The Jovian Moons |
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Astronomers starting with Galileo watched the ever-changing parade of the four large satellites around Jupiter. They were able to determine the orbital periods of these satellites, just by observing how long any one satellite took to move back and forth in front of the planet and return to the same relative position. Io, the satellite closest to Jupiter, has a period of about 2547 minutes. Knowing this, astronomers could predict the exact arrangement of satellites months in advance. But Danish astronomer Ole Roemer noticed a problem. At times when Earth was closer to Jupiter, the satellites (in particular, Io) seemed to be ahead of schedule. This corresponds to point A in the diagram. Six months later when Earth was farthest from Jupiter, the satellites were behind schedule. This is point B. The difference in timing from A to B was about 16.5 minutes. Roemer figured that it meant that light needed the additional 16.5 minutes to cross the diameter of the Earth's orbit from A to B. Roemer realized that he could find the speed of light by dividing the distance across the Earth's orbit by the 16.5 minute time difference. Unfortunately his determination was off a good bit because he did not know the diameter of the Earth's orbit accurately. Others later calculated a reasonably good speed of light. (This diagram is not to scale!) |
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