Here's why:

* Natural form usually refers to a physical object or structure found in nature, like a mountain, a tree, a cloud, or a rock.

* Sunset is a phenomenon – a visual effect created by the interaction of light, the Earth's atmosphere, and its rotation.

However, you could argue that the *shape* of the setting sun itself is a natural form, since it is the outline of the sun as seen from Earth.

Ultimately, whether you consider a sunset a natural form depends on your definition of "form."