[–] nicky_haflinger 0 points 1 point 1 point (+1|-0) ago
If the two foci of the ellipse are coincident then the result is a circle. Ellipses with foci that are fairly close look very circle-like. It might be an ellipse but it would have been nice to have an explicit demo.
[–] sillyquiet ago (edited ago)
not all ellipses are circles. Circles, however, are a special case of ellipse in which the both foci coincide, which means some ellipses are in fact circles.
[–] sqeak ago (edited ago)
Please demonstrate, I don't understand, all I can find is this.
[–] sillyquiet ago
As usual, Wikipedia illustrates better than I can. See here: https://en.m.wikipedia.org/wiki/Ellipse, under "elements of an ellipse". The last paragraph mentions that when the eccentricity, that is the ratio of the distance between the foci and the length of the major axis, is zero the ellipse is a circle. An analogy would be circle:ellipse as coupe:automobile. Not all automobiles are coupes, but all coupes are automobiles.