1. The problem statement, all variables and given/known data Halley’s Comet passed perihelion on 9 February 1986. It has a semi-major axis a = 17.96 AU and eccentricity e = 0.9673. (One astronomical unit (AU) is the distance between the Earth and the Sun.) Calculate the period of Halley’s Comet and the approximate date on which you might next see it. 2. Relevant equations Period = T = 2pi(sqrt(a^3/u) 3. The attempt at a solution I realize all this information is available over the internet but I need to work it out by hand. I attempted this question first using the value for 'a' given and the standard gravitational parameter (u) of earth 3.986E14 but realised this is wrong and that halley's comet would have a different value of (u). I assume the equation for the value of (u) of halley's comet will be related to its eccentricity but can't seem to work it out. Any ideas?