, a disc of comets and icy planets including Pluto, which periodically sends icy material hurtling into the solar system.

Ford said the oldest confirmed observation of Halley's comet was from Chinese recordings on 25 May in 240 BC.

Art had often been the source of evidence of sightings of Halley's comet, he said.

For example, the Bayeux tapestry depicted the comet in the lead up to the Battle of Hastings in 1066. But art had also mislead astronomers, Ford said.

"Giotto painted it into his nativity scene, probably because he has recently seen Halley's comet and he was impressed," Ford said. "But the comet only appeared in 12 BC, way before the birth of Jesus."

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