[Wavytone]

Paul,

In Donald Yeomans 1981 paper the conclusions at the bottom of page 644 contain two interesting points.

Firstly that the positions of comet - earth - sun aren't often favourable for it to be easily seen - ie in dark sky (elongation angle from the sun), and at about 0.25 AU. As he points out, from 1404 bc to 315 bc, there were only two favourable (1266 bc and 1404 bc).

Secondly there is the problem of whether the close approach is before (not so bright) or after perihelion (when its brighter). "... from 240 bc to 1910 there were eight apparitions for which the comet was observable at A < 0.25 au while for the 1404 bc to 315 bc apparitions there were none."

This is interesting because having seen it in 1985-6, there certainly was a huge disparity between its pre and post perihelion appearance. The pre-perihelion appearance was not great and you had to know exactly what you were looking for; many ordinary people simply never noticed it at all. Post perihelion, before dawn it was as plain as your index finger standing above the eastern horizon. But how many would be awake and outside at 3am.... let alone would notice a comet in the sky.

Add to that, two issues regarding the practical aspects of actually seeing something like a comet - from having spent some time there (including the Lake District and in the highlands), and having seen Halley in 1985-6.

Given the high latitude of the UK (and the Lake District) from May - August the nights are very short and twilight lasts for hours. Through summer, the hours of real darkness are extremely limited, so the chances of seeing a less than spectacular comet at low elongations from the sun are quite slim (nil, I suggest) unless it was a really bright "Great Comet" like that of 1910. But... Halley was never a Great Comet, as far as anyone knows. Which suggests an apparition in summer is unlikely to have been seen in the UK, while an apparition from October through winter and spring is more likely to be possible.

And then there's the weather - I can recall some pretty awful winters in the UK where weeks passed between any chance of seeing a clear sky, so its entirely possible that even if it was theoretically visible, it may have been missed entirely. How anyone ever did much astronomy in the UK astounds me.