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.

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.

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© Barry Gray March 2022