Add Jupiter and Saturn.












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Remember that pressing the F11 key may improve your view on a small screen. In this diagram you can see the four rocky planets (Mercury, Venus, Earth and Mars) as the four tiny dots nearest to the sun. The outer two objects are the planets Jupiter (11 times the size of the Earth) and Saturn (9 times the size of the Earth) - look carefully at Saturn and you'll see that it has a ring round it. These are the biggest two of the four gas giants. Gas giants are made mainly of dense gas (you can't stand on the surface because you'd just sink into them and keep on sinking for tens of thousands of miles until you hit a liquid or solid core), so they are a very different kind of planet from the rocky ones like ours. The letter keys V, P, C, H and A can be used to show or hide four asteroids in the asteroid belt between Mars and Jupiter: V is for Vesta, P for Pallas, C for Ceres (the biggest asteroid), H for Hygiea, and A does all four at once. These asteroids are much smaller than the four rocky planets, so they have to be shown much brighter than they deserve in order that you can see them.

The asteroid belt contains many thousands of asteroids, but Ceres is the biggest: it's about 600 miles across (nearly 1000Km), so it's quite a significant piece of rock (about a third the size of our moon). Ceres makes up a third of all the material in the asteroid belt all by itself. The other three asteroids I have shown are half the size of Ceres, each containing about an eigth as much material. If you watch the way they move, you'll notice that their orbits cross, but collisions are rare, and these four aren't at all likely to hit each other. Some asteroids do collide (and sometimes little asteroids crash into the big ones), but only very rarely. When this happens, little bits of rock can be thrown into orbits that cross the Earth, and many chunks of them have landed here in the form of meteorites, including little bits of Ceres. Whole asteroids can occasionally be pulled into a new orbit, perhaps by getting close enough to Ceres for its gravity to affect them, and that can send the small asteroid into a new orbit which may allow it to collide with one of the inner planets. Rocks from Mars have actually landed on the Earth after collisions between Mars and asteroids. Asteroids have also hit the Earth, the most famous one leading to the extinction of most dinosaur species, though luckily birds survived (birds, if you hadn't realised it, are dinosaurs).

As you can see, Jupiter is five times as far away from the sun as the Earth, and Saturn is nearly twice as far away as Jupiter. Jupiter has four large moons going round it (not shown here) which can be seen through binoculars, though you'll need to clamp them to something to keep them steady. You can watch them change position from night to night and plot their orbits if you get enough clear nights to keep track of which one's which. Jupiter has numerous other moons which can only be seen through a big telescope. Saturn is best known for its rings, and you can just about see that they are there with binoculars. A birdwatching telescope will give a slightly better view showing you that they are indeed rings, and you may also be able to pick out Saturn's biggest moon Titan as well, though you'll probably only see it by looking a little bit away from it as the light sensors in the very middle of your retina aren't so good at seeing dim objects.

Names of the Days of the Week

Before the invention of the telescope (and binoculars), there were only six significant things seen in the heavens other than stars, comets and meteors. Those were the sun, moon, Mars, Mercury, Jupiter, Venus and Saturn, and these were used as names for the seven days of the week: Sun-day, Moon-day, Mars day, Mercury day, Jupiter day, Venus day and Saturn-day. Four of them were then given the names of Norse gods instead, but you'll still find the original planet names for those days in French and Spanish. The other two planets, Uranus and Neptune (we'll see them later), weren't noticed back then, so that's why we have a seven-day week instead of a nine-day week. It's also why seven is thought of as being a special, magic number, tied up in religion as well, and that in turn is why the rainbow is usually thought to have seven colours in it, even though you're likely to count only six if you look at it carefully yourself. Newton was the first person to discover how to split white light into a spectrum with a prism (a glass wedge), and for religious reasons he wanted there to be seven colours in it. Red, orange, yellow, green, blue, indigo and violet? So far as I'm concerned, it's simply red, orange, yellow, green, blue, purple.