By Bruce McClure
EarthSky Communications

Friday, April 17, 2009

Tomorrow, on Saturday morning, April 18, the dazzling planet Jupiter lies to the east (left) of the waning crescent moon at and before dawn. Both the moon and Jupiter light up the rather faint constellation Capricornus the Seagoat.

One day later, on Sunday April 19, the lunar crescent and Jupiter will pair up together for a spectacular display in the early morning sky.

Jupiter has more known moons than any other solar system planet. At the last count, there are 63 moons, though only four of them are of significant size (like Earth’s moon: 2,160 miles in diameter). Jupiter’s four major moons are called the Galilean moons, because Galileo first observed these moons in 1610. The largest non-Galilean moon is potato-shaped Amalthea. Its length is only about 160 miles.

In the 1670s, the Danish astronomer Ole Romer found that the eclipses of Jupiter’s moons happened earlier than expected when Jupiter was closer to Earth, yet later than expected when Jupiter was farther away. Ole Romer correctly surmised that this must be due to the finite speed of light. In Ole Romer’s day, many people thought the speed of light was “instantaneous.” Light does travel very fast, but not infinitely fast. The speed of light is 186,000 miles per second.

When Jupiter is closest to Earth, this planet is about 33 light-minutes distant. At its farthest, Jupiter resides some 54 light-minutes away. At present, Jupiter is about 45 light-minutes from Earth.

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