For the last number of months, Venus had dominated the western sky after sunset. Now it is moving into the twilight on it’s journey around the Sun. In the evening sky, the planet Jupiter will dominate proceedings over the coming months. Jupiter, a gas giant consisting primarily of hydrogen and helium gases is the largest planet in our solar system with a diameter of almost 140,000 km. These nights, Jupiter rises in the east shortly after eight o’clock and there is no mistaking it, as only the moon or Venus shines brighter in the night sky. It glows with a silvery brilliance that is unmistakable.
Turn a pair of binoculars on Jupiter and you can catch a glimpse of celestial mechanics in action. Jupiter is orbited by many moons, 67 at the last count, most of which are relatively small, but four are large and easily spotted in binoculars. These four moons are known as the Galilean Moons after Galileo’s discovery of them in 1610. In binoculars, the four moons are strung out on either side of the planet and appear as tiny dots of light. These four moons are worlds in their own right and range in diameter from 3,130 km to 5,268 km.
One of the interesting things about these moons is that it is possible to spot them change position in the sky with respect to each other and to Jupiter. The moons take from between 1.75 to 17 days to orbit Jupiter and we see this edge on from Earth and so the moons seem to disappear and appear as they pass behind of and in front of Jupiter. You can notice them moving over the course of a few hours. At ten o’clock tonight, the 4 moons should be visible, but by 12.30 am one of the moons will pass behind Jupiter and so only three will be visible for a few hours. Pretty amazing to think that you can see this with a set of binoculars. Rest against a wall or tripod mount the binoculars to keep them steady
Before these moons were discovered it was believed that the Earth was at the centre of the universe and that every other star and planet orbited it. Galileo’s observation of the orbiting moons of Jupiter helped us realise that the Earth is not the centre of the universe and was one of the key steps in defining our place within it.