How Halley's Comet sightings changed history over the past 2500 yearsS

Ancient Greek texts reveal the earliest recorded sighting of the solar system's most famous comet 2,500 years ago. Since then, Halley's Comet has repeatedly cameoed in history, getting credit for toppling armies, birthing empires, and even killing Mark Twain.

Halley's Comet is the most famous of the short-period comets, which are comets that complete their eccentric orbits in 200 years or less. It's the only short-period comet that's visible to the naked eye, and its 76-year circuit means it's the one comet that pretty much everyone can hope to see once, if not twice, during their lifetime. Because of this uniqueness and its often dazzling appearances, it's become something of humanity's companion throughout human history, popping up again and again in historical records.

Ancient sightings

Astronomers recently modeled the comet's approach in 466 BC, then looked for ancient texts that matched its probable appearance. The previous earliest recorded appearance - or, as it's more technically known, apparition - was by Chinese scholars noting the passage of the comet in 240 BC, which is three apparitions later than the 466 BC event. The model suggests the comet would have been visible for about 82 days in 466 BC, from the ancient equivalent of June 4 to August 25.

Amazingly, the ancient Greeks recorded an event that pretty much perfectly matches that description. Authors from that year described a wagon-sized meteor that struck northern Greece one day, which quite understandably terrified the neighboring population and created one of the ancient world's most popular tourist destinations. Significantly, the authors note the presence of a comet in the sky at the time the meteor struck, and the comet remained in the sky for about 75 days. Considering atmospheric conditions might have slightly reduced the 82 day presence of Halley's Comet, that's an excellent match.

It gets better. The records say the comet appeared in the western sky, and it came at a time of high winds and shooting stars. Strong winds are common in Greece during the month of July, and Halley's Comet moved into the western sky on July 18. At that time, Earth would have been directly under the comet's debris field, which explains the shooting stars. As for that wagon-sized meteorite, it's possible the comet pushed an asteroid off course and made it hit Earth, but as researcher Eric Hintz explains, it's probably more likely that "it was just a really cool coincidence."

So that's the earliest known record of Halley's Comet, at least for now. The comet has probably been in its current orbit for anything from 16,000 to 200,000 years. Indeed, if it's the latter, then Halley's Comet has been visiting Earth since the first emergence of anatomically modern humans, which would be another really cool coincidence. Either way, Halley's Comet has been visiting humanity for a long time, and writing predates the 466 BCE apparition by at least three millennia, so it's possible an even earlier sighting will turn up in the records of ancient Sumer or Egypt.

How Halley's Comet sightings changed history over the past 2500 yearsS

Babylon and the Bible

After these first recorded appearances in 466 and 240 BCE, Halley's Comet starts appearing with regularity every 75 or 76 years. Babylonian tablets mark its apparitions in both 164 and 87 BCE, and that second appearance may actually have been recorded in the local currency. A coin featuring the Armenian king Tigranes the Great features a star with a curved tail on his crown, and there's some thought that this is meant to be Halley's Comet. Tigranes could have seen the comet during its closest approach to the Sun on August 6, 87 BCE, in the eighth year of his reign, and uses its brilliant appearance as a sign that his rule marked the beginning of a new era, a time of the king of kings.

How Halley's Comet sightings changed history over the past 2500 yearsS

Speaking of which, there's some thought that the comet's apparition in 12 BCE provided the basis for the biblical tale of the Star of Bethlehem. Although it's not impossible, there's nothing about the New Testament's descriptions that clearly indicate it was Halley's Comet, and there were other comets that passed Earth closer in time to the assumed date of Jesus's birth. Still, even if it didn't inspire the original Star of Bethlehem, its 1301 appearance probably did influence Giotto di Bondone's depiction of the star in his 1305 depiction of the Nativity.

The Battle of Hastings

Halley's Comet made its closest approach in 837 CE, coming within 3.2 million miles of Earth. That's only about ten times the distance between the Earth and the Moon, and its tail would have stretched across about a third of the night sky. The comet made the ancient equivalent of international news, showing up in records from Japan, China, the Middle East, and northern Europe.

