King of the planets Jupiter is never a difficult object to find in the night sky thanks to its dazzling brilliance but when opposition comes around, it is at its most spectacular. Roughly every 13 months, the Sun, Earth and Jupiter are in alignment and the giant gas planet is visible all night as it rises in the east at sunset and sets in the west at sunrise.
This year's opposition occurs on the 29th October 2011. Jupiter will shine at magnitude -2.9 with the planets disk spanning a whopping 50 arc seconds in apparent diameter. Currently located in the constellation of Aries, 12 degrees north of the celestial equator, it is an especially good apparition for Northern Hemisphere observers as Jupiter soars high in the night sky.
Common 7x50 and 10x50 binoculars will show the disk of Jupiter and its non-stellar nature but no detail will be visible on the planet's surface. However, they will easily show the four large Galilean moons: Io, Europa, Ganymede and Callisto. These moons will continuously change position. Sometimes all four will be visible, on other occasions some or all of the satellites will be temporarily obstructed and not visible as they pass behind or in front of Jupiter's disk.
Sky and Telescope magazine has an excellent tool to predict the positions of the Galilean moons: Sky and Telescope - Galilean moons
Through a telescope a wealth of detail is visible on the planet disk. Even a small 60mm (2.4-inch) telescope at medium to high magnification will begin to show some detail including the great belts of Jupiter. Keen eyed observers under good seeing conditions may even be able to notice the shadow transits of the Galilean moons. A larger 100mm (4-inch) telescope will reveal more details in the belts including bright and dark spots. Also visible is the most famous storm of all on Jupiter "The Great Red Spot", and the South Equatorial Belt which recently completely disappeared and is now back to its full glory. Even larger telescopes reveal more subtle details.
Generally, if the seeing conditions are good it is possible to push a telescope close to the magnitude limit when observing Jupiter. The accepted values are 50x for every 25mm (1-inch) of aperture. So up to 200x for a 100mm (4-inch) telescope and 400x for a 200mm (8-inch) telescope.
Whatever optical instruments you use, take time to observe Jupiter and it's moons this opposition as they are always fascinating and you never quite know what to expect.