In 1609, Galileo pointed a telescope at the sky and changed the world for every human who has lived since. One of his first observations was of the craters on the moon. These days, it’s difficult to see that discovery as earth-shaking, but it was.
Galileo was not the first person to point a telescope at the sky. But those who tout, for example, the British stargazer William Harriot’s observations and drawings of the moon miss the point entirely.
Harriot observed the moon six months before Galileo pointed his telescope. However, an examination of Harriot’s lunar sketches shows the miserably poor quality of his telescope, which was essentially a toy imported from Holland. It resolved so poorly that it reveals no features a sharp-eyed stargazer can’t see with the unaided eye.
Galileo had a similar toy, which he saw was worthless. He improved the design so much that he looked at the moon and saw what even simple opera glasses will show us today. Galileo saw craters.
Galileo’s craters were nothing short of heresy. The prevailing wisdom at the time came from the fourth-century BCE Greek philosopher Aristotle. He believed that Earth may be corrupt and evil, but the heavenly spheres were pure and perfect.
The moon may have dark splotches, forming our mythical “Man in the Moon,” but its surface was smooth and unsullied by the corruption below.
Still, those dark markings, which Galileo dubbed maria (Latin for “seas”), were an intellectual conundrum, even in Aristotle’s time.
Aristotle himself admitted that some of the corrupt imperfections found on Earth might have sullied the moon. Luna was embedded in the celestial sphere closest to Earth and its atmosphere. Apparently, proximity breeds corruption.
Aristotle’s beliefs held sway in the Western world for over a millennium. However, a few divergent opinions appeared.
In the first or second century CE, the Greek historian Plutarch suggested in “On the Face in the Moon’s Orb” that the dark markings resulted from cavities in the moon so deep that light did not reach their bottoms.
During the early parts of the Middle Ages, Aristotle reigned supreme. As the pursuit of knowledge waned in Europe, the Islamic world took up the defense of Aristotle.
They resurrected an idea from Aristotle’s time that the moon was a perfect mirror that reflected the features of the Earth below. However, they soon rejected the idea because the lunar features never change as the moon moves across the sky.
The most common explanation was finally accepted in both the Islamic world and Europe as it awoke from its academic torpor. The moon was indeed perfectly smooth, but its elements varied in density. Areas of different densities resulted in Luna’s various shades.
Along came Galileo, who destroyed the perfection of God’s heaven. He saw not only craters but mountains and valleys.
Of course, Galileo had opponents. For example, mathematician Christoph Clavius, a Jesuit faculty member of the Collegio Romano, resurrected the old density argument.
However, the handwriting was on the wall. According to historians from the Galileo Project, the three other mathematicians, Jesuits all, on the Collegio Romano faculty admitted that the moon’s surface was uneven.
Galileo had direct observation on his side, but his craters and other discoveries eventually led him to be tried and convicted by the dreaded Inquisition. He abandoned his telescope and spent the rest of his life under house arrest doing gravitational experiments.
Nowadays, lunar maria and craters are the primary evidence that planets formed by the accretion of smaller “planetesimals” into planets.
Relatively minor impacts formed the craters. The maria were created from enormous impacts that caused the moon’s then-molten interior to ooze forth giant lakes of lava from the moon’s still molten interior.
The lakes cooled very slowly, thus accounting for their darker color. The lunar mountains are, for the most part, the raised rims of the maria.
The heavens are hardly perfect. They were born out of large-scale destruction and violence that make the possibility of an atomic war on Earth seem petty by comparison.
But out of the chaos of accretion came a new order, the solar system, with at least one solid surface blessed with liquid water and an atmosphere. The upshot of destruction was re-creation, and that re-creation eventually resulted in life on our planet.
Tom Burns is the former director of the Perkins Observatory in Delaware.
