comet observed over longest historical time

Observations of Halley's Comet, formally known as Comet 1P/Halley, have been traced back as far as 239 BC. There is no historical record for any other periodic comet that can compare with that of Halley's Comet. Halley's Comet uniquely has been observed at 30 apparitions over more than 2000 years because it is much larger and more active than other periodic comets. It was named for Edmund Halley who in 1705 realized the connection between several previous cometary apparitions, and predicted its return in 1758- 59. In 1986, the Giotto spacecraft succeeded in imaging its nucleus from a distance of only 10,000 km, and found it to be 15 km long by 8 km wide. The coma and tail, which make the comet so conspicuous, form when heating by the Sun causes gas and dust to erupt as jets from the dark crust overlying the icy interior of the comet nucleus.