Question

Why during sunset and sunrise the sun is seen even when it is slightly below the horizon?

Answer 1

Technically, we can't. Sunrise is defined as the moment that the Sun first appears over the horizon. So, by definition, you can't see the Sun before it appears. But you're right that we can see the Sun even when it is *geometrically* just below the horizon, at both sunrise and sunset. This is because of the refraction of the light from the Sun by the Earth's atmosphere--the Earth's atmosphere bends the path of the light so that we see the Sun in a position slightly different from where it really is. The magnitude of this effect varies with latitude, but it's strongest at the equator, where the Sun rises 2 minutes earlier than it would if the Earth had no atmosphere, and sets 2 minutes after it would if the Earth had no atmosphere. 

Answer 2

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1. The earth is surrounded by an atmosphere which is denser near the surface of the earth.

2. When the rays of light from the Sun enter the earth's atmosphere from outer space, they travel from a rarer medium to a denser medium.

3. Hence, they bend towards the normal on refraction.

4. Hence, even when th sun is below the horizon while rising or setting, its rays reach is due to refraction and it appears to be on the horizon.

Therefore, the sun is seen on the horizon a little before sunrise as well as for some time even after sunset.

Hope it helps!

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