Atmospheric duct by the ionosphere. The reduced refractive index due to lower densities at the higher altitudes in the Earth's atmosphere bends the signals back toward the Earth. Signals in a higher refractive index layer, "i.e.," duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material. In some weather conditions, such as inversion layers, density changes so rapidly that waves are guided around the curvature of the earth at constant altitude. Phenomena of atmospheric optics related to atmospheric ducting include the green flash, Fata Morgana, superior mirage, mock mirage
Age of the Earth His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality"). In the 1790s, William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age. William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old. In the mid-18th century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe. Lomonosov's ideas