On Friday night (Jan. 23), observers all across North
America will witness a rare event when three of Jupiter's moons, and their
shadows, pass across the face of the giant planet.
How rare are these Jupiter triple transits? I have seen
two in the last 15 years personally, but the next one will not occur until
2032.
As Galileo found early in the 17th century, Jupiter has
four large, bright moons that are usually seen as points of light on one side
of the planet or the other. These satellites — Io, Europa, Ganymede and
Callisto — are known as the Galilean moons, after their discoverer. Jupiter has
at least 63 smaller moons, most of which are too small to be seen with amateur
telescopes. [Photos: The Galilean Moons of Jupiter]
As the Galilean moons revolve around the giant planet,
they sometimes pass in front of it and sometimes are lost in its shadow or
behind the planet itself. In addition, all four moons cast their own shadows on
Jupiter's cloud tops.
Because of the complex gravitational interaction between
Jupiter and its moons, their motions are not independent, but are locked in
specific patterns. The orbital periods of the three inner moons are as follows:
Io, 1.769 days; Europa, 3.551 days; Ganymede, 7.155 days. More
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