For Leonardo, the Earth was also a great organism whose movements are regulated by Nature's universal laws of mechanics. He subjected its parts to the same detailed anatomy that he applied to machines and living matter. Thus he discovered that, like man, it had an internal, continuous circulation of water that travels from the bottom of the oceans through a process of distillation to the tops of the mountains from whence it flows back to the sea. His notes about problems of the Earth cover a variety of disciplines, now called geology, geomorphology, palaeontology, cartography and astronomical geography. Water (whose movements fascinated Leonardo) was the fundamental dynamic aspect of his concept of geology: it scours the earth's crust, strips the mountains by erosion and causes volcanic activity and earthquakes.
He continually evoked the body-earth analogy, attributing a vegetative life to the Earth: "Its flesh is its soil, its bones the orders linking the rocks, its blood the veins of water." He explained the Earth's structure by rigorously mechanical reasons and laws, whether he was examining its centre of gravity or its tides or exploring the reasons for the creation of mountains and rock stratification, or whether he was studying wind erosion or the destructive dynamics of floods and earthquakes. Leonardo's interest in the plant world was linked to his work as a painter. He studied nature so that he could paint it faithfully and his observations were so accurate as to assume scientific value. He recognised many phenomena including the growth of branches and the regular arrangement of fruit and leaves.
His botanical studies are limited, however, because there are physical and chemical principles at the base of plant life and these were unknown to Leonardo and his contemporaries, but he has the merit of being one of the first to refer to a plant classification system. Meticulous drawings and notes describe the process of rock stratification that he attributed to the deposit and sedimentation of material eroded by rivers, lakes and the sea, the formation of the earth's crust by the effect of water in all its forms, and fossils, which he regarded as ancient organisms that had lived in the seas which at one time covered the mountains. Leonardo's innovative contribution was his observation that the layers of fossils must have been formed at different times, allowing different "year cycles" to be identified, and it was on this concept that division into geological eras would later be based. He was also involved in astronomy and the reflection of light between celestial bodies interested him greatly. He demonstrated that the ring of light seen round the new moon is not an autonomous emanation of lunar light but produced by the reflection of the terrestrial seas on the moon.