What does the circular (sometimes crescent-like) symbol on top of Islamic Minarets and Domes represent?

This circular figure appears on the top of the minarets and domes of a great number of Islamic worship places around the world (even as far back as the middle ages—the first image is from the Book of Wonders / Kitab al-Bulhan). What does it represent? A common theory is that it's a stylized crescent; but many Muslims say the crescent is not intrinsically a symbol of Islam. I've seen someone call it "the horns of Dhul Qarnayn"; which sounds too far-fetched, as Dhul Qarnayn isn't that much of a major figure in Islam. So, what are the most common explanations in Muslim literature/historiography for this symbol?