The sun doesn't actually appear smaller in the evening than in the morning. This is an optical illusion caused by the angle at which we view the sun and the scattering of light in the atmosphere.

Here's why:

* Atmospheric Refraction: As sunlight passes through the atmosphere, it bends due to the varying density of air. This bending is called refraction. At sunrise and sunset, the light travels through more of the atmosphere at an angle, causing it to appear elongated and distorted.

* Perspective: When the sun is low on the horizon, our brains perceive it as being further away than when it is high in the sky. This makes it appear smaller.

* Scattering: The blue wavelengths of sunlight are scattered more by the atmosphere than the red wavelengths. This is why the sky appears blue during the day. At sunset and sunrise, the light travels through more of the atmosphere, resulting in more blue light being scattered away, leaving the red and orange wavelengths to reach our eyes.

In reality, the sun's size remains constant throughout the day. The changes we perceive are just tricks of the light and our perspective.