On This Day: Scientist Roemer presents finding that light travels at finite speed - Nov. 21, 1676
(edited from article)
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Speed of Light
it wasn't always known that light has a measurable speed. Light travels so fast that, at the short distances we are used to in everyday life, it appears to travel instantaneously. From the time of Aristotle until the seventeenth century, it was generally believed that light traveled at an infinite speed.
However, in the 1600s, Italian astronomer Galileo became convinced that light, like sound, had a measurable speed. He tried to measure this speed by clocking how long it took for flashing lantern signals to travel back and forth between two hilltops. Unfortunately, because light travels so fast and human reaction time over such a short distance is unreliable, this method failed.
Not long after Galileo's experiment, Danish astronomer Ole Roemer studied the time it took for one of Jupiter's moons, Io, to complete its orbit. He observed that the amount of time it took Io to reappear from behind Jupiter—that is, for the light reflected from Io's surface to be seen by Roemer—varied, depending on the distance between Earth and Jupiter. If the speed of light was infinite, there should have been no such delay. Using the diameter of Earth's orbit and this time lag, Roemer calculated the speed of light as 227,000 kilometers per second (140,000 miles per second)—about one-quarter below the modern value of 300,000 km/s (186,000 mi/s). Inaccurate though it was, this was the first physical evidence that light had a finite speed.
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https://www.pbslearningmedia.org/resource/lsps07.sci.phys.energy.lightspeed/speed-of-light/
(edited from article)
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Ole Roemer and the Speed of Light
Roemer, working at the Paris Observatory, was not looking for the speed of light when he found it. Instead, he was compiling extensive observations of the orbit of Io, the innermost of the four big satellites of Jupiter discovered by Galileo in 1610. By timing the eclipses of Io by Jupiter, Roemer hoped to determine a more accurate value for the satellite’s orbital period. Such observations had a practical importance in the seventeenth century. Galileo himself had suggested that tables of the orbital motion of Jupiter’s satellites would provide a kind of “clock” in the sky. Navigators and mapmakers anywhere in the world might use this clock to read the absolute time (the standard time at a place of known longitude, like the Paris Observatory). Then, by determining the local solar time, they could calculate their longitude from the time difference. This method of finding longitude eventually turned out to be impractical and was abandoned after the development of accurate seagoing timepieces. But the Io eclipse data unexpectedly solved another important scientific problem—the speed of light.
The orbital period of Io is now known to be 1.769 Earth days. The satellite is eclipsed by Jupiter once every orbit, as seen from the Earth. By timing these eclipses over many years, Roemer noticed something peculiar. The time interval between successive eclipses became steadily shorter as the Earth in its orbit moved toward Jupiter and became steadily longer as the Earth moved away from Jupiter. These differences accumulated. From his data, Roemer estimated that when the Earth was nearest to Jupiter (at E1), eclipses of Io would occur about eleven minutes earlier than predicted based on the average orbital period over many years. And 6.5 months later, when the Earth was farthest from Jupiter (at E2), the eclipses would occur about eleven minutes later than predicted.
Roemer knew that the true orbital period of Io could have nothing to do with the relative positions of the Earth and Jupiter. In a brilliant insight, he realized that the time difference must be due to the finite speed of light. That is, light from the Jupiter system has to travel farther to reach the Earth when the two planets are on opposite sides of the Sun than when they are closer together. Roemer estimated that light required twenty-two minutes to cross the diameter of the Earth’s orbit. The speed of light could then be found by dividing the diameter of the Earth’s orbit by the time difference.
The Dutch scientist Christiaan Huygens, who first did the arithmetic, found a value for the speed of light equivalent to 131,000 miles per second. The correct value is 186,000 miles per second. The difference was due to errors in Roemer’s estimate for the maximum time delay (the correct value is 16.7, not 22 minutes), and also to an imprecise knowledge of the Earth’s orbital diameter. More important than the exact answer, however, was the fact that Roemer’s data provided the first quantitative estimate for the speed of light, and it was in the right ballpark.
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https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/ole-roemer-speed-of-light#
(excerpt from article)
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OTD in Space - Nov. 21: Astronomer (Accidentally) Discovers Speed of Light
On November 21, 1676, the Danish astronomer Ole Rømer discovered the speed of light. Before Rømer figured it out, scientists thought that light travels instantaneously, or infinitely fast. Rømer disproved this almost by accident when he was studying Jupiter's moon Io. He was trying to figure out how long it takes Io to orbit Jupiter in hopes of using it as a cosmic clock. He watched Io disappear behind Jupiter and reappear on the other side. He did this over and over every 42 hours for years. To his surprise, the timing of the eclipses was not consistent. When Earth was closest to Jupiter, the eclipses happened 11 minutes early. Likewise, when the two planets were farthest away, the eclipses were 11 minutes behind schedule. Rømer figured out the pattern and made an accurate prediction for Io's eclipse on November 9, 1676. Then on Nov. 21, he took his findings to the Royal Academy of Sciences and explained that a finite speed of light must be responsible.
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(Wikipedia reference)
https://en.wikipedia.org/wiki/Ole_R%C3%B8mer
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