Carefully observe the difference between the two.
The moon though a sphere in reality does not look like a sphere. Instead it looks like a disc. A sphere has lesser intensity in the sides. But the beauty of the moon lies in the uniformity of its intensity. Similarly the sun also looks like a disc.
This is because natural sources like the sun and moon do not emit isotropically(same intensity at all directions). Instead they emit as lambertian sources (a plane that emits a flux proportional to the cosine of the angle of the normal).
