currently called "planets" that were known to the Greeks were those visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Graeco-Roman cosmology commonly considered seven planets, with the Sun and the Moon counted among them (as is the case in modern astrology); however, there is some ambiguity on that point, as many ancient astronomers distinguished the five star-like planets from the Sun and Moon. As the 19th-century German naturalist Alexander von Humboldt noted in his work Cosmos, Of the seven cosmical bodies which, by their continually varying relative positions and distances apart, have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of the "fixed stars", which to all sensible appearance preserve their relative positions and distances unchanged, five only—Mercury, Venus, Mars, Jupiter and Saturn—wear the appearance of stars—"cinque stellas errantes"—while the Sun and Moon, from the size of their disks, their importance to man, and the place assigned to them in mythological systems, were classed apart. In his Timaeus, written in roughly 360 BCE, Plato mentions, "the Sun and Moon and five other stars, which are called the planets". His student Aristotle makes a similar distinction in his On the Heavens: "The movements of the sun and moon are fewer than those of some of the planets". In his Phaenomena, which set to verse an astronomical treatise written by the philosopher Eudoxus in roughly 350 BCE, the poet Aratus describes "those five other orbs, that intermingle with [the constellations] and wheel wandering on every side of the twelve figures of the Zodiac." In his Almagest written in the 2nd century, Ptolemy refers to "the Sun, Moon and five planets." Hyginus explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta." Marcus Manilius, a Latin writer who lived during the time of Caesar Augustus and whose poem Astronomica is considered one of the principal texts for modern astrology, says, "Now the dodecatemory is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven." The single view of the seven planets is found in Cicero's Dream of Scipio, written sometime around 53 BCE, where the spirit of Scipio Africanus proclaims, "Seven of these spheres contain the planets, one planet in each sphere, which all move contrary to the movement of heaven." In his Natural History, written in 77 CE,