When asked to give this lecture I was given to understand that this Society would like a change from the discussion of pedagogical methods and school problems, and could do with something that had nothing to do with teaching at all. In other words, I was to talk about stars and not to mention schools. Well, I promise to do that, though I must confess that the fact that members of the Society are teachers was at the back of my mind when choosing the subject. The schoolboy and schoolgirl and the man in the street know that celestial distances are very large; in fact the adjective “astronomical” has been sadly degraded by being applied to such mundane affairs as the figures of the national debt—and a schoolboy of enquiring mind may ask how the astronomers measure such distances. Such an enquiry is often tinged with a certain scepticism—sometimes politely veiled, sometimes not. When you tell a member of the public that a certain nebula is many million light years away and that a light year is 6 billion miles, he is apt to adopt the attitude of mind indicated by the phrase, “You’re telling me”, and to wonder what meaning such a monstrous distance can have: and how does the astronomer know, anyway?