Heavenly mirrors: Leonardo’s theory of cosmology

Leonardo’s theory of cosmology concerns the relation of the sun, earth and moon to one another. But without a telescope, he had to reach his conclusions on the structure of the moon by applying his knowledge of optics and perspective. By Dr. Domenico Laurenza, Museo Galileo.

Distinct from the dominant view at the time, Leonardo believed that the moon has no light of its own, but reflects sunlight because its surface, like the earth, has seas whose waves reflect sunlight.

What we now know is that Leonardo was half right, the moon does not emit its own light, and the seas are of solid lava, not water. Leonardo’s theory of the earth was also very original for the time. The land, according to Leonardo, is in continuous metamorphosis. The water erodes the soil and deposits it elsewhere and in this way digs the valleys and creates the mountains, modifies the course of the rivers and the shape of the coasts. The earth has history of continuous metamorphoses and will eventually have an end, just like the body of man and animals.

Here, Leonardo studies how the waves of the lunar seas reflect the “simulacrum” or image of the sun.

This drawing shows the waves of the moon, by which the sun, represented by the circle at the top, is reflected.

If the seas of the moon were calm, or if the moon was a simple convex mirror, the terrestrial observer (placed at the point "f") would see a small image of the sun, equivalent to the circle represented below.

Instead, the waves, each reflecting an image of the sun, determine an enlargement of the solar image reflected towards the earth, as big as the entire represented sea surface.

The two drawings in the centre represent various forms of sea surface furrowed by waves. Leonardo believed that in the presence of small waves the image of the sun is reflected more intensely.

In this drawing, Leonardo shows how moonlight is sunlight reflected by the lunar sea waves.

In his studies of the sun-earth-moon system, Leonardo deduced that the moon has no light of its own but reflects sunlight like a convex mirror.

The sun is top left, the observer on the right and the moon at the bottom. If the moon were a uniform convex mirror it would reflect sunlight only with a small portion of its surface, as shown in the drawing. Instead in the full moon, we see the entire lunar surface reflecting sunlight.

This happens because the lunar mirror is actually made up of many single mirrors (the various circles of the drawing): the waves of the lunar seas. The individual images reflected by each wave seen from the distant earth appear as a uniform image of sunlight.

Aristotle believed that the moon was a perfect crystalline sphere with its own luminosity. Leonardo rejected this. He argued that the moon has no light of its own and is an opaque body like the earth. 

A small drawing of an egg shows Leonardo musing on the Moon’s gravity: “If the egg yolk is lighter than the albumen, it ought to rise above the albumen and attach itself to the shell; if it is heavier, it should sink; and if it is equal in weight, it could be positioned in the middle or below.”

Other drawings study the various brightness of the waxing and waning moon, based on the different angles of the incidence and reflection of light.

The drawings in the upper half of this sheet represent the waves of the lunar seas that reflect and amplify the luminous image of the sun towards the terrestrial observer.

Almost all the drawings in the lower half of the sheet consider various reciprocal positions of the moon, the sun and the earth (for example, the sun to the west of the earth, the moon to the east).

The irregularity of the moon’s surface, as seen from the earth, is also what Leonardo imagined the earth would look like from the moon.

His vision of the reciprocity of phenomena and structure between moon and earth is the most important aspect of this field of his research.

This sketch represents the spheres of the elements, as they were popularly conceived in Leonardo’s time: the earth in the centre, then the water, air, and fire. The small circle “o” represents the forces of motion and gravity.

Based on the analogy between macrocosm and microcosm, the body of the earth and the body of man, Leonardo is convinced that water circulates in the earth like blood in the animal body.

The water from the bottom of the sea enters subterranean canals and, thanks to the internal heat of the earth, similar to that of the animals, rises up to mountain springs (represented in the drawing) which give rise to the rivers that bring water back to the seas.

Leonardo studies how water acts like a sculptor on the rivers and valleys it passes through.

It erodes matter from one side of the bank and deposits it on the opposite side. In this way the course of the rivers varies continuously, even more so as it is further from its source.