Flat Earth theory
Earth's roughly spherical shape has been established since antiquity and is confirmed by satellite imagery, circumnavigation, gravity measurements, and direct observation of Earth's curvature and shadow during lunar eclipses.
What we know
The scientific case for Earth's spherical shape, technically an oblate spheroid slightly flattened at the poles due to rotation, rests on multiple independent lines of evidence developed across more than two thousand years. The Greek scholar Eratosthenes calculated Earth's circumference around 240 BCE using shadow lengths measured at two different latitudes in Egypt, arriving at a figure within about 5 to 15 percent of the modern measured value using nothing more than geometry and two measuring sticks, evidence that a spherical Earth was demonstrable using observational methods available well before modern technology.
Modern satellite imagery, produced by numerous independent government space agencies, private companies, and even amateur high-altitude balloon projects that any individual can replicate with a consumer camera and weather balloon, consistently shows Earth as a sphere from every vantage point in orbit. More than 24 crewed missions have traveled far enough from Earth to observe its curvature directly with the naked eye, including the Apollo astronauts, and thousands of professional and amateur astronauts and pilots have independently reported the same visual observation. Commercial airline pilots and high-altitude U-2 reconnaissance pilots also routinely observe Earth's curvature at cruising altitudes using the horizon line, an observation replicable by anyone with access to sufficiently clear high-altitude conditions and a wide-enough field of view.
Circumnavigation, the act of traveling continuously in one direction and returning to the starting point, has been performed by ship since Magellan's expedition in the 16th century and by aircraft numerous times since, a feat only possible on a round or otherwise curved and closed surface, not a flat plane. GPS satellite systems, which underlie modern navigation, shipping, and aviation industries worth trillions of dollars globally, rely on precise orbital mechanics calculations that assume a spherical, gravity-generating Earth; these systems function correctly on a daily basis, which would not be possible if the underlying physical model were wrong.
During lunar eclipses, Earth's shadow cast on the Moon is always circular regardless of the time of year or Earth's orientation relative to the Moon, which is only geometrically possible if Earth is a sphere, since a disc-shaped or flat object would cast an elliptical or otherwise variable shadow shape depending on its orientation. Flat Earth claims typically explain away this and other evidence with additional ad hoc claims, such as that space agencies worldwide are coordinating a hoax, or that gravity does not exist and objects are pushed down by acceleration instead, explanations that require an implausibly large and perfectly coordinated conspiracy spanning thousands of scientists, pilots, astronauts, and amateur experimenters across every country on Earth, with no verified defector or leaked evidence ever surfacing in more than a century of spaceflight and aviation history. Ships disappearing hull-first over the horizon before their masts vanish from view is another everyday, repeatable observation consistent only with a curved surface, and can be reversed by viewing the same ship through a sufficiently powerful telescope, which brings the hull back into view by compensating for the curvature-induced obstruction rather than by revealing some trick of distance alone.
Common claims
- Water always finds its level, so oceans must be flatFalse, water conforms to Earth's gravitational field, which produces a curved surface at large scale
- You can't feel Earth spinning, so it must not be rotatingFalse, Earth's rotation is smooth and constant, producing no perceptible acceleration at the surface
- NASA photos of a round Earth are fakedNot supported, images are independently corroborated by other countries' space agencies and private and amateur projects