Halley's Comet

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Halley's comet orbits the sun about every 76 years in a long elliptical orbit that takes it out beyond the furthermost planets in the Solar system.
Its direction is retrograde. When it comes near enough to see we are travelling like express trains in opposite directions so we don't see it for very long, (usually less than 50 days).
If we know the True Anomaly, (given the Greek letter n, 'nu'), of its orbit we can date its closest approach to the sun, (called the perihelion date).  We can then work out where our earth will be in its orbit while we can see the comet and what sort of view we will have.
In the rest of this presentation we will use n loosely to mean the perihelion date.
n varies because the orbit changes slightly all the time.
The gravity fields of other planets tug at the comet,
The solar wind buffets it about,

Dust and gasses escaping from it act like little jet engines.

Until fairly recently we knew it was due to appear at around 164 BCE, but  the best guess for n had an uncertainty of some 150 days.
In 1985, Stephenson, Yau and Hunger* looked at fragments of clay tablets containing the records of Babylonian Astronomers  and narrowed n down to the 17 days  between November 9th and 27th, 164 BCE.

*Records of Halley's Comet on Babylonian Tablets" - Nature (Vol. 314, April 1985 - pp 587 - 592)