# Thread: Questions about points on the surface of a sphere & great circles...

1. ## Questions about points on the surface of a sphere & great circles...

Hi,

Is it true the for any 2 points on the surface of a sphere it is possible to draw a great circle that passes through both points?

B

2. ## Re: Questions about points on the surface of a sphere & great circles...

The shortest path between two points on a sphere, also known as an orthodrome, is a segment of a great circle.
source ...

Great Circle -- from Wolfram MathWorld

3. ## Re: Questions about points on the surface of a sphere & great circles...

Originally Posted by Bwts
Hi,

Is it true the for any 2 points on the surface of a sphere it is possible to draw a great circle that passes through both points?

B

Of course, any three points form a plane (your two on surface and the center of sphere)

4. ## Re: Questions about points on the surface of a sphere & great circles...

Thanks my follow up question is this, if using the same 2 points I construct a another circle on the sphere's surface (not a great circle this time) how would I prove that the segment of this new circle is greater then the great circle segment between the same points?

I think it must have something to do with the curvature of the circles but am having trouble visualising it?

5. ## Re: Questions about points on the surface of a sphere & great circles...

Originally Posted by Bwts
Thanks my follow up question is this, if using the same 2 points I construct a another circle on the sphere's surface (not a great circle this time) how would I prove that the segment of this new circle is greater then the great circle segment between the same points?

I think it must have something to do with the curvature of the circles but am having trouble visualising it?

the shortist distance between points on the earths surface is along a great circle.Navigators have to change course regularlyto follow such a route.Points are places like New york and London and are not changable

6. ## Re: Questions about points on the surface of a sphere & great circles...

Yes but how do I prove this is the case?

There are many arcs on the surface of a sphere that will pass through the same two points.

7. ## Re: Questions about points on the surface of a sphere & great circles...

Originally Posted by Bwts
Yes but how do I prove this is the case?

There are many arcs on the surface of a sphere that will pass through the same two points.

Look up the definition of great circle and remember that 3 points create a plane. Two surface points and center of sphere.Take a ball and mark two points less than a diameter apart.Measure the distance by using a string. Try to find a route shorter than the string