## Physics Questions People Ask Fermilab

The first measurement of the light speed

You Wrote:
Can you please inform me who the first person was to measure the speed of light and how it was done.

Thanks,
Johan Venter

Hi Johan,
the fact, that the light travels with a finite speed was shown first by a Danish guy, Olaf Romer in 1675. He also made the first actual measurement of that quantity. Now you probably wander, how he was able to measure such a high speed without all these high-tech instruments available to the physicists of our era. Well, since he was an astronomer, he used the world's biggest lab, the universe.

I made up a little story how the discovery could have been done.

On the summer of 1675, when the Earth was on the line between the Sun and the Jupiter, (position A on the enclosed speed_light.gif) Romer had a date. He was walking in the night showing to his girlfriend the beauties of the night sky. At 9:00 p.m. he brought to her attention that one of the Jupiter's moon just went to the shade of the Jupiter (see point B), and therefore got invisible. She said wow, and everybody was happy.

Roughly after half a year, Olaf had another chance to take out the same woman, so he planned it in such a way, that they will walk in the night of the 104th enter of the Jupiter's moon to the Jupiter's shade. (Note: Romer knew the orbiting time of the Jupiter's moon ( 42 hours and 28.6 minutes), he calculated at home that the 104th disappearing will happen after 184 days, 1 hour and 34 minutes i.e. after about half a year ( when the Sun was on the line between the Earth ( see point C) and the Jupiter) at 10:34 p.m. again.) Indeed, after 184 days, he took his girlfriend out again, and tried to show off by predicting that the Jupiter's moon will get invisible, because it will enter the Jupiter's shade at 10:34 p.m. You can imagine, how surprised he was, when the moon was still visible at 10:34 and entered the shade only ~1000 seconds later, i.e. at ~10:50 p.m. !!!! ( The story does not say what she said at that time :-)) He started to think about this delay, and came up with the solution, that these 17 extra minutes must have came from the fact, that the light needed extra time to travel the extra distance ( AC) between the summer position and the winter position of the Earth with respect to the Jupiter. He knew this distance ( about 300 million kilometers), he knew the needed extra time ( the above 1000 seconds) therefore the light speed had to be about 300 000 km/s.

P.S. The main idea of the above is correct. This was the way how the discovery was done. However, you cannot see the Jupiter's moon with naked eye. Also the Jupiter moves around the Sun a bit in half a year (to point D), but this distance is negligible. The above discovery was made after plenty of hours spent by watching the sky with primitive optical systems available at that time, and by careful observation of the motion of planets.

If you do not mind, I will bring to your attention a recent, extremely exciting project called Sloan Digital Sky Survey, in which Fermilab plays an important role and the physicists hope for a deeper understanding of our universe and perhaps new discoveries. You will find interesting details and pictures on their web page.

Keep wondering,

- bye, Arnold Pompos

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