Light waves are not always moving in a straight line. When it passes through a medium of a different refractive index, the waves will deviate. The phenomenon is called refraction and described according to Snell’s Law.
Earth’s atmosphere has variation in air density that depends on the altitude. As the refractive index changes with the density of the medium, light waves passing through Earth’s atmosphere also experience refraction.
Refraction causes the following natural phenomena:
- The Sun appears elliptical near the horizon.
- The Sun is still visible when physically, it should already set.
- The Moon appears reddish during a total lunar eclipse.
- The Moon appears yellowish when it is near the horizon.
- Objects could still be visible even it is physically already behind the curvature of the Earth.
- The stars twinkle.
- The stars move slightly slower when they are closer to the horizon.
The atmospheric refraction explains all the above phenomenon consistently and elegantly.
Flat-Earthers think the atmospheric refraction is not well-explained, and abuse it to explain away the fact that sunrises and sunsets happen every day. Their claim does not correspond to real-world observations. In the real world, refraction bends light to the opposite direction: during sunset, it makes the sun still visible for some time, when it is physically already below the horizon.