![water creatures that change age mythology water creatures that change age mythology](https://ychef.files.bbci.co.uk/976x549/p03yx5ck.jpg)
The answer he gave was remarkably accurate. Sacrobosco went on to explain how you could calculate the size of the earth, simply by measuring the height of the sun above the horizon in two different cities. A beautiful diagram in many manuscripts illustrates how this shows that the seas must be round. As a ship sails away from harbor, noted Sacrobosco, a lookout at the top of the mast will still be able to see land long after the sailors on deck have lost sight of it. It was written in about 1230 by Johannes de Sacrobosco-John of Hollywood. They recorded the proofs in textbooks, handwritten on smooth animal-skin parchment.
![water creatures that change age mythology water creatures that change age mythology](https://www.nhm.ac.uk/content/dam/nhmwww/discover/sea-monsters/carta-marina-sea-monsters-full-width.jpg)
Get your history fix in one place: sign up for the weekly TIME History newsletter Of course we mustn’t generalize about what “people” thought-there was as much disagreement and debate as there is today-but anyone who studied the question realized immediately that the earth was round. For example, it’s still commonly assumed that medieval people thought the earth was flat until explorers like Columbus proved them wrong. Historians have shown it to be a period of impressive innovation and ingenuity. Recent research has, however, exploded almost every myth about the scientific stagnation of the Middle Ages.