On the next page of his book Dürer draws an ellipse correctly by elongating a circular arc.[br]But, he doesn't know it's an ellips...[br]We have to wait until 1640 when [url=https://nl.wikipedia.org/wiki/Paul_Guldin]Paul Guldin[/url] (1577-1643), een Swiss Jezuit, mathematician and astronomer who had contacts withJohannes Kepler, proved that an elongatet circle is an ellipse.

[list][*]Draw a semi circle within the rectangle ABCD.[/*][*]Draw vertical lines and decide the rectangle in twelve equal parts.[/*][*]Adjacent to the ractangle ABCD draw a second rectangle EFGH, longer than the first one and devide it as well into 12 equal strokes.[/*][*]Define the intersection points of the circle and the vertical lines and draw horizontal lines through these intersection points.[/*][*]In rectangle EFGH define the intersection points of the horizontal and the vertical lines and connect them by a continuous curve.[/*][/list]Drag the point H and see how the shape of the ellipse changes.