But the comet's most celebrated appearance probably came in 1066. Although not quite as close as the 837 apparition, it was a remarkable sight, four times bigger than Venus and about a fourth as bright as the Moon. English astrologers took its appearance as omen for the upcoming battle between the English and invading Normans.

Of course, it was only after the Battle of Hastings that the English learned what type of omen it was - a good one for the new king William the Conqueror, but a very bad one for the now dead King Harold II. William the Conqueror certainly liked the look of it - he's said to have called it a ''a wonderful sign from heaven" when he sighted it off the French coast and took it as final proof that his invasion was destined to succeed.

How Halley's Comet sightings changed history over the past 2500 yearsS

How do we know it was actually Halley's Comet that these people saw? The iconic Bayeaux Tapestry actually features the comet, attracting fearful attention from both commoners and King Harold himself, who seems to cower in its presence. (Although one could be forgiven for thinking the comet looks more like a strange alien craft come to tip the battle in the Normans' favor, but that's just how celestial phenomena tended to be drawn back then.)

As a final bit of proof, the long-lived monk and astrologer Eilmer of Malmesbury - who is rather awesomely best known for trying to fly using a pair of artificial wings - wrote of Halley's Comet as the destroyer of Anglo-Saxon Europe in 1066 while also recognizing it as the same comet that had appeared 75 years before in 989:

"You've come, have you? ... You've come, you source of tears to many mothers, you evil. I hate you! It is long since I saw you; but as I see you now you are much more terrible, for I see you brandishing the downfall of my country. I hate you!"

Genghis Khan's inspiration

But spurring on the Norman invasion of the British Isles wasn't enough for Halley's Comet - its encore in 1222 was even more world-changing. Genghis Khan is said to have considered the comet his own personal star, and its westward trajectory inspired him to head west himself, launching his invasion of southeastern Europe that would leave millions dead. In fairness to the comet, invading Europe is the sort of idea Genghis Khan probably would have thought up without any celestial intervention.

Beginning in the sixteenth century, the comet moved from the domain of astrology to that of astronomy. In 1705, British astronomer Edmond Halley published Synopsis Astronomia Cometicae, demonstrating the comets of 1456, 1531, 1607, and 1682 were all one and the same object, and it would return again in 1758. He didn't live to see that he was correct, but he at least gained the immortality of having the comet named after him.

The death of Mark Twain

Anyone who lives a decently long life should have the opportunity to see the comet for themselves, although very few people actually plan their lives around its apparitions. Mark Twain, as with so many other things, was an exception. The legendary humorist was born two weeks after it appeared in 1835, and he said it would be the greatest disappointment if he didn't die when it returned in 1910. Thankfully for him, he kept his promise, dying a day after its return.

How Halley's Comet sightings changed history over the past 2500 yearsS

Unfortunately, practically no one alive today has firsthand experience with the full brilliance of Halley's Comet - its 1986 appearance was the dimmest ever recorded. Of course, the people of 1986 had one thing going for them that their predecessors didn't - they could send up spaceships to get a look up close at the comet. Tragically, the ill-fated Challenger mission was slated to investigate the comet for NASA, so it fell to the rest of the so-called Halley Armada, made up of Soviet, European, and Japanese probes, to investigate the comet. You can see one of the images snapped in 1986 above.

Tracking the comet today

These days, Halley's Comet can be found in the outer solar system, its eccentric orbit taking it far from the orbital plane on which the planets revolve. You can track its current progress through the solar system here. Those waiting to see the Halley's Comet again - or those who weren't around in 1986 - have got another 51 years to wait, as it won't be back until July 18, 2061.

Thankfully, the 2061 apparition will be quite a bit more impressive than the 1986 vintage, and those planning on living another 124 years should be in for a real treat on May 7, 2134, when the comet comes within 14 million miles of Earth. Who knows what future invasions it'll predict, future generals it will inspire or terrify, and authors it will kill off. Whatever happens, Halley's Comet should definitely be worth waiting for.

[Journal of Cosmology for information on the Greek records of 466 apparition; arXiv for information on the coin of Tigranes the Great.]