James Clerk Maxwell's theory of electromagnetism tells us that light is an electromagnetic wave traveling at 300,000 kilometers per second (kps). Maxwell's equations tell us that changing electric and magnetic fields create and sustain each other even in regions where there are no electric charges to accelerate or magnets to move. Maxwell showed how these two fields, interlocked in a dance, create their own light show. The fields spread out through space as light or as any other electromagnetic wave.
But before Maxwell, other scientists made their own attempts to identify the nature of light and to calculate its speed.
Galileo: Hanging lanterns
How fast does light travel through space? The modern value for the speed of light is 300,000 kps (186,000 mps). Actually, it's 299,792.458 kps, but that's a tough number to remember. The circumference of the earth is about 40,000 km (25,000 mi), so it would take light slightly longer than a tenth of a second to travel around the world.
With modern instruments, the extremely large value of the speed of light can be measured. But how did anyone measure it before those instruments were created?
Galileo was the first person who tried. He had two people stand on distant hills flashing lanterns. Clearly, Galileo's experiment didn't work — he couldn't even measure seconds accurately, much less the tiny fraction of a second that it took for light to travel between the two hills.
But Galileo was Galileo, and with his crude approach to this very difficult experiment, he was still able to show that the speed of light is finite. His contemporary, French philosopher René Descartes, had been saying that it was infinite.
Roemer: Timing a satellite
Some 70 years after Galileo's experiment, the young Danish astronomer Olaus Roemer was able to get the first value of the speed of light. But he had to go farther than a distant hill to get it. He used the satellites of Jupiter instead. And he also had to fight with his boss — the famous astronomer Jean-Dominique Cassini, for whom the Saturn rings are now named.
Tackling an inconsistency
Roemer was a bright 21 year old who was hired by one of Cassini's assistants to help at the Paris Observatory, which was headed by Cassini. But Roemer didn't just help; he tackled one of the observatory's major problems.
Cassini's observations were showing a problem with the motion of one of Jupiter's satellites, the one named Io (after one of the many lovers of Zeus, who is called Jupiter in Roman mythology). It seemed as if Io's orbit was a bit unpredictable. The times when the satellite came out from behind the planet changed inexplicably. Cassini ordered his assistants to make better observations and to do more calculations.
Roemer doubted that the observations or calculations were the problem. The problem was that no one had taken into account the relative distance of the Earth and Jupiter as the two planets went around the sun. At different places in their orbits, the planets are sometimes closer and sometimes farther apart. When Io comes out from behind Jupiter, the distance that light travels from the satellite to the Earth depends on the separation of the planets at that time.
Cassini didn't agree with his assistant. He believed that light traveled from place to place instantaneously, without delays. It didn't matter how far Jupiter was.
Roemer stuck with his idea. He went back and reviewed many years' worth of data taken in Cassini's observatory. With this data, he was able to calculate the changes in the eclipsing times for Io as it went around in its orbit. He was sure that he was right and wanted to go public.
Going around the boss
What to do? Normally, the lab director would make the public presentation of new findings, along with the researcher who made the discovery. But Cassini didn't agree with Roemer's work, so Roemer decided to go alone. He'd been in Cassini's observatory for five years and felt cocky. He appeared before the Academy of Sciences in Paris and announced that Io was going to come out from behind Jupiter exactly ten minutes after Cassini said it would.
Cassini had calculated that Io was going to come out of the eclipse on November 9, 1676, at 5:25:45. The astronomers went out to look that night. 5:25:45 came and went, and Io wasn't there. At 5:30, there were still no signs of it. But at 5:35:45, Io reappeared. Roemer had been right.
Roemer's friend Christian Huygens used this data to come up with the first measured value for the speed of light. His number was 227,000 km (140,000 mi) per second, which is about 24 percent lower than the modern value.
Cassini never admitted his error. Most European astronomers followed Cassini and didn't believe that the speed of light was finite. Some 50 years later, other methods to measure the speed of light showed that Roemer had been correct.
By the time Einstein was in school, the speed of light had been measured with fairly good accuracy. This speed, represented in Einstein's work by the letter c, ended up at the very foundation of his special theory of relativity.