Why is this the answer?

The answer is moon because this wonderful riddle uses beautiful, secret words to describe the different ways we see the moon move across the night sky. It tells us about the moon's shape, the kind of light it gives us, and how it helps us keep track of time. Let's look at the first line: "Scythe of darkness." A scythe is a tool with a long, curved blade that farmers sometimes use. When the moon is just a sliver, like a thin fingernail, it is called a crescent moon. If you look closely at that crescent shape, it looks exactly like the curved blade of a scythe hanging up in the dark night. This is a perfect picture of the moon's shape during a part of its cycle, cutting through the black sky. Next, the riddle says, "Shadow's light." This is a clever way of telling you that the moon is not like the sun. The moon does not make its own light; it just reflects the bright light of the sun down onto Earth. This light is very soft and gentle, not bright or hot like the sunshine during the day. It is just enough light to help you see things in the dark and make long, soft shadows as you walk or play outside at night. It is a quiet light that works perfectly with the shadows. Finally, the most mysterious clue is: "Guiding eye of thirteenth sight." The moon is like a powerful, guiding eye in the sky because people long ago used it to help them travel across the ocean or know when to plant their food. It has always helped people keep track of time. A regular year has 12 months, and usually, we see one full moon in each month. But every few years, our planet gets an extra full moon, which means we have a total of 13 full moons in that special year. This extra, thirteenth cycle is what the riddle calls the moon's "thirteenth sight," making the moon a wonderful object that guides us with its constant, yet changing, glow.