x Abu Dhabi, UAESaturday 29 July 2017

Jupiter's moons to put on big no-show

Space enthusiasts in the Emirates will have the chance to observe most of a rare event tomorrow night, when, one by one, Jupiter's four biggest moons disappear from view.

Space enthusiasts in the Emirates will have the chance to observe most of a rare event tomorrow night, when, one by one, Jupiter's four biggest moons disappear from view. The phenomenon, which occurs only a few times every century, is a result of "occultation" and "transit" - when the four Galilean moons, Io, Europa, Ganymede, and Callisto, are invisible at the same time as a result of passing behind or in front of the solar system's largest planet.

Unfortunately, sky-watchers in this part of the world will be able to catch the vanishing moments for only three of the moons. Jupiter itself will be out of sight below the horizon by the time the fourth - Ganymede - disappears at 8.43am on Thursday. Io and Callisto will go behind the gas giant, while Ganymede and Europa will be indistinguishable as they pass in front of it, casting small shadows on the planet behind them.

"Since their orbits are of various sizes, the moons being at different distances, their diverse orbital periods make them achieve this special alignment rather rarely," said Dr Nidhal Guessoum, an astrophysicist and professor of physics at the American University of Sharjah. "We will not get all four moons to disappear for us, for when that happens exactly, morning will have come," he added. During the UAE night Callisto would hide first, then Io followed by Europa.

The four Galilean moons were discovered by Galileo Galilei in 1610. In all, Jupiter has 63 moons. Hasan al Hariri, chief executive of the Dubai Astronomy Group, an association of astronomy enthusiasts mostly in the UAE, said the group would organise a photo shoot at its observatory on Sheikh Zayed Road, but was not planning an elaborate gathering because it was hard to get people into the desert during Ramadan.

The transit could be harder to appreciate fully because of the nearly full Moon and its proximity to Jupiter, though the giant planet's brightness in the night sky makes it easier to see. Jupiter will rise in early evening from the east and set in the west at 5am, said Dr Guessoum. He recommended that for the best chance of a clear view enthusiasts should find dark places that are free of artificial light and dust haze.

Dr Guessoum said each of the moons was special in its own way. "For instance, Galileo is the most volcanic place in the whole solar system, Europa is covered with a thick blanket of ice and probably has a huge ocean of liquid water, and perhaps some life forms, underneath the ice; Ganymede is the largest moon in the solar system, even bigger than the planet Mercury." newsdesk@thenational.ae

Editorial, page a19