In the 19th century, the discrepancy between the age of Earth and the age of the cosmos posed a great problem to scientists. Geologists had calculated, using methods like erosion or sedimentation rates, ages for Earth spanning from three million to fifteen billion years. Physicists and astronomers, based mostly on the energy output of stars, calculated an age for the universe spanning from twenty million to ten billion years - so in many models of the cosmos, Earth seemed to be too young or too old to fit in. In August 1893, during a meeting of the American Association for the Advancement of Science, geologist Charles D. Walcott (1850-1927) summarized the debate as follows:
"Of all subjects of speculative geology, few are more attractive or more uncertain in positive results than geological time. The physicists have drawn the lines closer and closer until the geologist is told that he must bring his estimates of the age of the earth within a limit of from ten to thirty millions of years. The geologist masses his observations and replies that more time is required, and suggests to the physicist that there may be an error somewhere in his data or the method of his treatment." In 1896 the French physicist Henri Becquerel (1852-1908), based on Conrad Röntgen's (1845-1923) research, discovered that naturally occurring elements, like uranium, also emit X-rays and in 1897 Polish physicist Marie Curie (1867-1934) coined the term radioactivity to describe this energy of unknown origin. Her husband, Pierre Curie (1859-1906), realized that this energy from radioactive decay must be considered when calculating the age of Earth. Physicists supporting a young Earth based their calculations on a quickly cooling Earth. However, radioactive decay in Earth's interior provided a continuous source of energy and heat, therefore Earth was cooling slowly and so could be quite old.
Radioactive decay or another similar long-lasting and high-energy source (nuclear fusion was discovered later) could also explain how stars could produce light and heat for very long periods of time. The notion that stars or the sun had to be young (in most calculations younger than Earth) could also be dismissed.
But even better - the discovery of radioactivity provided not only indirect evidence of an old Earth but by measuring the constant decay it was also possible to calculate the exact age of a mineral, a rock and even of Earth.
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In 1896 the French physicist Henri Becquerel (1852-1908), based on Conrad Röntgen's (1845-1923) research, discovered that naturally occurring elements, like uranium, also emit X-rays and in 1897 Polish physicist Marie Curie (1867-1934) coined the term radioactivity to describe this energy of unknown origin. Her husband, Pierre Curie (1859-1906), realized that this energy from radioactive decay must be considered when calculating the age of Earth. Physicists supporting a young Earth based their calculations on a quickly cooling Earth. However, radioactive decay in Earth's interior provided a continuous source of energy and heat, therefore Earth was cooling slowly and so could be quite old. 
In 1896 the French physicist Henri Becquerel (1852-1908), based on Conrad Röntgen's (1845-1923) research, discovered that naturally occurring elements, like uranium, also emit X-rays and in 1897 Polish physicist Marie Curie (1867-1934) coined the term radioactivity to describe this energy of unknown origin. Her husband, Pierre Curie (1859-1906), realized that this energy from radioactive decay must be considered when calculating the age of Earth. Physicists supporting a young Earth based their calculations on a quickly cooling Earth. However, radioactive decay in Earth's interior provided a continuous source of energy and heat, therefore Earth was cooling slowly and so could be quite old. 