Feynman's lost lecture
In 1964 Feynman gave a guest lecture The Motion of Planets around the Sun to the Caltech freshman class, the aim of which was to present a geometric proof of the elliptical motion of planets around the Sun.
Inspired by Newton’s geometric way of proving Kepler's laws of planetary motion, which is presented in Principia (1687), Feynman derived his own geometric proof based only on elementary knowledge of plane geometry combined with the selected statements of Newton's laws of motion and gravity.
Owing to the temporary disappearance of records of this lecture it became known as Feynman’s Lost Lecture. After finding it among other documents from the Physics Department of Caltech it was published in a book Feynman's Lost Lecture, The Motion of Planets around the Sun by D. L. Goodstein and J. R Goodstein in 1996.