Diogenes Laertius Vitae philosophorum 7.1.155

[155] They maintain that the parts of the world are arranged thus. The earth is in the middle answering to a centre; next comes the water, which is shaped like a sphere all round it, concentric with the earth, so that the earth is in water. After the water comes a spherical layer of air. There are five celestial circles: first, the arctic circle, which is always visible; second, the summer tropic; third, the circle of the equinox; fourth, the winter tropic; and fifth, the antarctic, which is invisible to us. They are called parallel, because they do not incline towards one another; yet they are described round the same centre. ¹ The zodiac is an oblique circle, as it crosses the parallel circles.