measure the speed of light with Kepler

Okay this idea is dumb but I would love to see it done: As Kepler goes around the Sun (no, not the Earth, the Sun), it is sometimes flying towards its field and sometimes away. This leads to classical stellar aberration (discovered by Bradley in the 1700s; Bradley was a genious, IMHO), which leads to a beaming effect, in which the field-of-view (or plate scale) changes with the projection of the velocity vector onto the field-center pointing vector. A measurement of this would only take a day or two of hard work, and would provide a measure of the speed of light in units of the velocity of Kepler in its orbit.


  1. This is a fun idea — did you know that you can do it with a small telescope from the ground?

    I've always wanted to try it, but it would take a few years of observation, a good filter set and camera, an alt-az telescope well protected from the wind, and a pier that was not subject to swaying from a building (so, most rooftop observatories might not work.)

    Someday I'll write up the lab...

    1. I guess it is obvious you can do it from the ground, since Bradley did it before 1729! But yes, I agree that this is a great undergrad project...

  2. An alternative interpretation of the result, BTW, would be to measure the orbital motion of Kepler (or the Earth, in my experiment) in units of the speed of light. With nothing but a camera!

  3. OK, you inspired me to write up the idea I had for my lab.