That's no star!

Judging by all the enquiries I am getting this week, many of you are seeing a bright star in the eastern sky. This is not a star, especially not the star of Bethlehem, rather it is the mighty planet Jupiter.

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 you are seeing it as it was 35-52 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 asteroid 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 binoculars as brilliant points of light in a neat line around the planet. If you watch over several nights their movement as they orbit Jupiter will be apparent.

The image above, made with Stellarium software (freely available from our Free Stuff page), shows the sky about 10.30pm BST from Armagh. Jupiter (marked with a cross) is below the Moon.

So there you are, go out tonight and see a planet a thousand times as massive as the Earth!