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A circle tranforming into ellipse

  1. Sep 20, 2012 #1
    I don't know what category this question falls into. I have two parallel planes, on one I draw a circle and on the other I project it orthogonally. Now I incline the plane with the circle. The projection on the other plane will be an ellipse. I need to find out, the relationship between the angle of inclination and the eccentricity of the ellipse.
  2. jcsd
  3. Sep 20, 2012 #2
    I've figured it out. If a is the semi-major axis, b is the semi-minor axis of the ellipse and r is the radius of the circle, then a= r and b= rcosθ (θ is the angle of inclination). Now it isn't difficult to find the eccentricity of the ellipse. But.. how to prove that an inclined projection of a circle is an ellipse in the first place?
  4. Sep 20, 2012 #3


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    Look at the projection of a circle between parallel planes as a cylinder. then the angle of the second plane intersects the cylinder.
  5. Sep 21, 2012 #4
    I was looking for a mathematical proof of "a non-uniform scaling of a circle changes its shape into an ellipse".
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