Somehow I missed this anniversary, and the Google Doodle shown below isn’t visible from the U.S. Here’s the skinny from CNet:
While my car has trouble going over 65 mph, the speed of light is much faster — approximately 186,282 miles per second. How do we know that? Well, we can all thank Danish astronomer Olaus Roemer.
To honor Roemer and the 340th anniversary of the determination of the speed of light, Google made a Doodle.
Roemer determined the speed of light in 1676 by observing the planet Jupiter eclipsing its moon Io 140 times. His measurements were taken from Copenhagen while his peer Giovanni Domenico Cassini took measurements of the same eclipses in Paris. Roemer compared the results to determine the speed of light.
In the Google Doodle, you see Roemer repeatedly and thoughtfully pacing back and forth after viewing his telescope. This simple illustration shows just how much detail and thoughtfulness Google applies to its Doodles.
There’s more on this experiment on a Wikipedia page, which describes the method—a very clever one. It’s based on the assumptions that the period of revolution of Jupiter’s moon Io around its planet would be constant, but that as the Earth moved farther from Jupiter over a period of time, the time that Io appeared to us from behind its planet would be greater because light would take longer to travel to Earth. By measuring the time differential over many revolutions of Io, Rømer calculated that the speed of light was 220,000 kilometres per second. That’s pretty accurate: about 26% lower than the true value of 299,792 km/s. His observations were controversial, but the order-of-magnitude accuracy was supported by other astronomical observations, and then refined to the present value by experiments involving measurements solely on Earth:
More data, and why we’re celebrating on this date, comes from Wikipedia (my emphasis):
On 22 August 1676, Rømer made an announcement to the Royal Academy of Sciences in Paris that he would be changing the basis of calculation for his tables of eclipses of Io. He may also have stated the reason:[note 4]
This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.
Most importantly, Rømer announced the prediction that the emergence of Io on 16 November 1676 would be observed about ten minutes later than would have been calculated by the previous method. There is no record of any observation of an emergence of Io on 16 November, but an emergence was observed on 9 November. With this experimental evidence in hand, Rømer explained his new method of calculation to the Royal Academy of Sciences on 22 November.
The original record of the meeting of the Royal Academy of Sciences has been lost, but Rømer’s presentation was recorded as a news report in the Journal des sçavans on 7 December. This anonymous report was translated into English and published in Philosophical Transactions of the Royal Society in London on 25 July 1677.