Jupiter – king of the planets

Jupiter is the big daddy of the Solar System – the largest of the planets with a diameter at the equator of 142,984 km, and the first of the so-called gas giants.

An image of Jupiter taken by the Cassini space probe in 2000 as it passed by on its way to Saturn. The dark blot is the shadow of one of its main moons. Image credit: NASA/JPL/University of Arizona

Oddly its diameter pole-to-pole is a lot less – 133,708 km – so that it rather resembles a pumpkin. This is due to Jupiter’s rapid rotation – it has a day just 9.8 Earth hours long.

It is always a brilliant sight in the night sky except for the couple of months a year when it lies on the other side of the Sun to us.

Jupiter is a fine sight through amateur telescopes and can show fascinating detail in its gaseous surface, particularly when the atmosphere is steady. Its complex, high-speed weather systems produce light and dark belts and a Great Red Spot which is a storm that has been raging for hundreds of years.

The smallest telescope will show Jupiter’s four largest moons among the retinue of rocks caught in its orbit. First spotted by Galileo in 1610, Io, Europa, Ganymede and Callisto perform a fascinating dance as they orbit their parent planet like a mini solar system.

Even if you only have binoculars and can hold them steady on, say, a tripod, you can see these moons stretched in a line around Jupiter like a string of pearls. It can be fascinating to watch their changing pattern over the hours as they orbit their home world.

Many amateur astronomers sketch the detail they see on Jupiter – they have to be quick because of its rapid rotation. Increasingly, amateurs are using adapted webcams to produce images of Jupiter that can show a surprising amount of detail.