Mountains on the Moon: Empirical Astronomy and the Imperfect Heavens

Mountains on the Perfect Moon

When I first turned my improved spyglass toward the Moon in 1609—having refined the Dutch design to achieve thirty-fold magnification—what I observed shattered two millennia of celestial doctrine. The Aristotelians proclaimed heavenly bodies perfect: smooth spheres of quintessence, fundamentally distinct from our corrupted terrestrial realm. Yet through my lens, the Moon revealed itself as a world like Earth—rough terrain, bright highlands, dark lowlands, sharp shadows cast by mountains.

I measured these peaks using geometry. Observing the terminator—that irregular boundary between lunar day and night—I noted how mountain summits caught sunlight while their valleys remained dark. From shadow length and Sun angle, I calculated height through similar triangles. The result: peaks rising some four miles high, comparable to Earth’s own mountains. I sketched what I saw: craters large and small, jagged relief, a surface bearing the mark of material substance, not ethereal perfection.

The implication undermined everything: if the Moon consists of rocks and terrain like Earth, then the heavens are not fundamentally different from our sublunary world. Same matter, same physics. The ancient distinction between celestial perfection and terrestrial imperfection collapsed under direct observation.

Jupiter’s Four Companions

In January 1610, I noticed three stars near Jupiter, aligned and moving with the planet. The next night, their configuration had changed. Tracking them for weeks, I confirmed: four stars orbit Jupiter itself. I named them Medician stars—honoring my Medici patrons—though others now call them the Galilean moons.

This single observation demolished geocentrism more effectively than any theoretical argument. Ptolemy insisted all celestial bodies orbit Earth. Yet here: Jupiter commands its own satellites. Not everything orbits Earth. Multiple centers of motion exist. If Jupiter can have moons, perhaps planets orbit the Sun—Copernicus’s heliocentric hypothesis becomes not merely mathematical convenience but physical possibility.

My opponents refused to look through the telescope. They dismissed these observations as optical illusions, or argued that if God intended us to see Jupiter’s moons, He would have granted us sharper eyes. But the Babylonians discovered eclipse patterns through patient observation spanning centuries—no instruments required, only systematic record-keeping and pattern recognition. Tycho Brahe achieved two arc-minute precision through naked-eye observations using massive quadrants and sextants, collecting twenty years of data that later enabled Kepler to decouple Earth and Mars motions through opposition observations. These examples prove: careful observation reveals nature’s patterns whether through instrument-aided or unaided sight.

Eyes Over Ancient Authority

My method places observation above authority. Do not trust ancient texts merely because Aristotle wrote them. Test nature directly—observe, measure, calculate. The scientific method Feynman later articulated validates this: first guess the law, compute its consequences, then compare predictions to experiment. If observation contradicts the guess, the guess is wrong, no matter how elegant or venerable.

The Church condemned heliocentrism in 1616, ordering me to teach it as mathematical hypothesis only, not physical reality. I complied initially. But my Dialogue Concerning the Two Chief World Systems compared Ptolemaic and Copernican models too favorably toward Copernicus. Pope Urban VIII felt mocked—saw himself in Simplicio, the Aristotelian fool—and ordered my trial. The Inquisition forced my recantation in 1633: “I abjure, curse, and detest.” House arrest followed until death.

Yet observations eventually prevail. The Moon has mountains. Jupiter has moons. Earth moves. Nature provides objective feedback—experiments refute incorrect theories while supporting correct ones. Authority yields to evidence. The telescope opened the heavens, and what we found there was not crystalline perfection but a universe subject to the same mathematical laws governing our imperfect Earth.