Mechanism of Ellipse Envelope

This video was made based on the design numbered as 995 in volume 2, Mechanisms in modern technic, I. Artobolevski.
Angle between two sides of blue bar = angle FAO = φ
When yellow crank rotates, a side of the blue bar is always tangent to the black ellipse.
O: ellipse center.
F: a focus of the ellipse.
Semi-major axis of the ellipse = L.sin φ
L is radius of yellow crank.

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