Ancient Greeks believed Earth was spherical based on star observations. Aristotle argued Earth was spherical based on star movement patterns
Eratosthenes measured Earth's circumference using sun shadows at Alexandria. Earth's shape is oblate spheroid due to centrifugal effects. Geoid is Earth's shape best fitting global mean sea level
Geoid is Earth's model coinciding with mean sea level and spirit level. It approximates regular oblate spheroid shape perpendicular to gravity. Geoid is irregular due to buried mass concentrations and elevation differences. Geoid is equipotential surface with constant potential function
Earth's shape is approximately spherical due to gravity. Rotation causes centrifugal force, transforming sphere into ellipsoid. Earth's shape is slightly lumpy due to different material densities. Modern ellipsoid flattening is approximately 1/298.25
Earth's figure is used to model planet Earth's size and shape. Spherical Earth is a historical approximation suitable for many purposes. Earth's radius is approximately 6,371 km, deviating from spherical by 0.3%
Many ancient cultures believed in flat Earth, including Greece, Near East, India, and China. Ancient Near Eastern civilizations depicted Earth as flat disk floating in ocean. Ancient Chinese maintained flat Earth until 17th century, using egg analogy. Norse and Germanic peoples believed Earth surrounded by ocean with world-tree