Context:
Aristotle and his contemporaries believed that the celestial bodies, particularly the Sun, is made of quintessence (fifth element) to be able to float that high up in the atmosphere. It is not the Sun that is hot, rather its rays having friction with the atmosphere that makes it hot.
Leonardo da Vinci disproved this by focusing sunlight in a concave mirror, arguing that the sun rays have intrinsic heat. He also observed how very hot metal turns white, the colour of the Sun.

I actually nicked the whole idea from Vsauce's short: https://youtube.com/shorts/gNVj-FIxoe4
Just watch that instead, and spot the comment where I got this meme from.