An Etymological Dictionary of Astronomy and Astrophysics
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The belts of latitude over the oceans, located around 30° north and south of the equator, characterized by predominantly calm or light winds and hot and dry weather.

→ horse; → latitude.
The origin of the term horse latitudes is not clear, despite numerous speculations. A likely explanation appears in Spanish in a natural history text (Historia General y Natural de las Indias by Lopez de Gomara) published in 1535. Therefore, the term derives from El Golfo de las Yeguas, which translates to "The Mares' Sea." The sailors called it this because in the 1500's there was active shipping of horses, particularly brood mares, from Spain to the Canary Islands, and many of the horses died during the transit of this area.