The astrologer’s cosmology is, of course, geocentric, reflecting the general consensus in antiquity. Though Aristarchus hypothesised a universe centred round the Sun in the third century BCE, this hypothesis never found general support. The geocentric cosmos accorded with perception from Earth: we look up to the sky and see the Sun, Moon and planets revolving round us. It seems as if there is a dome above us in which the stars are fixed, and thus a celestial sphere was envisaged revolving round a stationary Earth (Figure 3). The Earth was regarded by ancient astronomers as spherical, as a result of observation of its curvature.
If you were to watch the sky at sunset over the period of a whole year, making a note of the stars which appeared just after the Sun, by the end of the year you would have made a map of a line through the heavens known as the ecliptic (Figures 2, 3). The planets can be seen to remain within about 8 degrees (measuring the celestial sphere as 360 degrees in circumference) on either side, though the Moon may move outside this band of the sky occasionally. This band on either side of the Sun’s path is the zodiac, and is divided into twelve equal sectors of 30 degrees each, named after constellations identified by the Greeks, or in some cases by the Babylonians, which lay in the area of each sector. The 360 degrees of the zodiac are measured clockwise from the First point of Aries. The positions of the planets are plotted in relation to the ecliptic, in degrees of celestial longitude (Figure 4). The planets are seen to move in the same direction as the Sun at different speeds, Mercury taking only eighty-eight days to go round once, while Jupiter takes twelve years to go round, thus being