Dear Juliet,

Now that you are ten, I want to write to you about something that is importantto me. Have you ever wondered how we know the things that we know? How dowe know, for instance, that the stars, which look like tiny pinpricks inthe sky, are really huge balls of fire like the sun and are very far away?And how do we know that Earth is a smaller ball whirling round one of thosestars, the sun?

The answer to these questions is “evidence.” Sometimes evidence means actuallyseeing ( or hearing, feeling, smelling….. ) that something is true. Astronautshave travelled far enough from earth to see with their own eyes that it isround. Sometimes our eyes need help. The “evening star” looks like a brighttwinkle in the sky, but with a telescope, you can see that it is a beautifulball – the planet we call Venus. Something that you learn by direct seeing( or hearing or feeling….. ) is called an observation.