On my Lathe...
I have turned smaller balls, Dont go direct between spur and live center, leave a stub on each end. and work toward as small a point as possible. Use a template, cut a "Perfect1/2 circle" in the bandsaw or Jig saw. (allowing for the stubs). Center up and usuing your best lathe skills begin working down the shape, test fitting the template. till the shape is correct. Sand and smoothe as much as you can, then hand cut each stub closer to severed untill you fear it will break,cut off stubs and then file & sand the area.
I also used a cup chuck (for loss of a better name" I used a faceplate and some thick stock and turned a "bowl", the inside and rim become the driving force and I also turned a smaller cup that will fit over the live center. Place the ball between these two cups (slower speeds) tighten the tailstock and turn on lathe, Of courswe there will be some off center turning, turn off this lump, then re locate the ball in th chuck and again smoothe out the hump, keep relocating and turning off the lump, you willfind that the lumps are less and the ball is smoother (althought shrinking in size) When you have it "perfect" repeat the process with sanding.
Or at least that is the way I do (did) it...
On that note, I see fellows who get all excited building segmented bowls and vessels, but haven't seen any segmented "Balls" I bet they would make a great art form, and conversation piece, yet I see none.
Sorry, I didn't look at Matt's post (lazy me) but "Thats what I'm Talking about" better presentation, nice pictures, and good advice. :Thumb