Astronomy, Book I.

Mention must be made of astronomers; therefore, some of the most prominent among them should not be passed over in silence.

Vertical circles are those that pass through the Zenith, the point directly overhead, and through the Nadir, the point directly opposite it. These circles cut the horizon at right angles. It is customary for these circles to be called by the Arabic name Azimuths original: "Azimutha".

Since countless circles of this kind can cut the horizon as one moves from the Meridian (which is considered one of the vertical circles) toward the east or west, the Prime Vertical is considered to be the one that passes through the intersections of the Horizon and the Equator.

Circles of altitude are those taken above the horizon and parallel to the horizon itself. They gradually decrease in size, becoming smaller and smaller until they end at the point of the Zenith. These are also commonly called by the Arabic name Almucantars original: "Almicantarath".

It is the custom for both of these sets of circles to be represented on Astrolabes or Planispheres, as they are called. They can also be understood from the preceding diagram representing half of the sphere as it appears on a flat surface. In that diagram, A-B is the Horizon; C is the Zenith; D is the Nadir; and C-A-D-B is the Meridian. The other circles drawn from the Zenith to the Nadir through every ten degrees of the horizon are the Vertical circles. Among them, C-E-D is the Prime Vertical. Meanwhile, F-G, H-I, and others parallel to the horizon and drawn through every ten degrees of the meridian, are the circles of Altitude.

Circles of distance are the names given to the great circles that pass through two stars. Consequently, the mutual distance between them is nothing other than the arc of one of these circles intercepted between the stars.

In the previously mentioned diagram, we can understand such a circle to be the great circle K-L, passing through M, the star Pollux, and N, Spica in the constellation Virgo. The arc M-N, which measures 90 and three-quarters degrees, is the distance between those stars.

The circles of Position, or of the celestial Houses, is the name given to the Horizon and the Meridian, as well as four other circles. These intersect each other and the others at the common sections of the Meridian and Horizon. Since there are six of them, they divide the whole sphere and the entire sky into twelve parts. They call these the celestial Houses. Six of these are below the Horizon and six are above.

The first house is the one located immediately below the horizon to the East. It is called the Horoscope and the House of Life. The next one below it is called the House of Wealth. The third following is the House of Brothers. The fourth, at the bottom of the sky, is the House of

Parents. The others follow in order, as indicated by these two lines of verse:

Life, Wealth, Brothers, Father, Children, Health,
Wife, Death, Religion, and Duty, Friends, Enemies.

It should be noted that these circles of position, or of the Houses, divide the sphere into equal parts. According to some, they divide the Equator; according to others, the Zodiac; and according to others, the Prime Vertical.

One may understand this from the attached diagram. In this diagram representing the Eastern Hemisphere, A-B is the Horizon; C-A-D-B is the Meridian; A-E-B, A-F-B, and A-G-B are the circles of Position. I-K represents either the Equator or the Ecliptic; C-D is the Prime Vertical. The order of the Houses is designated below the Horizon as I, II, and III, and above the horizon as X, XI, and XII. One must understand the number and series of the remaining houses to be in the Western Hemisphere.

I pass over the fact that some prefer the circles of Position to intersect each other not where we have said, but at the poles of the World. In that case, the Horizon is rejected and a fifth circle is added to the Meridian. Others place the intersections at the poles of the Zodiac. In that case, the Meridian is also rejected and a sixth circle is added, so that there are always twelve Houses.

A circular astronomical diagram represents the celestial houses and coordinate systems. It features a central vertical axis (C-D) and horizontal axis (A-B). Various arcs intersect at points A and B, dividing the sphere into segments labeled with Roman numerals (I, II, III, X, XI, XII). Points are labeled with letters C, X, E, XI, F, XII, A, B, I, G, II, H, III, D, and K. The diagram illustrates the division of the heavens into twelve houses and the relationship between the horizon, meridian, and vertical circles.

C A Signature of the printer