When an educated observer around 1300 went outside on a cloudless night and peered into the sky, he might have marvelled at the vast expanse of the star-spangled firmament, and, awestruck, praised the Lord as the creator of the universe. But he would not have been confused. What he saw was not bewildering chaos, but order that made sense. First of all, he would have thought of himself as being in the middle of things. The earth on which he lived was the immovable centre of the universe; around it there moved in concentric order ten transparent convex spaces or spheres, beginning with those of the seven known planets, in which the Sun and the Moon were included. Beyond the sphere of the last planet, Saturn, began that of the fixed stars (stellatum). Then came the vaguer circles of ‘the chrystalline’, presented as a thin fluid mass encircling the entire firmament, and that of the primum mobile, the first of the spheres to show movement and to pass that movement on to the lower spheres. Beyond the primum mobile extended the immovable empyreum, where heaven was located. While earth then formed the centre of the universe, it was nevertheless fully understood that it was insignificant on the cosmic scale. One estimate showed that a complete revolution of the stellar sphere round the earth took 36,000 years. An English chap-book (popular book) from the fourteenth century calculated that a journey from the earth to the stellatum would take 8,000 years, at an average travelling speed of 40 miles per day. This is nothing compared to the actual 40 trillion kilometres (a 40 with 12 zeros) which separates our sun from the nearest sun in our galaxy, Proxima Centauri, but was still a vast distance to imagine for our medieval observer.