Question

The sun appear oval at sunrise and sunset but appears circular at noon why

Answer 1

At sun rise,and sun set,sun is near to horizon.The rays of light from upper part and lower part of the periphery of the sun blend unequally on traveling through earth's atmosphere.That is why sun appears oval or flattened during sun rise and sun set.
At noon the sun is over head.The rays of light from sun,enter earth's atmosphere normally.Therefore they suffer no refraction or bending on passing through earth's atmosphere.Hence,the sun appears circular at noon.
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Answer 2

The sun appears oval at sunrise and sunset but appears circular at noon is best explained by phenomenon called refraction.

• At sunrise and sunset, the sun is at horizon.
• The light rays have to travel a longer distance.
• It has to travel through Earth's atmosphere whose density is not uniform.
• The upper layers of atmosphere are thinner when compared to lower layers.
• As a result when the light rays are travelling through these non-uniform layers of atmosphere, they undergo refraction making the sun to look oval in shape.
• At noon, the sun is directly overhead and light rays need not travel long distance.
• Hence, they don't tend to bend at noon and the sun appears circular in shape.