![]() ![]() Roemer estimated that light required twenty-two minutes to cross the diameter of the Earth’s orbit. 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. In a brilliant insight, he realized that the time difference must be due to the finite speed of light. Roemer knew that the true orbital period of Io could have nothing to do with the relative positions of the Earth and Jupiter. And 6.5 months later, when the Earth was farthest from Jupiter (at E2), the eclipses would occur about eleven minutes later than predicted. 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. 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. By timing these eclipses over many years, Roemer noticed something peculiar. The satellite is eclipsed by Jupiter once every orbit, as seen from the Earth. The orbital period of Io is now known to be 1.769 Earth days. But the Io eclipse data unexpectedly solved another important scientific problem-the speed of light. This method of finding longitude eventually turned out to be impractical and was abandoned after the development of accurate seagoing timepieces. Then, by determining the local solar time, they could calculate their longitude from the time difference. 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). Galileo himself had suggested that tables of the orbital motion of Jupiter’s satellites would provide a kind of “clock” in the sky. Such observations had a practical importance in the seventeenth century. By timing the eclipses of Io by Jupiter, Roemer hoped to determine a more accurate value for the satellite’s orbital period. 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. Roemer, working at the Paris Observatory, was not looking for the speed of light when he found it. The dominant view, vigorously argued by the French philosopher Descartes, favored an infinite speed. Until that time, scientists assumed that the speed of light was either too fast to measure or infinite. ![]() Roemer measured the delay and, knowing approximately the diameter of the Earth's orbit, made the first good estimate of the speed of light. This causes a delay in the timing of the eclipses. When the Earth is at E2, the light from the Jupiter system has to travel an extra distance represented by the diameter of the Earth's orbit. In this figure, S is the Sun, E1 is the Earth when closest to Jupiter (J1) and E2 is the Earth about six months later, on the opposite side of the Sun from Jupiter (J2). Roemer measured the speed of light by timing eclipses of Jupiter's moon Io. ![]()
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