Astronomer Edmund Halley (1656–1742), who is usually thought of as the discoverer of the comet that bears his name, was also interested in cartography, oceanography, and the atmosphere. For instance, he created tidal charts and maps illustrating the path of ecliptic shadows. In 1868, he formed a theory to explain why we have the trade winds. Halley correctly guessed that it had to do with warm tropical air mixing with cooler air from more northern and southern latitudes. His idea, though, did not adequately explain why the winds blow from east to west, rather than south to north, as his theory would have indicated. It took English meteorologist George Hadley’s (1685–1768) discovery of convection cells to amend the theory correctly in 1735.