Herodotus counted layers of earth near the Nile River in 450 B.C.E. The Bible was used in the Middle Ages to compute the age of the Earth. The 18th and 19th centuries brought a more scientific look into determining the age of the Earth by studying the salinities of the oceans, the rates of sedimentation, and models of cooling of different materials in order to calculate the Earth's age. In 1862, William Thompson calculated the Earth's age to be 98 million years and recounted that age to determine that it was actually between 20 and 40 million years old. He used a very thorough method and ended up being wrong on both counts because he was unaware of the existence and effects of radiation. There was another method of aging the Earth utilizing the relative positions of rock layers. Ensuring that unconformities were accounted for, that fossils were used as benchmarks, and understanding what rocks are older than others, this relative aging helped to develop a geologic time scale. The discovery of radioactivity allowed scientists to understand heat better. The radioactive decay of elements helps scientists calculate the age of an object by using the statistics of large numbers.
2. What makes Earth a habitable, relatively stable environment within which we exist and survive? Review the early development of the solar system, including the Big Bang theory, to support your answer.