A scale estimate from local observations
The traditional Eratosthenes account compares noon solar angles and the separation of two places. Converting ancient stadia into a modern kilometre figure involves uncertainty about the unit and reported baseline; distinguish that historical estimate from a newly measured experiment.
A sphere becomes an ellipsoid
Measuring meridian arcs at different latitudes revealed that a single spherical radius was insufficient for precise surveying. An oblate reference ellipsoid captures the systematic equatorial and polar difference. The geoid describes the additional gravity-related shape.
The Columbus story does not establish a medieval flat consensus
The historical dispute over an Atlantic crossing included the size of Earth and the distance to Asia. A claim that educated Europeans universally expected a flat edge confuses those disagreements with a different proposition. Follow specific texts and measurements rather than treating one popular story as a whole era.
The modern flat-Earth movement in its own archive
Samuel Shenton’s correspondence uses a letterhead for the International Flat Earth Research Society, dated to its 1956 founding and based in Dover, England. A 23 February 1967 letter discusses preparing posters for a science fair and appearances on television and radio. This is evidence of an organised modern campaign and its communication methods, rather than evidence about Earth’s geometry.
The society’s library preserves Shenton’s letters alongside Charles K. Johnson correspondence from 1971 and later newsletters. These named records let a reader distinguish the modern movement from earlier writings such as William Carpenter’s 1885 pamphlet and Samuel Rowbotham’s nineteenth-century arguments. Assess a proposed measurement separately from the group that circulated it.