We know that the Sun is much bigger than the Earth but appears very small because it is very far from the Earth. The Sun is about 150 million kilometres away from the Earth. When the Sun is overhead, its distance is 6000 kilometres less than the rising and the setting Sun, which is equal to the diameter of the Earth. The distance of 6000 kilometres is negligible as compared to 150 million kilometres and would not make any difference in the size of the solar disc. In case of any difference, the Sun at noon would have looked a bit bigger than that of the size of rising and setting Sun. Do you know why do we observe exactly the opposite effect? The Sun looks bigger in the morning and evening due to optical illusion. The optical illusion works for the lunar disc also, and because of this, the rising moon appears quite big. The reason for this optical illusion is probably because we tend to compare the size of the rising or the setting Sun with earthly objects. When we see a tree, or a building or a ship near the solar disc then the size of the Sun appears to be big. At noon, when the Sun is overhead, we are not able to compare it with any nearby object. In short, when we look at the rising and the setting Sun, it is nothing but just an illusion. In fact, the Sun’s size always remains the same.
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