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 Plasticine®, a 30 cm school ruler, a jamjar of granulated sugar, some pieces of white card and a felt tip pen, and their mobile 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 Plasticine 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 of Plasticine about 2 mm in diameter to represent Venus.

• The rest of the children walk another 10 paces and Adam makes a ball of Plasticine 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 of Plasticine 20 mm in diameter.

• Another 162 paces takes Sam, Nicholas and Abigale to Saturn. Here Abigale makes a ball 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 fifteen minutes. Sam explains that the Sun is our nearest star, and that on this scale it would take them three months to walk to our next nearest.

In real life the Sun is not the size of a football so paces is not the best way of giving sizes and distances in astronomy. In eveeryday speech most people use kilometres for the sizes of objects in the Solar System and the distances between them, and light-years (ly) for the distances between the stars and the size of the galaxies. A light-year is the distance light can travel in a year. Light travels at about three hundred thousand kilometres a second so if you want the size of a light-year in kilometres you can do the sum for yourself. A scientific calculator will give the answer in standard form, a basic calculator will give you a wrong answer. If you are at school you need to know the difference between different sorts of calculator and how to use them and here is a Page to help you.

Our nearest star is a little more than four light-years from the Earth, the Sun is about eight light-minutes from the Earth.

Astronomers have their own sets of units when talking among themselves, and these are described on another Page FTPO.