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
Figure 3 Ecliptic and equator against the visible sky.
in the same zodiac sign for a year, and Saturn stays two-and-a-half years in one sign. The planets Uranus, Neptune and Pluto were not known to antiquity. From the fifth century at least the planets' motions were understood to be basically circular. However, as they were seen to stop this overall motion (described as being 'in their stations') or even go backwards (retrograde), attempts were made to refine the model to explain this.
The apparent path of the Sun, the ecliptic, lies at an angle of 23 l/ 2 degrees to the celestial equator, the projection of the Earth's equator on to the celestial sphere (Figures 2, 4). This is the reason for the varying seasons and hours of daylight. At the spring equinox, when the Sun is at the First point of Aries, it is directly overhead at the equator, and night and day are of equal length everywhere. In the days which follow, its angle to the equator, measured in degrees of declination, increases to a maximum of 23 l/2 degrees at the summer solstice, midsummer in the northern hemisphere. It then appears to stand still before returning to the equator, which it reaches at the autumn equinox. As the angle increases again, in the northern hemisphere, the Sun's maximum altitude at noon gets lower until the winter solstice (Figure 5).
Each day the Sun is seen to rise in the East at dawn, and to move upwards until it is directly overhead at noon, and then downwards until it sets in the West (Figure 6). It can be deduced that it continues to describe this circle during the night on the other side of the Earth. The planets too are moving in an East-West direction, and so is the whole sphere in which the stars are fixed. During the night, a new zodiac sign can be seen to rise in the East about once every two hours. However, the starry sphere moves round slightly faster than the Sun, achieving a full revolution in about 23 hours and 56 minutes. This is why given constellations will not always be visible in the night-time, but for part of the year are only above the horizon in day-time. To go back to our initial observation of the night-sky over a year, it is the stars' greater speed which means that the Sun gradually falls behind the point in the zodiac from which it started. So the Sun can be seen to move very slowly in the opposite direction, completing a revolution in a year. This is what gives us Sun-signs in our horoscopes.
But before considering the birth chart and its relation to the heavens, we need to consider the relevance of space as well as time. Clearly, perception of the stars will vary at different points on the Earth's surface. The Sun takes four minutes to pass over an arc of 1 degree. Thus the Sun rises twenty minutes earlier in Amsterdam than in Greenwich, which is 5 degrees east of the Greenwich meridian. Similarly, which sign is rising and setting depends on where you are on Earth. And since all rising occurs parallel to the celestial equator, and as the ecliptic lies obliquely to the equator, some signs can be seen to rise more quickly than others. In northern latitudes the signs from Capricorn to Gemini take less time to rise. In the polar regions, some signs never rise at all. The ancient astrologers were aware of the variation in rising times, and offered tables, which were calculated for different climata, or zones based on the ratios of daylight to night on the longest day in the year. The rising time of a given sign is the number of degrees of the equator which cross the horizon of a given locality at the same time as the sign. Accurate tables of rising times, calculated by spherical trigonometry, are found in Ptolemy's astronomical work, the Almagest.1 Astrologers, especially those earlier than Ptolemy or contemporary with him, used rising times calculated arithmetically according to Babylonian schemes, which were less accurate.
Now, of course, it is not the Earth which lies at the centre, but the Sun. It is the Earth's rotation on its axis joining the poles which produces the effects of day and night rather than the Sun's movement, and it is because the Earth does not 'stand up straight' in its orbit, but is tilted in relation to the plane of the ecliptic, that we have the varying seasons (Figure 7). The ecliptic, the apparent path of the Sun, is actually the plane of the solar system. The planets' motions are approximately in this plane; they are also not circular but elliptical, and, apart from the Moon, orbit round the Sun rather than around the Earth. This is why they appear to stop and go backwards from Earth. As for the Sun itself, it is because the Earth makes a circuit of the Sun once a year that the Sun seems, from a point of view on Earth, to complete a journey round the ecliptic in the same period, appearing against a background of the zodiac. This does not matter, as astrology is concerned with movements relative to the Earth.
In addition, the 'fixed stars' within and outside the zodiac only appear fixed because of their great distance from the Earth. A moment in the Sun's journey is schematically represented in Figure 8, where the inner circle represents the months. Here, in the third month, the Sun is seen against the sign of Pisces.
Now in fact the relationship of each area of the sky designated as a zodiac sign to the constellation after which it was named is even more arbitrary than in the original division of the zodiac. This is because of a wobble in the earth's axis as it spins, like a spinning top which is running down. Thus the North Pole describes a circle, taking about 26,000 years to reach the point where it started. The First point of Aries, or the point of the vernal equinox, from which the zodiac begins, has slowly moved backwards well into Pisces. This phenomenon, known as precession, was probably discovered by Hipparchus in the second century BCE. Ancient astrologers, naturally, did not have to consider the problems posed for their model by modern findings about the universe. However, precession was an issue: it was used as an argument against astrology by Origen, presumably drawing on an earlier source. Ptolemy recommended a correction of 1 degree a century for precession; this seems to have been used by some successors.2
Was this article helpful?