In 1609, Galileo Galilei learned of a telescope that had been made in Holland, and he immediately set about building one of his own. One night after midnight, he trained his spyglass on celestial objects and was astounded by what he saw: nebulosities in the Milky Way gave way to a multitude of faint stars so close together as to appear nebulous to the naked eye; the Moon cratered; and planet Jupiter with four specks of light revolving about it, which Galileo correctly identified as moons. He made sketches of what he observed and of Jupiter's moons as they revolved and published the results in March, 1610 in a booklet known as The Starry Messenger. Shakespeare wrote Cymbeline before the end of that year.
In the play, Giacomo emerges from a trunk after midnight and observes Immogen's bedroom, its stars and Jupiter's moons, and in Shakespeare's day, another word for "telescope" was "trunk."
Giacomo happens to be the nickname of the person who transmitted to Galileo news of the Dutch telescope invention. Giacomo wagers that he can compromise the virtue of Posthumus's beloved Immogen. He travels to England, but she spurns his advances and he resorts to deceit to access her bedchamber. He tells her that he has purchased jewels and has stored them in a trunk, which he persuades her to keep safe for him. He hides in the trunk, has it delivered, and soon after Immogen falls asleep at midnight, he emerges. Auditors presume that he intends to win his bet forcefully, but instead he makes sketches and takes notes on the contents of her bedroom. Of interest to him are "ten thousand meaner movables" that are "above," and in Hamlet, 10,000 represents the number of stars in the sky visible to the naked eye. i.e. Galileo estimated that there were 10,000 or more stars beyond the reach of the naked eye. Furthermore, Giacomo identifies four moons whose periods are less than the Earth's Moon, as are all four of Jupiter's moons.
Posthumus masquerades as a soldier in the Roman army that had invaded Britain, and the troops of the British King Cymbeline capture him. He is incarcerated awaiting trial when he falls asleep and dreams of the four ghosts of his deceased parents and two brothers who dance around in imitation of celestial orbits. They beseech the god Jupiter to intervene on his behalf, and with spectacular theatricality, Jupiter descends from the heavens seated on the back of an eagle,
the Bird of Jove. He brings with him a tablet that morphs linguistically into a book, and lays it on Posthumus's breast. In 2001 and again in The Shakespeare Newsletter in 2003, I suggested that the godly gift is Galileo's book The Starry Messenger in which he announced observations of the four ghostly satellites of Jupiter. The Diggeses were telescopists long before 1610, and we wonder next whether the bard refers to the Jovian satellites in an earlier play.