
RE: Just for fun:
ptc1181807 Jun 20, 2007 6:27 AM (in response to ptc1181807)Man, you guys are no fun at all....... Am I posting things no one even wants to think about?
Defining a sphere.
ptc3840842 Jun 20, 2007 7:48 AM (in response to ptc1181807)if one thinks about a 3legged milking stool which will fit onto any surface or sphere, it follows that 4 points are needed to define a sphere
Re: Defining a sphere.
ptc1181807 Jun 20, 2007 8:18 AM (in response to ptc3840842)"Richard Alcock" wrote:
if one thinks about a 3legged milking stool which will fit onto any surface or sphere, it follows that 4 points are needed to define a sphere

Re: Defining a sphere.
GrahameWard Jun 20, 2007 9:33 AM (in response to ptc1181807)Make a datum plane thru pts 1,2,3. Then sketch a circle thru the 3 pts on the datum plane. Uh... sketch a point at the center of the circle? Uh... make an axis thu the point normal to the datum plane? Do the same with another combination of the the 4 points, like say 2,3,4. where the axes intersect is the center of the circle. Maybe.
Re: Defining a sphere.
ptc1181807 Jun 20, 2007 9:50 AM (in response to GrahameWard)"Grahame Ward" wrote:
Make a datum plane thru pts 1,2,3. Then sketch a circle thru the 3 pts on the datum plane. Uh... sketch a point at the center of the circle? Uh... make an axis thu the point normal to the datum plane? Do the same with another combination of the the 4 points, like say 2,3,4. where the axes intersect is the center of the circle. Maybe.

Re: Defining a sphere.
GrahameWard Jun 20, 2007 9:56 AM (in response to ptc1181807)Definitely make it a Guinness! Yes, I'd put a line from the perimeter to the center of the circle too. That would be apparent if I was doing it on Pro rather than in my head.
Re: Defining a sphere.
ptc1181807 Jun 20, 2007 10:02 AM (in response to GrahameWard)"Grahame Ward" wrote:
Definitely make it a Guinness! Yes, I'd put a line from the perimeter to the center of the circle too. That would be apparent if I was doing it on Pro rather than in my head.

Re: Defining a sphere.
GrahameWard Jun 20, 2007 10:04 AM (in response to ptc1181807)"Grahame Ward" wrote:
Definitely make it a Guinness! Yes, I'd put a line from the perimeter to the center of the circle too. That would be apparent if I was doing it on Pro rather than in my head.


Re: Defining a sphere.
ptc1275435 Jul 10, 2007 9:07 PM (in response to ptc1275435)fun ended .......
Re: Defining a sphere.
ptc1181807 Jul 13, 2007 6:29 AM (in response to ptc1275435)Wow! Seems this is a popular thread! Anyone else try/gigure this out besides Grahame?











Re: Just for fun:
FrankS.Schiavone Mar 12, 2012 6:45 AM (in response to ptc1181807)Man, this is funny, I'd forgotten I'd started this under a different name way back in the day. Interesting.

Re: Defining a sphere.
ckeller Apr 7, 2017 9:59 AM (in response to ptc1181807)Funny this conversation was from 10 years ago.... hahaha.
you only need 2 points if you already know it's a sphere.
All axes that are perpendicular to the surface of a sphere will go through the center of that sphere.
Since all axes go through the center, then any 2 will intersect at the center.
Therefore, you only need 2 such points to define 2 such axes to find the center.
Then you can measure from that intersecting point to either of the 2 points on the surface, and you will have your radius.
However in order to define a sphere from points, you'd need 4 points.
Of course, not just any 4 points, you'd need 4 points that are not all coplanar, or colinear, or coincident.
It won't work if any 3 points lie on a line, or if all 4 are coplanar, or if any of the points are coincident.
2 points define a line
or a point if they are coincident.
3 points define a plane unless they all lie on the same line
or a line if they are all colinear
4 points define a sphere
or a plane if they are all coplanar
of a line if they are all colinear