Measuring the Sun's Size, and Mocking Epicurus
A sundial-like device to count the sun's diameters, with a note on the first water-wheel
The page opens with a heading on the size, shape, pivots, and circumstances of the first wheel that receives water in water-driven machines. Its main passage then describes a sundial-like apparatus, a circle turned to the south with a central rod, whose shadow is used to count how many times the sun's diameter fits into its daily course. From this Leonardo derives a mocking reductio against Epicurus's claim that the sun is only as large as it appears: if its diameter were a foot and it passed a thousand times through its course in 24 hours, this 'venerable snail of a sun' would have crawled only about 25 braccia an hour. A small circle diagram sits at the upper right.
On this page
Counting how many suns fit in the daily course
Make a circle turned to the south like a sundial, set a rod at its centre, and note the sun's shadow (out to n) on the circumference. The number of times that shadow fits around the circle equals the number of times the solar body fits into its 24-hour course.
Against Epicurus: the 'venerable snail' of the sun
If, as Epicurus held, the sun were only as large as it appears, a foot in diameter, and passed a thousand times through its course in 24 hours, it would travel 500 braccia, a sixth of a mile. Such a sun would have crawled just 25 braccia per hour, an absurd 'venerable snail.'
The first wheel of water-driven machines
A heading introduces a study of the size and shape of the first wheel that receives the water in water-driven machines, together with its pivots and other circumstances pertaining to that wheel.