Spherical Earth

Spherical Earth is the understanding that Earth is approximately spherical, more precisely an oblate spheroid. Its shape is supported by observations of the horizon, celestial bodies, and measurements of Earth's circumference and gravity.

Horizon: The apparent boundary where the sky meets the land or sea, shaped by an observer’s position and Earth’s curvature. Ships disappearing hull-first provide a visible clue to curvature.
Eratosthenes' experiment: A method for estimating Earth’s circumference from the Sun’s shadow angles at two separated locations. It is a clear early example of calculating global size from local observations.
Pythagoras: An ancient Greek philosopher and mathematician traditionally associated with early Greek arguments for a spherical Earth. Later accounts connect his school with the idea, though the evidence is uncertain.
Flat Earth: The belief that Earth is flat rather than spherical. It is the direct alternative contradicted by observations of curvature and global geometry.
Lunar eclipse: An event in which Earth passes between the Sun and Moon, casting its shadow on the Moon. Earth’s consistently curved shadow on the Moon supports a spherical shape.
Meridian arc: The measured distance along a north–south line between two points with known latitude difference. Comparing arc length with angular change tests models of Earth’s curvature.
Aristotle: An ancient Greek philosopher who presented observational arguments for Earth’s spherical shape. His evidence from lunar eclipses and changing stars made the case explicit.
Geoid: An equipotential surface of Earth’s gravity field that approximates mean sea level. It differs from a smooth sphere by accounting for uneven gravity and mass distribution.
Latitude: An angular coordinate measuring a location north or south of the equator. Changing latitude changes which stars are visible and how high they appear.
Foucault pendulum: A freely swinging pendulum whose apparent rotation reveals Earth’s rotation beneath it. Its motion demonstrates Earth’s rotation, one consequence of a rotating globe.

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