Measuring the Speed of Light ...DIY Style!

Want to measure the speed of your light from your own back yard? Can't afford the PASCO Foucault kit? Well this post is for you!

First, don your powdered wig because we're going to do it like they did it in the 17th century. Ole Rømer was the first person to painstakingly record the orbital period of Io for this purpose, starting in the year 1671.

You should be able to pull this off from your backyard if you have a tripod and a small telescope, or a good pair of binoculars that can be mounted.

The following vignette lets you play with time in the year 2021.

• The top canvas shows how the positions of Earth, Jupiter, and Io are roughly related. Not to scale!
• The bottom canvas depicts Jupiter's moons as seen from Earth, with a built-in time delay that accounts for the speed of light. This includes Io, Europa, Ganymede, and Callisto.

This experiment can performed using the above applet or from your back yard. Measure the orbital period of Io around Jupiter at two different times of year.

If you're using the applet, a good way is measure orbital period is to record two subsequent eclipse events. Note that you can record either the entry or the exit time of the eclipse, but if you were doing this experiment in real life, you'd need to choose entry vs exit based on the time of year!

Press the "Stash Time" button to copy the selected time in the form below. After stashing the time, scrub to the next event of interest and note the value for "Selected Time - Stashed Time". This value should correspond to Io's observed orbital period.

Now it's time for some math... MORE TEXT HERE, TBD

@prideout
January 2022

• I used the astronomy-engine library, written by Don Cross, to determine absolute positions of various celestial bodies.
  (github link)
• English Translation of Rømer's paper, dated 1677 - 1678.
  (download pdf)
• An analysis of Rømer's paper by James H. Shea, published in 1998.
  (download pdf)
• Simplified version of this page