Complete Works, Volume Four

through many observations, hypotheses are constructed, or circles and orbs are assumed. It is probable that the stars move according to these. This is done so that the things observed appear as they are.

I pass over for now the creation of what are called Astronomical Tables. These are calculations original: "Abacos" expressing in numbers or arithmetic signs the times that the stars spend in completing their circuits according to the assumed hypotheses.

I also pass over the preparation of Ephemerides, or Diaries, from these Tables. These show where the Sun, Moon, and other stars are in the sky on individual days. They also show how they are positioned relative to each other based on certain established starting points of motion and time.

Furthermore, since Astronomy is a primary part of Cosmography, or the mapping of the world, nothing can be appropriately said or understood about the Heaven and stars unless a general System of the World is kept before the eyes. Therefore, a Diagram original: "Schema" must be added below to represent the arrangement of the parts of the world as it is commonly conceived and taught.

In this diagram, the innermost small circle represents the globe of Earth and Water. The two succeeding circles represent Air and Fire, which are the two superior elements.

The eleven following circles show the eleven moving Heavens. There are seven specific to the planets, called those of the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. One belongs to the Fixed Stars, which is therefore called the Aplanes from the Greek word for unwandering and the Firmament. There are two Crystalline heavens, so called because they are starless original: ἄναστροι (anastroi) and transparent in a special way. Finally, there is one Supreme heaven, also starless original: ἄναστρον (anastron), called the First Mover original: Primum Mobile.

I say moving Heavens because theologians establish a twelfth heaven which is itself immobile. This is the Empyrean Heaven, which is the seat of the Blessed souls. It is considered to have a square external shape because the Holy City described in the Apocalypse is said to be laid out as a square.

To remember the number and order of the Heavens, this couplet may be helpful:

Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn,
Firmament, two Crystals, the First, and the Empyrean.

Since a twofold motion is generally observed in the moving Heavens, one is called the Primary or Diurnal motion, common to all. The other is the Secondary motion, specific to some or each individual star. To explain the first, the material sphere (as they call it) was devised. For the latter, they use what is called the Theory of the Planets. From this, it follows that two parts of Astronomy are usually distinguished: one named the Spherical Doctrine and the other the Theoretical.

Therefore, we shall speak briefly of both. It is worthwhile to first understand the subject according to the commonly received opinion, which assumes the System of the World described just before. For that reason, we will carry this out in two books. In the first, we will hand down those things pertaining to the Spherical Doctrine. In the second, we will cover those looking toward the Theoretical.

Immediately after, however, because two other systems have now become very famous, namely that of Copernicus and that of Tycho Brahe (both of which have gained noble supporters), we will attach a third book as a kind of appendix. This is so that one may also learn what those systems are and how they are defended by their followers.

An ornate woodcut tailpiece features a central grotesque mask or face flanked by cornucopias, floral swags, and architectural flourishes, including a small bird perched on the left side.