Earth’s atmosphere scatters more short-wavelength (blue) light than longer-wavelength (red) light, so the thicker the atmosphere the more blue light is removed. As the Earth rotates throughout the day the amount of atmosphere between you and the sun changes. At sunrise and sunset, light passes through more atmosphere to reach your eye than at midday leaving the sky red in the morning and evening and blue during the day.

Click below to see how the amount of atmosphere between you and the sun changes depending on the time of day.