# Student Colloquium, Christoffer Ravn Søndergaard

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Fys. Aud.

**Supervisor:** Thomas Tram

**Title:** Feynmans Lost Lecture; The elliptical motion of the planets around the sun

The motion of the planets around the sun is elliptical. This very simple fact described in Kepler's laws of planetary motion between 1609 and 1619 lead Isaac Newton to his description of gravity. Newton observed that the gravitational force must be acting along the line connecting the planet and the sun, and it must be inversely proportional to their separation squared.

In 1964 Feynman held a lecture on the motion of the planet around the sun, he started his lecture by stating that “Simple things have elementary demonstrations”. In this colloquium we follow the demonstration given in Newtons ‘*Principia Mathematica’*, and in Feynman's lost lecture, of the simple fact that the motion of the planet around the sun is elliptical. Feynman described this demonstration as elementary. Elementary in the sense that one goes as far back as possible in regards to required prior knowledge. The demonstration will be a purely geometrical one and will be without calculus.