NavList:

This is off-topic for navigation but "in the aura" around it (or the "fog" around it, if you prefer).

How fast is Halley's Comet moving away from the Sun today in knots [see PS]? And when will that speed be zero? This is normally called aphelion, but it's a little tricky since the orbits of comets are frequently perturbed by planets and also affected by "propulsive" forces from out-gassing on the comet's surface. A related question: when is the halfway date? There was a perihelion of Halley's Comet in early 1986 --call it 1986.1, and the next one will be in the summer of 2061 --make that 2061.6. Average those years, and you'll get 2023.85, which is right about now. But what's the date between the two perihelion dates when you go for the actual day of the year? Does this match the date when the comet starts falling back toward the Sun? Probably not quite. Is there a proper quantitative explanation for the difference?

Frank Reed

PS: Why ask for the speed of a comet in knots? A good reason? No. A lighthearted reason: that at least gives us a modest connection to traditional navigation. :) Also, Edmond Halley himself was an experienced ocean-sailing mariner, and I think he would approve.