Question

why does sun appears flattened during sunrise and sunset but not during noon?

Answer 1

The sun appears flat when near the horizon on account of refraction of light. The atmosphere is made up of different layers of air each having a slightly different refractive index. This change in refractive index with layers causes uneven bending of sunlight. This bending causes the light coming from top layers to bend less then light coming from bottom layers of the atmosphere. This gives sun an apparent deformed, flattened look.