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The brilliant point of light above tonight’s waxing crescent moon is Jupiter, the 5th planet outward from the sun. These two worlds – our companion moon and the solar system’s largest planet – are the brightest objects in the evening sky now. Look for them in the west, shortly after sunset.
Jupiter – the king of planets in our sun’s system – has over 60 known moons of its own. Only four of these moons are large enough to be seen through a small telescope. In their order outward from Jupiter, these four moons are Io, Europa, Ganymede, and Callisto. You might glimpse them through binoculars if you peer at Jupiter tonight, but it will be tough – because Jupiter is close to the evening twilight, relatively low in the sky, and not up all that long after the sun goes down.
Two of Jupiter’s moons, Io and Europa, are about the size of our moon. The other two, Ganymede and Callisto, have diameters that are approximately 1.5 times the moon’s diameter. Ganymede, the largest moon in the solar system, is actually larger than the planet Mercury.
These four major moons are called the Galilean moons because Galileo first observed them in the early 1600s. Galileo believed that the eclipses of Jupiter’s moons would enable sailors to find longitude at sea, if an accurate almanac giving the eclipse times for the moons could be provided.
The Danish astronomer Ole Romer studied the moon Io in the late 1600s for the purpose of creating a reliable almanac. However, Romer ran into a snag. He found that when Jupiter was farther away from Earth, the eclipses came later than expected and when Jupiter was closer to earth, the eclipses happened sooner than expected.
Ole Romer correctly surmised that the finite speed of light must be responsible for the observed phenomena. Light takes less time to travel when Jupiter is closer to Earth, and more time to travel when Jupiter lies farther distant. This explanation proved shocking to people in Romer’s day because many took for granted that the speed of light was instantaneous.
After sunset, look westward for the moon and Jupiter, whose moons first told astronomers the surprising story about the finite speed of light.
Written by Bruce McClure