Foci (focus points) of an ellipse
Two points inside an ellipse that are used in its formal definition. See Ellipse definition.
Try this Drag any orange dot. As you reshape the ellipse, note how the two focus points (F1 and F2) move.
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An ellipse has two focus points. The word foci (pronounced 'foe-sigh') is the plural of 'focus'. One focus, two foci.
The foci always lie on the major (longest) axis, spaced equally each side of the center.
If the major axis and minor axis are the same length, the figure is a circle and both foci are at the center.
Reshape the ellipse above and try to create this situation.
Note how the major axis is always the longest one, so if you make the ellipse narrow,
it will be the vertical axis instead of the horizontal one.
