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Because of atmospheric refraction, the Sun is visible to us:

Because of atmospheric refraction, the Sun can be visible to us a little bit before it actually rises and a little bit after it has already set below the horizon. This is because the Earth's atmosphere bends and scatters some of the Sun's light, causing it to curve around the Earth's curvature and reach our eyes even when the Sun is still slightly below the horizon. 

This effect is more pronounced at lower altitudes, where the atmosphere is denser, and it is also more prominent during sunrise and sunset when the Sun's light has to travel through more of the Earth's atmosphere to reach us.

The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.