chapter eighteen
In his essay on astrological houses as defined by medieval Islamic astronomers, E. S. Kennedy maintained that astrology “attracted numerous serious and able practitioners. This alone makes it worthy of study by historians” (Kennedy 1996, p. 535). There are indeed a great many tables in astrological texts, some of which involve sophisti- cated mathematical techniques encountered nowhere else in medieval literature, as well as lists with no mathematical structure; we will focus our attention for the most part on mathematical astrology, that is, the part of astrology that relates to astronomical concepts and requires the use of mathematical tools.
1. Horoscopes and Astrological Houses
Astrologers claimed to be able to make predictions about an individual and about the world based on the configuration at a particular time of the five planets, the Sun, the Moon, and the lunar ascending node. For an individual astrologers focussed their attention on the time of birth, and a key element for their predictions was the arrangement of the twelve astrological houses at that time for a specified locality. The ecliptic was usually divided into 12 consecutive unequal sections, called astrological houses, numbered from 1 to 12 in the direction of increasing longitude, beginning at the ascendant (called ascendens or horoscopus in Latin), that is, the point on the ecliptic that is rising at the eastern horizon at a particular time. These astrological houses had names that, broadly speaking, indicated the significance they had for the “native”: (future) life, business, brothers, parents, children, illness, marriage, death, travel, honors, friends, and enemies, or variants of them. Thus, future life was affected by the planets in the first house, business by those in the second house, and so on, with many varia- tions (North 1986, p. 1). In addition to the first house, three others begin at points with astronomical significance. The 4th house begins at the lower midheaven (Latin: imum medium caelum), i.e., the inter- section of the ecliptic and the meridian below the horizon; the 7th 206 chapter eighteen house begins at the descendant, i.e., the intersection of the ecliptic and the western horizon; and the 10th house begins at midheaven (Latin: medium caelum), i.e., the intersection of the ecliptic and the meridian above the horizon. For a discussion of the difficult historical questions concerning the various mathematical rules for defining these astro- logical houses, see North 1996, pp. 579–582. In Figure 24 H is the ascendant, M is midheaven, and the cusps, that is, the initial points of the houses on the ecliptic, are indicated by black dots. Note that the numbers correspond to the houses and that the back of the sphere is not shown. There were several medieval systems for domification, that is, the division of the zodiac into astrological houses, and the data required for each procedure were the date, the time of day, a value for the obliquity of the ecliptic, and a geographical latitude. Each system generated a different set of 12 houses for the same data (North 1986, especially pp. 46–49; Kennedy 1996, especially pp. 535–545). Generally, the longitudes of the cusps were given (in degrees and minutes) as a function of the longitude of the ascendant (in integer degrees). This type of table is usually called a table for the equation of the houses and it is presented as 12 sub-tables, one for each zodiacal sign (see Table 18.1A). There are only a few different tables of this
Z mer idi C an
M
10
S 11 N φ̄ V 12 H horizon E 1 ecliptic eq 2 I ua 3 tor
Figure 24: The astrological houses