The Sphere of the World with Commentaries

Chapter 3: On the rising and setting of signs; on the diversity of days and nights; and on the diversity of climates.

The rising and setting of signs are understood in two ways: namely, with regard to the poets and with regard to the astronomers. Therefore, the rising and setting of signs according to the poets is threefold, namely: cosmic, chronic, and heliacal. For a cosmic rising, or mundane, is when a sign or star ascends above the horizon on the side of the east by day. And although in any given artificial day six signs thus rise, yet antonomastically, that sign is said to rise cosmically with which and in which the sun rises in the morning. And this is called the proper, principal, and daily rising. An example of this rising is had in the Georgics, where the sowing of beans and millet is taught in the spring while the sun is in Taurus, thus:

"The bright Bull opens the year with golden horns, and the Dog, yielding to the opposing star, sets."

The cosmic setting, however, is with respect to opposition—that is, when the sun rises with a certain sign, the opposite of which sign sets cosmically. Of this setting, it is said in the Georgics, where the sowing of grain is taught at the end of autumn while the sun is in Scorpio: when Taurus, the sign opposite to it where the Pleiades are, rises with the sun, it sets, thus:

"Before the Atlantides hide for you at dawn, let the due seeds be committed to the furrows."

A chronic rising, or temporal, is when a sign or star emerges above the horizon on the side of the east after the setting of the sun—that is, by night—and it is called temporal because the time of the mathematicians is born with the setting of the sun. We have an example of this rising in Ovid’s Epistulae ex Ponto, where he laments the duration of his exile, saying:

"The rising Pleiad makes four autumns."

Signifying by the four autumns that forty-four years had passed since he was sent into exile. But Virgil intended the Pleiades to set in autumn; therefore, they seem contrary. But the reason for this is that according to Virgil they set cosmically, [while] according to Ovid they rise chronically, which can well happen on the same day, but differently nonetheless.