On January 23, the Hubble Space Telescope took two photos of a rare celestial event involving the gas giant Jupiter – three of its four largest moons (Europa, Io and Callisto) and their shadows were spotted simultaneously passing in front of their host planet.
Astronomers said that such event occurs only once or twice in a decade, while the next one would not happen until 2032.
Jupiter has 67 moons, but only four of them are visible from Earth due to their large size: Europa, Io, Callisto, and Ganymede. These four moons were first discovered by Galileo Galilei in the 17th century, so they are called the Galilean moons. From Earth any star gazer using a telescope can identify these moons as bright spots on one of Jupiter side or the other.
The rest of the moons are too dim to be observed with a ground telescope. For instance, 51 of them are less than 6.2 miles (10 kilometers) wide.
On Jan. 23, three of the Galilean moons and their shadows passed across the surface of Jupiter. Ganymede, however, was not involved in the rare triple shadow transit.
Astronomers explained that due to their orbits, it is impossible to see all four Galilean moons perform a simultaneous shadow transit. One of them would be always left out, except Callisto, which must always be included in a triple transit.
Callisto is also the only Galilean moon that can sometimes miss annual single transits across its host planets disk due to a slight inclination of its orbit. This means that the moon doesn’t cast a shadow onto Jupiter every year.
But currently Callisto is in the middle of a visible cycle, which started two years ago and will end next year. Scientists noticed that these transit cycles of Callisto occur only twice in 11.8 years, so triple shadow transits must also happen during this period.
Astronomers also argue that Ganymede usually misses the transits because of a specific orbital pattern of the four moons. Io revolves around Jupiter in 1.769 days, Europa in 3.551 days, while Ganymede needs 7.155 days to perform a full revolution.
If you divide the last two figures by Io’s orbital period you’ll notice that they almost perfectly fit in a classical ratio of 1:2:4. Scientists say that this ratio causes Io and Europa to simultaneously pass in front of Jupiter, while Ganymede is usually left behind.
But for a three-fold transit, Callisto must also participate. However, Callisto’s orbital period has 16.689 days. So, a perfect junction of the three moons is often hard to perform.
Image Source: Phys.org
