The Milky Way and Comets

The so-called scattered stars, it is not possible to set down in the same way on the sphere because none of them have an evident permanent position. But if you look up to the sky, the point is clear. For in this circle alone are the intervals full of these stars. In the other circles there are obvious gaps. This, as we said before, explains why there are few comets and why they appear rarely. It is because at each revolution of the heavens this matter has always been and is always being separated off and gathered into this region. Let us go on to treat of the region which follows next in order after this and which immediately surrounds the earth.