Free Astronomy Magazine March-April 2015

SPACE CHRONICLES (these were among the first observa- tions ever made using a telescope. They revolutionised our understand- ing of the Universe, and finally laid to rest the theory that the Earth is the centre of the Solar System). They complete orbits around Jupiter rang- ing from two to seventeen days in duration. The moons can commonly be seen transiting the face of Jupiter and casting shadows onto its layers of cloud. However, seeing three of them transiting the face of Jupiter at the same time is rare, occurring only once or twice a decade. On the previous page, the image on the left shows the Hubble observa- tion at the beginning of the event. On the left is the moon Callisto and on the right, Io. The shadows from Callisto, Io and Europa are strung out from left to right. Europa itself can- not be seen in the image. The image on the right (yet on the previous page) shows the end of the event, just over 40 minutes later. Europa has entered the frame at lower left with slower-moving Callisto above and to T his simulation illustrates the orbits and positions of Jupiter’s four largest moons on 24 January 2015 during a rare triple-moon transit where three of Jupiter’s largest moons were seen parading across the banded face of the gas giant planet. The three moons are Europa, Callisto, and Io. We start with a polar view of Jupiter and its moons - including the moon Ganymede which is not seen in the transit - and tilt down to the orbital plane of the moons. The satellites move from left to right across the frame, casting shadows onto the face of Jupi- ter. [NASA, ESA, and G. Bacon, L. Frattare, Z. Levay, and F. Summers (STScI/AURA)] T hese new NASA/ESA Hubble Space Telescope images capture a rare occur- rence as three of Jupiter’s largest moons parade across the giant gas plan- et’s banded face. Hubble took a string of images of the event which are stitched together to show the three satellites — Europa, Callisto and Io — in action in this time-lapse video. [NASA, ESA, Hubble Heritage Team] the right of it. Meanwhile Io —which orbits significantly closer to Jupiter and so moves much more quickly — is approaching the eastern limb of the planet. Whilst Callisto’s shadow seems hardly to have moved, Io’s has set over the planet’s eastern edge and Europa’s has risen further in the west. The event is also shown from start to finish in a video. Missing from this sequence is the Galilean moon Ganymede which was outside Hubble’s field of view. The moons of Jupiter have very distinctive colours. The smooth icy surface of Europa is yellow-white, the volcanic sulphur surface of Io is orange and the sur- face of Callisto, which is one of the oldest and most cratered surfaces known in the Solar System, is a brownish colour. The images were taken with Hubble’s Wide Field Camera 3 in visible light on 23 Jan- uary 2015. Whilst Hubble captures these moons in great clarity they can also be seen with a small tele- scope or even a decent pair of binoc- ulars. Why not try it at home? n