It’s a little complicated, but for now, let’s just say that on this day in 240 BC, Halley’s Comet was discovered. And now that we’ve said that, let’s admit that it isn’t true. The main way that it isn’t true is that this isn’t the date it was noted. This is the date that the comet reached its perihelion — the position when it is closest to the sun. The comet was seen before and after this date.
This appearance was documented in Records of the Grand Historian, Sima Qian’s history of ancient China that was published some time around 109 BC. The reason we know it was the Halley’s Comet is because Halley calculated when the comet had come and when it would come. So we know that it should have shown up in May of 240 BC, and Sima Qian made note of a comet at that time that appeared in the east and moved north.
Interestingly, it is probably the case that this is not the first documented sighting of this most famous comet. Halley should also have dropped by for a visit in 467 BC. A comet between the years of 468 and 466 BC was noted both in ancient Greece and ancient China. But the dating is uncertain, so we can’t say for sure that it was Halley.
So happy sorta birthday Halley’s Comet!