Keep an eye on the sky in the next 5 days – Jupiter and Saturn are getting ever closer together until the 21st Dec when they’ll look like one bright point in the sky.

harmonic oscillator in 2D, Kepler problem in 3D and free particle on sphere in 4D. These are the same thing in different guises.

Mysteries of the gravitational 2-body problem

The Ellipse and the Atom

Washington Post, please get a new Canadian columnist.