The Inner Planets (text follows below).











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The diagram above shows the four rocky planets going round the sun. (Pressing the F11 key may give you a better view if you have a small screen.) The Earth is the blue one with a moon going round it, Mars is orange/red, Venus is white (because it's always covered in thick cloud), and mercury is the fast-moving one nearest the sun. There are four other planets (the gas giants, Jupiter, Saturn, Uranus and Neptune) which orbit much further out, but they are not shown here because they would be way off the edge of the screen (you'll see them in another diagram later). Beyond the four gas giants is a little icy ball called Pluto which used to be thought of as the 9th planet, but it's too small to count as a proper planet, and it turns out that there's a bigger one like it further out (called Eris). More about them later.

The planets and the sun have all been drawn far bigger than they should really be in comparison with the distances between them: if they were shown the right sizes they would be so small that they wouldn't show up at all on your screen, apart from the sun which would just be a tiny yellow dot in the middle. I've also put the moon further away from the Earth than it should be: its real position would put it less than a pixel (a screen dot) away from the centre of the Earth! The moon is 30 times further away from the Earth as the Earth is wide, so that gives you an idea of how tiny the Earth and moon would actually have to be drawn for the diagram to be accurate. It's impossible to show an accurate diagram of the whole solar system in a book or on a computer screen, but I will tell you later on how you can make a proper model of it which you can spread out along a road.

Mercury, Venus, the Earth, moon and Mars are all shown the right sizes relative to each other, but the sun would be far bigger if it was drawn to the same scale: it should be a hundred and nine times as big as the Earth, so it would fill the screen, which again wouldn't make for a useful diagram. Apart from the relative sizes of the planets and moon, the only other thing shown more or less correctly in the diagram is the orbits: you can see how fast the planets move relative to each other and how their years are different lengths. Notice too how Mercury speeds up and slows down: you can see that its orbit is elliptical rather than circular. Mars also has a fairly elliptical orbit, but the slow speed makes that harder to spot: if you look carefully you'll see that it goes much further out on one side than the other, and when it's directly above or below the sun, notice that it isn't quite moving horizontally across the screen. If you watch the diagram for a long time, you may find it interesting to see how close the Earth and Mars come together. Roughly every 15 Earth years, they have a relatively close enough encounter, but you have to remember that you aren't seeing the real sizes of the planets in the diagram, so if you look at mars through binoculars when it is at its closest to the Earth, you will still struggle to see that it is a disc rather than a point of light.

Another thing you should realise is that we're looking down on the Earth's North Pole, so the Earth is rotating anticlockwise, just as the moon goes anticlockwise round the Earth and all the planets go anticlockwise round the sun (or "counterclockwise" if you speak American English). If we were looking from the other side, we'd be looking at the South Pole instead, and all the rotations would be clockwise. The fact that planets and moons generally orbit and rotate in the same direction is evidence that they were formed billions of years ago from a rotating ring of gas and dust that once orbited a young star (to which we later gave the name "the sun").