The Milky Way and the setting moon over the Grand Canyon and the Desert View Watchtower. The Grand Canyon in northern Arizona is the longest canyon on the Earth, a steep-sided gorge carved by the Colorado River steadily in about 17 million year time span. The canyon is 446 km long, ranges in width from about 6 to 29 km and attains a depth of more than 1.6 km (a mile). Date: Nov. 2012