Determination of the speed of light: What's actually happening in Ole Roemer's Google Doodle

There's a lot going on in that little curly-haired cartoon's head

An image of Jupiter with one of its moons, Io, in the foreground, taken by the Cassini probe in 2001

The little curly-haired cartoon that is wandering around Google's homepage seems to be spending a lot of his time waiting. And that's what the real Ole Roemer, who the image depicts, mostly did – right up until he made one of the most important discoveries in the history of science, and secured his place in Google's image.

Today's Google Doodle depicts the way that humanity learnt about one of the most fundamental pieces of information in the universe – how we determined not only the speed of light, but that light had a speed at all.

The Doodle shows the structure of our solar system, and also Roemer looking up at it through a big telescope while wandering around and timing himself. Which is a fairly accurate depiction of how Roemer made the calculations that would go on to shape our understanding of the universe in the most fundamental ways.

It's really the two Os in the word Google that matter. In the drawing, those become the Sun and Jupiter, which are in turn being orbited by the Earth and the moon Io, respectively. It was the relationship between those four bodies that Roemer used to find the speed of light, and he did so in a way that is hinted at in the Google Doodle.

Roemer needed to use those planets because the speed of light is just too quick to do it with anything smaller – something that Gallileo found when he tried to measure its speed by sending light between two people stood on a hill. It needs to be measured at such huge scale because it's just impossible for humans to see its speed at any smaller level.

Instead, Roemer looked up – just as he's shown doing in the Google Doodle. By looking at how eclipses of Jupiter's moon and noting how long they took and when they happened, he established not only that light took time to move through space, but also had a very good guess at how quickly it did so.

The method worked by watching for those eclipses. He found that the interval between the eclipses seemed to be about seven minutes longer when the earth was moving away from Jupiter than when we were getting close to it. On that basis, he worked out that the time was increasing because it took longer for the light to travel to reach the earth.

By combining that information with what he knew about the size of the Earth and how quickly it was travelling, Roemer was able to work out that what we were seeing was the result of the time taken for the light to get to us – and to do a remarkably good estimate of the speed of light itself.

Roemer's calculations were a little out, and would go on to be refined. His working suggested that light travels about 220,000 kilometres per second, and so his estimate was about 26 per cent below the actual speed of travel.

All of those different pieces of working come together in the Google Doodle. The diagram showing the objects in our universe depicts how Roemer would have known where they all were while observing them, for instance, and the swinging pendulum in the bottom-right corner depicts the kind of timepieces that were necessary to understand how long each observation took – the precision of which were central to his work.