ATLAS

Official versionOMEGA-3-18-3Sheet 3 of 5 of the dossier Flat Earth

The round Earth: what measurement and observation show

From Greek astronomy to satellite images: the evidence that the Earth is a sphere.

A sequence of photographs of a tanker at sea, in which the ship gradually rises over the horizon line.
A tanker rising over the horizon at the Strait of Magellan, Chile, in a sequence of photographs. Dropus · CC BY-SA · Wikimedia Commons

Summary

The roundness of the Earth is not a modern discovery. The concept is documented from the fifth century BC in the writings of Greek philosophers, and in the third century BC Hellenistic astronomy established the roughly spherical shape of the Earth as a physical fact. Eratosthenes, later chief librarian at Alexandria, is best remembered as the first known person to calculate its circumference: comparing the shadows cast at noon on the summer solstice at Alexandria and at Syene, he measured an angle of about 7.2 degrees, one fiftieth of a circle. His method was sound, though two false assumptions fortuitously cancelled each other out.

Other lines of evidence accumulated. Since at least the time of Aristotle, people have noted that the Earth's shadow on the Moon during a partial lunar eclipse is circular. Ferdinand Magellan and Juan Sebastián Elcano's expedition circumnavigated the globe between 1519 and 1522. A sequence of photographs taken at the Strait of Magellan shows a tanker rising over the horizon, and the encyclopaedic list of effects that together disprove a flat Earth also includes gravity and weather systems. In July 2015, NASA released the first view of the entire sunlit side of the Earth from the DSCOVR satellite, a million miles away.

Verified facts

  1. Verified fact

    Ferdinand Magellan and Juan Sebastián Elcano's expedition achieved the first practical demonstration of the Earth's sphericity by circumnavigating the globe between 1519 and 1522.

    • Spherical Earth, Wikipedia
      A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's circumnavigation (1519–1522).
  2. Verified fact

    A sequence of photographs at the Strait of Magellan shows a 186 m tanker rising over the horizon; at about 18 km the theoretical horizon drop would be about 23 m.

    • Spherical Earth, Wikipedia
      Distance at first picture is about 18 km which would imply a theoretical horizon vertical drop of about 23 meters.
  3. Verified fact

    Many independent observations show the roughly spherical shape of the Earth, among them lunar eclipses, the visibility of distant objects, the sky seen from different locations and altitudes, surface navigation, weather systems and gravity.

    • Spherical Earth, Wikipedia
      The spherical shape causes a number of effects and phenomena that when combined disprove flat Earth beliefs.
A series of Moon images in which a dark circular shadow slowly covers part of the lunar disc.
Composite of the partial lunar eclipse of 16 July 2019, showing the curved edge of the Earth's shadow on the Moon. H. Raab (User:Vesta) · CC BY-SA · Wikimedia Commons

Official version

  1. Official version

    The sphericity of the Earth is documented from the fifth century BC in the writings of Greek philosophers, and in the third century BC Hellenistic astronomy established it as a physical fact and calculated the Earth's circumference.

    • Spherical Earth, Wikipedia
      In the 3rd century BC, Hellenistic astronomy established the roughly spherical shape of Earth as a physical fact and calculated the Earth's circumference.
  2. Official version

    Eratosthenes, chief librarian at the Library of Alexandria, is best remembered as the first known person to calculate the Earth's circumference.

    • Eratosthenes, Wikipedia
      He is best remembered as the first known person to calculate the Earth's circumference.
  3. Official version

    Eratosthenes compared noon shadows at Alexandria and Syene on the summer solstice, found an angle of about 7.2 degrees, or 1/50 of a circle, and multiplied the distance between the cities by 50; his method was sound, though two false assumptions cancelled each other out.

    • Eratosthenes, Wikipedia
      While Eratosthenes' method was sound he made two false assumptions which fortuitously cancelled each other out.
  4. Official version

    Since at least the time of Aristotle, people have noted that the Earth's dark shadow on the Moon during a partial lunar eclipse is circular, although never a whole circle.

  5. Official version

    In July 2015 NASA's Earth Observatory reported that the DSCOVR satellite, a million miles from Earth, had returned its first view of the entire sunlit side of the planet; other satellites had captured full-disc views before.

    • An EPIC New View of Earth, NASA Earth Observatory, July 2015
      From one million miles away, the DSCOVR satellite returned its first view of the entire sunlit side of Earth.
Play on YouTube (third-party content)
"How Do We Know the Earth Isn't Flat? We Asked a NASA Expert", a video from NASA's own channel.
NASA
Play on YouTube (third-party content)
"24 hours of Earth, as seen by DSCOVR's EPIC instrument", from the NASA Earth channel.
NASA Earth