Danish astronomer was first to estimate the speed of light 300 years ago

By Mark Weston
Tuesday, September 21, 2010; HE06

The man who first measured the speed of light died 300 years ago this week. That's right, 300 years. By the time Einstein published his Special Theory of Relativity in 1905, the speed of light -- that's the C in E=MC{+2} -- was old news.

A Danish astronomer, Ole Romer, figured out how to calculate it in 1676, using just a telescope and a clock.

In the 1630s, Galileo had tried and failed to measure the speed of light as it traveled from one mountain to another. Light took less than 1/20,000th of a second to move between the two peaks that he chose, and no clock was even remotely accurate enough to measure that tiny duration. Galileo was baffled. Light seemed to have no speed at all, but to be instant.

Forty years later, Romer was invited to France to work at the Royal Academy of Science, where he taught physics to the eldest son of King Louis XIV. Soon after he arrived, French astronomers perceived that the position of Mars in Paris's night sky was slightly different from the position of Mars in the night sky of French Guiana, on the northern coast of South America. Using this information, they triangulated the distance between Paris, French Guiana and Mars. Astronomers had studied the orbits of the planets for decades and knew the ratios of the distances between them. Once they knew the distance from Earth to Mars, they could calculate the distances to other planets. They also estimated the distance between Earth and the sun, which today we know to be nearly 93 million miles.

Meanwhile Romer, with the king's financial help, built a telescope 10 feet long. He liked to watch Jupiter, with its brown and white stripes, and Io, the fastest of its four large moons. He saw that Io orbits Jupiter four times a week, but soon observed something puzzling. The time Io seemed to take to move around Jupiter varied by a few seconds from one orbit to the next.

Romer found that every month, for six consecutive months, Io seemed to take an extra 3½ minutes to move around Jupiter, so that after half a year Io seemed to take 22 minutes longer to orbit Jupiter than it had taken before.

Then Io appeared to regain speed. Every month, for six straight months, Io appeared to take about 3½ minutes less to orbit Jupiter, so that after half a year, Io's orbit seemed to have shortened by 22 minutes. The amount of time Io took to orbit Jupiter now seemed the same as it had been a year before.

Romer doubted that Io was actually changing its speed, and he wondered what caused this illusion. Then it hit him. Io appeared to speed up and slow down not because of anything that was happening near Jupiter or Io every six months, but because of the changing distance between Earth and Jupiter as both planets orbited the sun. When Jupiter and Earth were on opposite sides of the sun, light from Jupiter -- and its moons -- needed more time to reach Earth. When Io seemed to take 22 minutes longer to orbit Jupiter, it was because light was traveling the extra distance to Romer's telescope.

Romer knew that Earth was roughly 90 million miles from the sun, making its orbit about 180 million miles in diameter. That was the extra distance the light had to travel when Earth and Jupiter were on opposite sides of the sun. When the Royal Academy published Romer's findings in December 1676, it divided this distance by 22 minutes, and estimated the speed of light to be 130,000 miles a second, an inconceivably high velocity.

In fact, light travels even faster than Romer thought.

Within 30 years of Romer's discovery, astronomers with better telescopes determined that light crosses Earth's orbit in only 16½ minutes, not 22, at a speed of more than 186,000 miles a second. They didn't change Romer's method of calculation; they just had better data to feed into it.

Romer returned home to Copenhagen to marry, to be his country's chief astronomer and to help a German glassblower, Daniel Fahrenheit, invent a better thermometer. On Sept. 19, 1710, he died at age 65.

Weston's most recent book is "Prophets and Princes: Saudi Arabia from Muhammad to the Present."

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