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In the previous section, we have discussed atmospheric refraction. In this section, we will see the advanced sunrise and delayed sunset.

Advanced sunrise and delayed sunset are events caused due to atmospheric refraction.


Advanced sunrise and delayed sunset:


Because of atmospheric refraction, the Sun is visible to all of us about \(2\ minutes\) before and after the actual sunrise and the actual sunset, respectively. 


By actual sunrise, we mean the actual crossing of the horizon by the Sun. The below figure displays the actual and apparent positions of the Sun with respect to the horizon. The time difference between the actual and the apparent sunset is about \(2\ minutes\). The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.


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Atmospheric refraction effects at sunrise and sunset


Advanced Sunrise:
The sun is rising above the horizon during sunrise. The rays from the sun refract as they move from denser (more dense) air to lighter air. Also, the human eye can see the sun rays as a straight line, which appears as the sun has risen. But it has actually not yet risen.


Delayed Sunset:
The sun, after sunset, is setting below the horizon. Now, the sun's apparent position is visible to us, not the actual position. This is because of the atmospheric refraction.