Horizons from On High and Low
April 19, 2007
Have you ever thought about the horizon? Here are two different views of it; one from the beach in Topanga, California, the other over the Pacific Ocean at an altitude of 35,000 ft (10,668 m). The horizon at the left is only 4 miles (6.4 km) away, while the one on the right is more than 230 miles (368 km) away. Both are very sharp although the horizon contrast on the right is less than 10% of the contrast on the left. Most remarkable of all, there's a contrast reversal between the two pictures; from sea level the sky is bright and the ocean is dark, yet from high elevation the sky is dark and the region below the horizon is bright. This happens because from sea level, most of the atmosphere is above us and scatters plenty of sunlight. But from high in the sky, very little of the atmosphere is above us and thus little sunlight is scattered. It's impossible to see the hard-Earth horizon from above an altitude of a couple of miles because of extinction by air molecules and aerosols. The high altitude “horizon” looks like a hard-Earth horizon but is really just a rapid vertical change in brightness, an indicator of how rapidly the number density of air molecules decreases with height.