Judging by all the enquiries I have received recently, many of you are seeing a bright star in the south-eastern sky. This is not a star, especially not the star of Bethlehem, rather it is the mighty planet Jupiter. As Jupiter is in so perfect a position in the sky for viewing this month and is the closest to Earth it has been since 1963 (it will not be as bright and close again until 2022), this a great chance to observe and admire it. You can even join in the international “Welcome the giant” campaign.
Jupiter is the fifth planet from the Sun and by far the largest within the Solar System. It orbits at an average distance of 778 million km from the Sun, about five times as far from the Sun as Earth. As it is so far from us, it takes Jupiter’s light a significant time to reach us –so when you see Jupiter tonight you are seeing it as it was 35 minutes in the past. Jupiter is huge, its diameter is over 142 000 km (compared to Earth’s 12 756 km). It is composed of a relatively small rocky core, surrounded by a layer of solid metallic hydrogen (a material which does not exist on Earth), covered by liquid hydrogen. Above this layer is a stormy atmosphere of mainly hydrogen with some helium and very slight traces of other materials. These other traces are enough to colour Jupiter’s turbulent cloud tops in a range of yellow and brown tones. Three times in the past year Jupiter has been hit by asteroids or comets and amateur astronomers have observed the vast explosions.
It is well worth using a pair of binoculars or telescope to view Jupiter, as it can be a magnificent sight. The planet will be clearly visible as a slightly flattened yellowish disc. If your hands are steady enough (or you use a tripod) you will probably also see the streaks of coloured clouds running across the planet.
Jupiter has at least 63 moons. The four largest (Io, Europa, Ganymede and Callisto) are collectively called the Gallilean satellites and are easily visible through small telescopes or even binoculars as brilliant points of light in a neat line around the planet. If you watch over several hours their movement as they orbit Jupiter will be apparent. In viewing them you will be recreating the observations of Galileo who discovered these moons exactly four centuries ago.
The image above, made with Stellarium software (freely available from our Free Stuff page), shows the south eastern sky about 9.00pm BST, 8 October 2010 from Armagh. Jupiter (marked with a cross) is below the Great Square of Pegasus.
So there you are, go out tonight and see a planet a thousand times as massive as the Earth!