 Scientific investigation has resulted in several culturally transformative shifts in our view of the planet . In the West , belief in a flat Earth was displaced by the idea of spherical Earth , credited to Pythagoras in the 6th century BC . Earth was further believed to be the center of the universe until the 16th century , when scientists first theorized that it was a moving object , comparable to the other planets in the Solar System . Due to the efforts of influential Christian scholars and clerics such as James Ussher , who sought to determine the age of Earth through analysis of genealogies in Scripture , Westerners prior to the 19th century generally believed Earth to be a few thousand years old at most . It was only during the 19th century that geologists realized Earth 's age was at least many millions of years . Lord Kelvin used thermodynamics to estimate the age of Earth to be between 20 million and 400 million years in 1864 , sparking a vigorous debate on the subject ; it was only when radioactivity and radioactive dating were discovered in the late 19th and early 20th centuries that a reliable mechanism for determining Earth 's age was established , proving the planet to be billions of years old . The perception of Earth shifted again in the 20th century when humans first viewed it from orbit , and especially with photographs of Earth returned by the Apollo program . 
