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I dedicated The Prince to Lorenzo de' Medici. It was a job application, essentially. I had been the secretary of the Florentine republic for fourteen years. The Medici had returned, dissolved the republic, and I was now a bankrupt farmer writing political theory in the evenings.

I offered him the distilled knowledge of ancient history and my own...

These are my drawings of the moon, made in November 1609. The telescope gave me a magnification of twenty times, sufficient to see that the lunar surface was not the smooth crystal sphere Aristotle had described. There were mountains. I could measure their height by observing where the shadow ended and calculating the geometry.

The highest lunar...

Ludovico Sforza commissioned the Last Supper for the refectory of Santa Maria delle Grazie in 1495. I worked on it for three years and ruined it almost immediately by choosing tempera on plaster rather than true fresco. True fresco must be painted quickly, wet into wet. I needed time — to observe the faces, to return to a passage, to reconsider a...

Pope Julius II gave me the Sistine Chapel ceiling in 1508. I told him I was a sculptor, not a painter. He told me to paint it anyway. I lay on scaffolding for four years, my neck twisted back, paint dripping into my eyes. I wrote a poem about it: 'My beard towards Heaven, I feel the back of my brain upon my neck.'

The Creation of Adam is the centre panel,...

I watched birds for thirty years. I filled notebooks with the mechanics of their flight — the articulation of the wing joint, the way a goshawk tilts its tail to change direction, the moment a kite folds before a dive. From this I designed machines that would allow a man to fly.

The ornithopter required a human to power the wing strokes. The aerial screw...

Santi di Tito painted this portrait after my death, based on descriptions. I do not know whether it resembles me. I was not handsome. I was thin, dark-eyed, with a faint ironic expression that everyone who met me mentioned. The expression was involuntary — it is hard to work in the Florentine chancery for fourteen years without developing a certain...

I published the Sidereus Nuncius — the Sidereal Messenger — in March 1610. Sixty pages. It sold out immediately. It contained four discoveries: the moon is not a perfect sphere but mountainous and cratered; there are far more stars in the Milky Way than the eye can detect; Jupiter has four moons that orbit it; Venus has phases that can only be explained if...

The David is seventeen feet tall, cut from a single block of Carrara marble that had been quarried in 1464 and left abandoned — damaged, they said, by a previous sculptor who had made a poor incision near the legs. I worked around the damage.

I showed him at the moment before the battle, not after. Not the triumphant boy holding a giant head. The hand is...

Vitruvius wrote that the human body, arms and legs extended, inscribes both a circle and a square. I wanted to draw this — not as geometry but as a problem. The circle and the square do not share the same centre. The navel is the centre of the circle; the genitals are the centre of the square. This meant that Vitruvius, or my understanding of him, had...

The Dialogo was published in February 1632. By October I had been summoned to Rome. The Inquisition said I had violated the injunction of 1616 by defending the Copernican system. I said I had merely presented both sides with fairness. They were not persuaded.

The book's title page shows three figures in conversation: Aristotle, Ptolemy, and Copernicus....

The proportions of man and the proportions of architecture are one and the same. Vitruvius was correct: the human body, perfectly proportioned, fits inside both a square and a circle. I drew this in my notebook around 1490. I was trying to understand what the ancient architect understood — that beauty is not arbitrary but mathematical.

I have since come...

The Last Judgment, altar wall of the Sistine Chapel, completed 1541. I was sixty-six years old. The Pope wanted it repainted because I had shown the saints nude. I refused. I told him: tell the Pope to reform the world first, and the painting will reform itself. He did not respond. The nudes remained.