The size of the Solar System

# The size of the Solar System and the distance to the stars

You can read about the diameters of the Earth, Sun, Moon and planets in thousands of kilometres and the distances between them in millions of kilometres, but Sam realises that the only real way for his friends to understand the size of the solar system is to make a model.

So one fine day he and ten of his friends from his school walk along the rocky path from the lighthouse onto the long sandy beach. They take with them a football, some modelling clay, a 30 cm school ruler, a jamjar of granulated sugar, some pieces of white card and a felt tip pen, and their phones. Sam’s Dad comes too, with his big video camera.

• Thomas holds the football. This is the Sun. He writes Sun on his piece of card.

• Thomas stays where he is, and everyone else walks 15 paces along the beach. Harry makes a ball of modelling clay about 1 mm in diameter. He writes Mercury on his piece of card.

• He stays where he is, and everyone else walks 12 paces along the beach. Natasha makes a ball about 2 mm in diameter to represent Venus.

• The rest of the children walk another 10 paces and Adam makes a ball about 2 mm in diameter for the Earth, and Simon a ball about 0.5 mm for the Moon. He holds it about 6 cm from the Earth.

• The rest of the children walk another 20 paces along the beach, where Victoria makes a ball about 1 mm in diameter for Mars.

• The Asteroid Belt is about 48 paces further along the beach. Julian holds up the jamjar of sugar to represent the asteroids. These are millions of lumps of rock varying in size from less that 1 m to nearly 1000 km in diameter which have never come together to form a proper planet. You cannot see any of them with the naked eye, but Sam can see the largest, called Ceres, with his telescope.

• The rest of the children walk another 93 paces to arrive at Jupiter. Sarah makes a ball about 20 mm in diameter.

• Another 162 paces takes Sam, Nicholas and Abigale to Saturn. Here Abigale makes a ball about 17 mm in diameter.

• Then another 360 paces takes Sam and Nicholas to Uranus. Mercury, Venus, Mars, Jupiter and Saturn are all very bright and easy to see, but Uranus is only just visible without a telescope. Nicholas makes a ball about 7 mm in diameter.

• Finally Sam walks another 410 paces by himself to the last planet, Neptune. You can never see Neptune without a telescope. He makes a ball about 7 mm in diameter. He calls up the children on his phone and they all wave their cards around while Sam's Dad tries to video it. Not easy of course, after all, Sam is 1130 paces from the Sun!

They all walk back to their starting point, where Thomas is still holding the Sun - it takes them less than twenty minutes. Sam explains that the Sun is our nearest star, and that on this scale it would take them several years to walk to our next nearest.

In real life the Sun is not the size of a football so paces are not really a good way of measuring the distances between the Earth and the other planets from the Sun. So we use kilometres instead. The other stars are so far from the Earth that ordinary people use light-years (ly) - a light-year is the distance light can travel in a year. As the speed of light is about three hundred thousand kilometres a second it is rather a long way. The Earth is about eight light-minutes from the Sun, our next nearest star is about four light-years.

Astronomers use their own units when talking among themselves but most young people are content with kilometres and light-years and these are the only units used on these Pages.