To know Earth is to love Earth. But efforts to secure our planet’s future require a knowledge of it, past and present. Only when we understand our wondrous world can we truly engage in the battle to save it. In honor of Earth Day we look at some of the very best titles in the App Store for learning about our home. From the sky, through a camera lens, through animals, nature, art, history and fun, we bring you the best apps to celebrate our planet and understand our place in it, on Earth Day and all year long.