NASA's Hubble Space Telescope has captured a rare image of a triple transit, as three of Jupiter's largest moons cast their shadows on the gas giant's planetary disk. The three moons captured in the image – Europa, Callisto and Io, were among the first celestial objects observed with a telescope, and were instrumental in debunking the long held belief that the Earth was at the center of the universe.
Whilst Jupiter has in excess of 60 moons, the four and most famous of these are undoubtedly the Galilean set. First observed by Galileo Galilei in the 17th century, the moons vary greatly in both size and orbital duration, which explains the rarity of the event.
Callisto has the longest orbit, taking 17 Earth days to orbit the gas giant. Ganymede is next taking 7.1 days, then Europa with 3.5 days, and finally Io, taking a mere 1.7 days to complete a circuit of the giant planet. Therefore it isn't overtly surprising that one of the moons is missing from the family portrait.
For those wishing to observe the gas giant and its four largest companions with their own eyes, Jupiter is easily visible in the night sky appearing as a bright, steady star, and the four Galilean moons become easily discernible through a telescope or even a good pair of binoculars.
Source: Hubble