foci

In an orbit made elliptical by the planetary attraction the sun necessarily occupies one of the foci of the ellipse.

That an ellipse may be given any required proportion of width to length by locating the foci at the requisite distance apart.

That the proportion of length to width in an ellipse is determined by the distance apart of the foci.

The curve thus traced will be an ellipse having the pins at the two points which are called its foci.

These are at points on the principal axis that are twice as far from O, the center of the lens, as are the principal foci.