Feynman's Lost Lecture

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Feynman's Lost Lecture: The Motion of Planets Around the Sun

Feynman's Lost Lecture is a short book based on a 1964 Caltech talk by Richard Feynman about why planets move in ellipses. The notes and photos for the talk were lost for years and only later recovered, with the chalkboard drawings missing. Caltech physicist David L. Goodstein and archivist Judith R. Goodstein helped restore the lecture and turn it into a book. The talk was given to a freshman audience and aims to show an elementary, geometric way to derive Kepler’s first law from Kepler’s other two laws.

In the lecture, Feynman presents a simple geometric argument (in the spirit of Newton’s Principia) that explains why planetary orbits must be elliptical. He uses the idea of a hodograph, the path traced by the planet’s velocity, to make the reasoning clear. He explains that he chose a geometric approach because he found Newton’s original proof in the Principia hard to understand. A similar line of thinking had already appeared in James Clerk Maxwell’s 1877 work, Matter and Motion.

The book also includes about twenty minutes of informal Q&A at the blackboard with students from the lecture. The edition was published by W. W. Norton & Company in 1996 and runs 191 pages.