Halley’s comet portrayed on ancient coin
Heather Catchpole, ABC Science Online
ABC Science Online, Australia
May 19 2004
Could the star shape on the king’s crown be Halley’s comet?
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 (Image: NASA/Ames Research Center) 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.”