PART ONE

Concerning terrestrial longitude.

Chapter VII.

The longitude of a place (alluding to Werner) is a segment of the equator or equinoctial circle, contained between the meridian of that same place and the meridian of the Fortunate Islands. Because those ancient geographers, wishing to describe the surface of the earth and to investigate the distances between places from measurements in stadia, placed the first degree of longitude in the west, that is, in the Fortunate Islands, which are now called the Canaries; then, according to the succession of numbers, they depicted the circuit of the earth degree by degree through the south toward the east. This longitude, I say, we have labeled in the plane figure (which they call a map) as follows, in Hebrew letters (because longitude, if we look toward the South, is directed from right to left in the Hebrew manner) as Helicha Haaretz, which signifies the passage or way of the earth. By what way the longitude of each and every place is to be found, we shall teach with some brevity in the following propositions. But others, God helping, we have decided to publish far more accurately from the treasury of our mathematical discipline in our work on the astrolabe, and also on the plane Meteoroscope. However, for the present, I shall explain how the longitude of the world is to be investigated in our plane representation, and also on a solid body. Longitude is therefore distinguished by meridians, which meet at the poles. Wherefore the degrees measuring the arc of the equator between the first degree of longitude (which marks the west) and the meridian which passes through the place of your habitation are called degrees of longitude.