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Atmospheric Refraction

Twinkling of Stars
Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of moving air in the earth’s atmosphere. Stars (except for the Sun) appear as tiny dots in the sky; as their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions. This random refraction results in the star twinkling at us. It looks as though the star moves a bit, and our eye interprets this as twinkling.
Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and this is subject to more refraction. Also, planets do not usually twinkle, because they are so close to us; they appear big enough that the twinkling is not noticeable.
Stars would not appear to twinkle if we viewed them from outer space or from a planet/moon that didn't have an atmosphere. 
Advance Sunrise And Delayed Sunset
Sunrise is defined as the moment that the Sun first appears over the horizon. So, by definition, we can't see the Sun before it appears, but 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 is 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.

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