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18 September 98

Seeing the Light from Jupiter

The planet Jupiter was in opposition this week. The Earth is on a line between the Sun and Jupiter. You can easily see bright Jupiter rising in the east just as the sun sets. In the 1600s, the Danish astronomer Ole Rømer noticed that the moon Io's orbits of Jupiter lose time for about 6 months, and then gain time over the next 6 months, losing and gaining about 8 minutes each cycle between successive oppositions. From this, and the fact that the earth is about 93 million miles from the sun, Rømer made the first reasonable estimate of the speed of light. What did he realize?


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