Spherical Earth

Lore & Background

The spherical shape of the Earth was known and measured by astronomers, mathematicians, and navigators from a variety of literate ancient cultures, including the Hellenic World and Ancient India. The knowledge of the Greeks was inherited by Ancient Rome, and Christian and Islamic realms in the Middle Ages. Circumnavigation of the world in the Age of Discovery provided direct evidence. Improvements in transportation and other technologies refined estimations of the size of the Earth, and helped spread knowledge of it.

Reader's Guide

The concept of a spherical Earth displaced earlier beliefs in a flat Earth, such as the Mesopotamian portrayal of the world as a disk floating in the ocean with a hemispherical sky-dome above. The realization that the figure of the Earth is more accurately described as an ellipsoid dates to the 17th century, as described by Isaac Newton in Principia. In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 (Delambre, Everest). The modern value as determined by the US DoD World Geodetic System since the 1960s is close to 1/298.25. The scientific study of the shape of the Earth is known as geodesy. Earth is massive enough that the pull of gravity maintains its roughly spherical shape; most deviation from spherical stems from centrifugal force caused by rotation, deforming the sphere into an oblate ellipsoid.

Did You Know?

• The earliest documented mention of a spherical Earth dates from around the 5th century BC in Greek writings.
• Ferdinand Magellan and Juan Sebastián Elcano's circumnavigation (1519–1522) provided a practical demonstration of Earth's sphericity.
• The modern flattening value of the Earth ellipsoid, determined by the WGS84 system established in 1984, is 1/298.257223563.
• Earth's shape is slightly lumpy because it is composed of different materials of different densities that exert slightly different amounts of gravitational force per volume.