[Physics] Why does the Sun appear larger during the sunrise and sunset

Question

Why does the Sun appear larger during sunrise and sunset compared to its size during midday even though its distance from the Earth remains fixed all the day? It is only during the Winter when the Sun is away from the Earth and that too is due to the motion of Earth around the Sun, not due to the diurnal motion of the Earth. So is it some optical effect? If yes, what is it really? I apologize if the question has already been asked.

Answer 1

The previous answer is not wrong per se, but it only explains a distortion of bodies on the celestial sphere, not the effect you are describing. Edited: Refraction does not reduce the size of objects on the horizon, but it is not the reason for the perceived size.

What you are referring to is often called the Moon Illusion and there are multiple perceptive effects at play. What is important to understand is that our brain is only capable of deducing distance, that is, perceiving three dimensions, up two a certain and very limited range. So everything that happens on the celestial sphere has no depth for us at all: The brain does not have any information about celestial distances at all, and has to fill in the blanks. There are some really interesting phenomena resulting from this.

So the most plausible explanations are:

1. Apparent Distance: When viewing clouds at shallow angles on the sky, they appear noticeably smaller, compared to when viewed directly above, because they are, of course, farther away. The real, measurable, angular size is smaller. The moon or the sun on the other hand do not appear noticeably smaller, because their real distance does not change much when they move to the horizon. But the brain would expect them to be smaller, much as the clouds, or a plane, and hence perceives them as larger than they 'should be'.

2. Relative Size: Since the Sun is surrounded by nothing but sky when above, there is nothing to compare it to. When viewed at the horizon, there is actually things to compare the size with, which are all smaller, and hence the brain corrects for this.