A horizon is not a physical object with a composition, but rather a visual phenomenon. It's the apparent line where the sky and earth meet, caused by the curvature of the Earth and our perspective.

Here's a breakdown of what contributes to the appearance of the horizon:

1. Curvature of the Earth: The Earth is a sphere, and this curvature means that the surface gradually slopes downward as you move away from a point.

2. Perspective: Our eyes perceive parallel lines as converging in the distance. Since the Earth's surface curves downward, it appears to meet the sky at a point on the horizon.

3. Atmospheric Effects: The atmosphere scatters and absorbs light, causing the sky to appear blue. This blue color extends down towards the horizon, blending with the colors of the ground or water.

4. Objects on the Horizon: The appearance of the horizon can also be influenced by objects that are close to the observer, such as buildings, mountains, or trees. These objects can block or obscure portions of the horizon.

5. Time of Day: The angle of the sun affects how light interacts with the atmosphere, influencing the color and clarity of the horizon.

In summary: The horizon is a product of our perception and the way we interpret the curvature of the Earth and the effects of the atmosphere. It's not a physical boundary with a specific composition.