✹ About Friday 23 December 1664 Martin on 24 Dec 2017 • Link Comet: pace Andy, it's not in fact particularly difficult to predict the maximum elevation of a celestial object whose declination (elevation with respect to the celestial equator) is known. Just depends on latitude, as any navigator would have been able to tell Sam. Presumably the astronomers of the day knew the celestial co-ordinates of the comet, which would change, of course, but not so much from day to day as to make them useless. Or, of course, someone might just have measured the elevation recently and told Sam. Sandwich was making observations among others. The orbits of comets would not be solved for another generation or so.