The size of the Solar System HorE.gif - 1338 bytes

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.

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.

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© Barry Gray June 2020