Proclus, Scholium on Hesiod’s Works and Days 383–387, “When the Pleiades, daughters of Atlas, are rising, begin your harvest, and your ploughing when they are going to set. Forty nights and days they are hidden and appear again as the year moves round, when first you sharpen your sickle.”
Hesiod is referring to Atlas, the child of Iapetus, who carries heaven and the pillars “that keep Earth and Heaven apart” (Odyssey 1.54), because he is established as their one connection and has been allotted the delimitative powers of Heaven and Earth, through which the former, who is separated from the Earth, is whirled around her for all time, and the latter, who is stationed fixed in the middle, brings forth all things maternally, as many as Heaven generates paternally. Because he steadfastly guards these powers of them both in delimitation, Homer called them pillars “that keep Earth and Heaven apart”, which shows that they are separate from each other and eternally unmixed with each other.
They mythologized that his children are the seven Pleiades: Celaeno, Sterope, Merope, Electra, Alcyone, Maia and Taÿgete. These are all archangelic powers set over the archangels of the seven spheres, Celaeno of the Saturnine sphere, Sterope of Jupiter’s, Merope of Mars’s, Electra of the Sun’s, Alcyone of Venus’s, Maia of Mercury’s and Taygete of the Moon’s. And the causes for these are manifest.*
[*The most obvious connections are between Maia and Hermes, her son according to myth, and between Electra and the Sun, also called Ēlektōr.]
The one arrangement of the seven is drawn onto the fixed sphere like a celestial cult statue (agalma), which they call the Pleiad. It is a visible constellation that is placed within Taurus, and through its rises and sets, it signifies the complete change of the weather (lit. ‘air’) to laypeople. So, Hesiod tells us to begin the harvest when these Pleiades are rising. And they ‘rise’ when they first appear before sunrise, because the morning rise of a star is the first visibility before the rays of the sun, while the evening rise is that immediately after sunset. And the morning set is the disappearance into the western horizon shortly before sunrise, whereas the evening set is the set immediately after the sunset. Now, the Pleiades rise when the Sun enters into Taurus, because they emerge before it (i.e., the visible constellation Taurus); and it is at this point that Hesiod tells us to begin the harvest, but when they are setting—meaning the morning set—he tells us to begin ploughing, that is, when he Sun is already leaving Scorpio; because at that point comes the morning set of Taurus. Having said this much, he continues by saying for how long a timespan the Pleiades are hidden from the sunrays, namely forty days; because they are hidden when the Sun is at the end of Aries, and they still are when it passes into Scorpio; and they are likewise not only visible when when the Sun is in Taurus, but also rise when the Sun is already in Libra.
[Note: in later scholia, it is said that, according to Proclus, the Pleiades are “archangelic powers, like the Muses”. This may be faulty extrapolation by a Christian Byzantine writer, but it could very well be a piece of information lost from the – very incomplete – text of Proclus, especially since the nine Muses have also been assigned to different spheres, including by Proclus himself in his commentary on the Timaeus (vol. 2 p. 210). He does not elaborate on this, but in Martianus Capella (Philologia I.28), they are correlated as follows: Polymnia to the Saturnian sphere, Euterpe to Jupiter’s, Erato to Mars’s, Melpomene to the Sun’s, Terpsichore to Venus’s, Calliope to Mercury’s, Clio the Moon’s, and Thalia to the Earth. Urania is not enumerated but may be connected to the sphere of fixed stars or to the cosmos as a whole. Note that for Proclus, the nine Muses are only members of the chain of the Muses, while their “very first cause” is the goddess whose name is simply Muse (Proclus, On the Republic vol. 1 p. 184).]