Expert: Azeem Hussain - 3/2/2009

Question
Dear Azeem:  I am trying to be sure that I am using the proper terminology.  An egg shape (like a chicken egg) seems to me a special type of ovoid form because it is tapered at one end.  Is there a proper term for that shape?  If the ovoid form is the same on both ends like a race track, is that known as an elliptical oval?  What do you call it if it is a three dimensional form rather than a two dimensional figure?  Is there an authoritative source that might illustrate the two forms?
Many thanks for any help you can provide.

Answer
Hi Lisa!

Ovals are loosely defined in mathematics.  I'd like to first say that the term ovoid is derived from "ovum," meaning egg.  An ellipse has a defined semi-minor and semi-major axes, while egg shapes do not, but they still have one axis of symmetry (in 2D).  Race track shapes have two straight edges and two curves.  If it has no straight edges, it is not a race track shape.
Here's what I call them:
egg shape: ovoid ("egg shape" is acceptable)
race track shape: oval ("race track shape" is acceptable)

In three dimensions, if it has three equal radii, it's a sphere.  Two, it's a spheroid.  It can be either an oblate spheroid (kind of flat) or a prolate spheroid, like a rugby ball.  If the three radii are unequal, it is a scalene spheroid.  I would classify an egg as a prolate spheroid.

Hope this has been helpful,
Azeem