Okay, so I get the concept behind Newton's version of Kepler's 3rd law, but everytime I go to do a problem with it, I get totally lost. This may be because I have never seen one done in a way that did not totally confuse me... I think it might be best if I just ask one of the questions that's bothering me... "The distance of closest approach of Halley's comet to the Sun is 8.9X10^10 m. Its period is 76 yr. Calculate the following: a) semimajor axis (b) eccentricity (c) aphelion distance (dist. farthest from sun)" So, here's what I know, semimajor axis = a = (Rmin +Rmax)/2, eccentricity = e = (Rmax - Rmin)/2a and of course Kepler's law T^2 is proportional to R^3, or newton's T^2/R^3 = 4(pi^2)R^3/GM So... I don't expect (or want) to be given the answer... i want to finally understand this stuff so that I dont have to continue struggling with it. Maybe there's some basic aspect of this that I've missed every time I've seen it...or maybe it's the continual use of different letters for variables that confuses me. I dont know... Any help would be appreciated.