Shakespeare was well aware of the fact that the moon lacked its own light and merely reflected the sun’s. He refers to the moon’s “borrowed sheen” in Hamlet Act III scene 2:

Full thirty times hath Phoebus' cart gone round

Neptune's salt wash and Tellus' orbed ground,

And thirty dozen moons with borrowed sheen

About the world have times twelve thirties been,

Since love our hearts, and Hymen did our hands,

Unite comutual in most sacred bands.

The idea that the moon is a rocky rather than a fiery object traced back in the West at least to the Greek philosopher Anaxagoras, who lived in the fifth century BCE. In one of the few surviving fragments of his writing he says: “It is the sun that puts brightness into the moon.” He believed that the moon was a fragment of the earth that has been thrown up to the heavens in early times, and was able to explain lunar phases and eclipse through the geometric arrangement of the celestial bodies.

Later Greek and Roman astronomers were able to refine these ideas and predict eclipses based on the calculations. They even manufactured clockwork computers such as the famous Antikythera Mechanism to automate the process of calculating the cycles and epicycles of the moon.

Shakespeare’s time, two millennia later, was the dawn of the scientific revolution and critical time in the development of astronomy, six decades after a Copernicus turned the world upside down by putting the sun in the centre of it. Shakespeare was a contemporary of two of the most famous early scientists, Galileo and Kepler. Within ten years of when Romeo and Juliet was first performed, Galileo would discover that not only the moon, but also Venus had phases, caused by the variable illumination provided by the Sun. This proved that Venus was also rocky, and that it revolves around the Sun, not (as previously thought) around the Earth on a separate celestial sphere. Kepler, meanwhile, around the same time, was working on a novel called The Dream, an early (possibly the first) science fiction novel in which the narrator travels to the moon on a flying ship and meets the lunar inhabitants. The conceit allowed Kepler to explain to the reader how the cosmos would look from the point of view of of the moon: the monthlong days, the earth hanging stationary in the sky. This was meant to function as a defence of Copernicus’s heliocentric model, but it show how far advanced beyond envisioning the moon as a fiery body the contemporaries of Shakespeare were.