FIRST SCIENCE · SECOND CHAPTER · NINTH SECTION · Second Article
Second Article
It describes the structure and movements of the carrier sphere of the beautiful-faced, luminous moon.
O noble one! It should be known that the scholars of astronomy have said as follows: The third sphere of the moon is the carrier sphere, which (as will be understood from the explanations given in the above articles) is, like the other planets that are eccentric spheres, situated within their own concentric spheres; likewise, the aforementioned carrier sphere is placed within the inclined sphere. And its convex surface touches the convex surface of the carrier sphere at the common point called the “inclined apogee.” Its concave surface also touches the concave surface of the carrier sphere at the common point called the “inclined perigee.” The center of this sphere is, together with its diameter and constituent parts, ten degrees and a quarter degree outside the apogee point from the center of the cosmos.
When the carrier sphere is separated from the inclined sphere, two spheres remain from the inclined sphere, which are called the complement of the inclined sphere. Apart from the three orbital movements of the carrier sphere mentioned above [52/b], its other movement is as follows: within its own inclined sphere, it rotates around its own center. And this rotation is tangent to the belt of the inclined sphere. On an axis parallel to its axis, it moves with its own particular motion from west to east. According to the rising of the zodiacal signs, it travels more than twenty-four degrees in one day and one night. During this movement, it also moves the epicycle sphere of the moon along with it. For, the movement of this carrier sphere, which is more than eleven degrees in accordance with the rising of the zodiacal signs and directed westward, is counteracted and opposed by the westward movements of the node and the inclined sphere, which are contrary to the movement of the rising of the zodiacal signs, thus causing it to be drawn back.
Thus, the daily movement of the carrier sphere of the moon in accordance with the rising of the zodiacal signs remains approximately thirteen degrees. The moon, whose movement is swift, whose own sphere is small, and which also encompasses the other spheres towards the earth, remains in each of the twelve zodiacal signs for about two full days and a third of a day, and by traversing the sphere of the zodiacal signs once every twenty-eight days, completes its cycle. In this manner, while revolving, it catches up with the sun and coincides with it once every twenty-nine and a half days. For this reason, the lunar months are calculated as either twenty-nine days or thirty days. The new moon is visible at sunset at the solstices (on June 21 and December 21) to the inhabitants living in the northern hemisphere. For those living in regions near the equator in the southern hemisphere, the moon is easily visible in every season. This is because the daily turning points of the sun are inclined in the northern hemisphere. In the southern hemisphere, however, the light is received at rather steep angles. For this reason, the moon does not set together with the sun; it is seen above the horizon.