Leonardo da Vinci (Figure 1) is not usually known for his astrophysics. Yet this remarkable individual -- the quintessential "Renaissance Man" -- made quite a few pronouncements related to astronomy and astrophysics.
Figure 1. Portrait of Leonardo da Vinci by the painter Francesco Melzi.
While many of Leonardo's statements proved eventually to be wrong, some of them do demonstrate his immense intellectual curiosity, his unusually keen power of observation, and his ability to distill simple conclusions even from complex data.
Take, for instance, his crisp description of the Sun: "The Sun has substance, shape, movement, radiance, heat and generative power; and these qualities all emanate from itself without its diminution." Indeed, the timescale on which the Sun evolves (about ten billion years) is so long, that it would appear unchanged during a human lifetime. Similarly, Leonardo addressed the question of whether the Sun is hot: "Some say that the Sun is not hot because it is not the color of fire is much paler and clearer. To these we may reply that when liquified bronze is at its maximum of heat it most resembles the Sun in color, and when it is less hot it has more the color of fire."
Generally, Leonardo's interest in astronomy stemmed primarily from his attentiveness to vision and to optics. Still, he owned a copy of Ptolemy's "Cosmography", as well as a book by the medieval Persian astronomer Abu Mashar.
One area in which Leonardo anticipated the later findings by Galileo, and clearly departed from the Aristotelian views of his time, concerned the nature of the Moon. Unlike Aristotle, who made a clear distinction between the "terrestrial" and the "celestial", Leonardo concluded that there is no real difference between the Earth and the Moon: "If you were on the Moon... , our Earth would appear to you to make the same effect as does the Moon." He further stated that: "The Moon is not luminous in itself. It does not shine without the Sun."
Leonardo also pointed out contradictions in the many erroneous ideas that existed about the spots seen on the lunar surface. Thus he declared, for instance, "some have said that vapors are given off from the Moon after the manner of clouds, and are interposed between the Moon and our eyes. If this were the case these spots would never be fixed either as to position or shape; and when the Moon was seen from different points, even although these spots did not alter their position, they would change their shape."
Figure 2. A page from Leonardo's notebook, showing a sketch and accompanying text that may be related to the design of a telescope.
A question that has intrigued many Leonardo scholars was whether or not he had ever attempted to design a telescope. After examining much of the evidence, my personal (non-professional) conclusion is that while Leonardo certainly explored some of the theoretical aspects involved in the construction of telescopes (and he even made such statements as: "Construct glasses to see the Moon magnified"), he never actually built one. Figure 2 presents a sketch from Leonardo's notebook, accompanied by text part of which reads: "eyeglass of crystal thick at the sizes at ounce of an ounce." Leonardo researcher Domenico Argentieri found the full text to be suggestive of a design of a telescope.
To conclude, Leonardo was definitely not an astronomer, but he was fascinated by nature, and he did not hesitate to attempt to provide explanations for natural phenomena, or to suggest tools that could help decipher nature's secrets.