The Sphere of the World with Commentaries

because the cosmic setting is with respect to morning time, whereas the chronic rising is with respect to evening time. The chronic setting is with respect to opposition. Whence Lucan says thus: "Then night drove the Thessalian arrows with small force." A heliacal rising, or solar, is when a sign or star can be seen because of the elongation of the sun from it, which previously could not be seen due to the sun's proximity. Ovid provides an example of this in the book Fasti thus: "Now the light Aquarius has subsided with his oblique urn." And Virgil in the Georgics: "And the star of the burning crown descends." Which, existing near Scorpio, could not be seen while the sun was in Scorpio. A heliacal setting is when the sun approaches a sign and does not permit it to be seen because of its presence and brightness. An example of this is in the verse mentioned above, namely: "The Bull and the Dog, yielding to the opposing star, sets."

ON THE RISING AND SETTING OF SIGNS ACCORDING TO ASTRONOMERS.

There follows [the discussion] on the rising and setting of signs as astronomers conceive them, and first in the right sphere. It is to be known that in the right sphere, just as in the oblique, the equinoctial circle always ascends uniformly—that is, in equal times, equal arcs ascend. For the motion of the heaven is uniform, and the angle which the equinoctial makes with the oblique horizon is not varied in any hours. The parts of the zodiac, however, do not of necessity have equal ascensions in either sphere, because the more directly a part of the zodiac rises, the more time is placed in its rising. The sign of this is that six signs rise in a long or short artificial day, and similarly in the night.

It is to be noted, therefore, that the rising or setting of any sign is nothing other than that part of the equinoctial [circle] rising which rises with that sign as it rises or ascends above the horizon, or that part of the equinoctial [circle] setting which sets with another sign as it sets—that is, as it tends toward the setting below the horizon. A sign is said to rise directly with which a greater part of the equinoctial rises, but obliquely with which a smaller [part rises]. Similarly