Jupiter, the Police, and the Speed of Light

Francois was an exchange student at a school where I taught.  He was originally from France, but moved to Montreal.  He had already taken every math and science course we had to offer, but he had never done any lab work!  So we decided to do lots of labs together.

Since I was interested in astronomy, I suggested trying to determine the speed of light by the method first used by Ole Roemer in the 17th century.  Eclipses of Jupiter’s moon Io occur at very regular time intervals, but they seem to occur sooner than expected when the Earth is traveling towards Jupiter, and later than expected when the Earth is traveling away from Jupiter.  The motion of the Earth (and to a smaller extent, the motion of Jupiter) changes the distance between the two planets, thus affecting the amount of time it takes for light to travel from one to the other.  For more info, click HERE.

The Earth travels approximately 1,600,000 miles in its orbit every day, and it takes light approximately 8.6 seconds to travel that far.  When the Earth is in position A, it is moving almost directly away from Jupiter, but when it is in position B, it is moving almost directly toward Jupiter.  Eclipses of Io take place approximately once every 1 and 3/4 days, so when the Earth is in position A, we should see eclipses take place 15 seconds late, but when the Earth is in position B, we should see eclipses take place 15 seconds early.  Francois and I tried the experiment.  I forget what we got for the speed of light - I think we were off by about 50%.  That really isn’t as bad as it sounds.  There are significant sources of error.  Eclipses occur gradually, and it’s tough to time them precisely.

More memorable, however, was our encounter with the police.  Some of our observations had to be made in the early morning (when at position B).  So there we were at about 5 AM, with an extension cord running through an open window of the school building, making an observation.  Up drove a policeman on patrol.  “What are you two doing here,” he asked.  “Observing the moons of Jupiter so we can determine the speed of light,” we answered.  “Do you want to take a look?”  Without saying a word, he shook his head, turned around, returned to his car, and drove off.  I guess our excuse was just too ridiculous not to be true!

Extra credit question: When in position A, we measured the time between consecutive eclipse reappearances; when in position B, we measured the time between consecutive eclipse disappearances.  Why?