“Awake, for it has come tossing torches in hand, Iacchos, Oh Iacchos, the light-bringing star of our nocturnal rite…. Beaming with your torch, lead forth to the flowering stretch of marsh the youth that makes your choruses, o blessed one!” – Aristophanes, Frogs
In Aristophanes’ “The Frogs,” the playwright puts the god Dionysus in the path of the subterranean celebrants of the Eleusinian mysteries a few times during his journey through the underworld. The Eleusinian initiates make many references to Iacchos or Iakchos, “an alter ego of Dionysus, going to Eleusis in search of his mother, Semele.” But who is Iacchos? And where do the appellations “lamptêr” (“the torcher bearer”) and “lampterus” (“of the torches”) come from? As with many famous myths surrounding Dionysus, we will have to look to the stars to answer these questions.
In the ancient world, wine-making was highly dependent on close observance of the stars, as different disappearances and reappearances of celestial objects would signal when to perform different stages of the finicky grape harvesting process.
In his “Works and Days,” Hesiod wrote: “When Orion and Sirius reach the middle of the sky…and when rosy-fingered Dawn sees Arcturus, then…you should cut off and take home all the grape-clusters. Show them to the sun ten days and ten nights. Then shade them over for five more, and, on the sixth, draw off into jars the gifts of joyous Dionysus.” Arcturus, the fourth brightest star in the sky and part of the hunter constellation Boötes, became irrevocably tied to Dionysus for this reason: its burning torch-like appearance signaled to farmers they were allowed to finally pick the grapes that needed to be hidden from the blistering Mediterranean summer sun until just the right moment.
But while this answers why Arcturus became linked to Dionysus, it doesn’t answer why it became specifically linked to his Eleusinian Mystery alter-ego Iacchos: a persona with much earlier roots.
It is believed that Iacchos, like the Mysteries themselves, may have their origins in the fertility and agrarian rituals of Bronze Age Crete (itself a famous producer of high-quality wines through the Classical era and beyond). Arcturus’s heliacal rising in mid-September was not merely a sign to begin the wine-making process to Cretans, but also a signal that laborers should begin planting figs, apples, olives, and other fruits. Arcturus’ heliacal rising also heralded in the ascendancy of Spica, the brightest star of the constellation Virgo which would take center stage in the sky during the harvest season sacred to Demeter and Persephone.
Spica and the Virgo constellation have both been linked to both the vegetation goddess and her chthonic daughter, and even now amateur astronomers are told to “follow the arc of Arcturus” to find Spica in the summer night sky.
The smaller blazing Arcturus star “guiding” the larger Spica into view was likely codified in both myth and mystery as a smaller male presence heralding the path of the great goddesses. This is preserved in many depictions of the Eleusinian Mysteries in art, as a smaller “torch-bearing” male figure is usually standing beneath, below, or behind Persephone or her mother.
While it is not certain if this origin was common knowledge to the Greeks (as much knowledge of the Mysteries has been lost to time), we do know that initiates left Athens for Eleusis on the 27th or 28th of September: right when Arcturus is at its peak.
 http://www.perseus.tufts.edu/hopper/text?doc=urn:cts:greekLit:tlg0019.tlg009.perseus-eng1:340-353   Eleusis: Archetypal Image of Mother and Daughter, 64  Ancient Skies: Constellation Mythology of the Greeks