Related Program: Our Island Universe Our Island Universe: Why Are There 7 Days in a Week? By Greg Jenkins • Oct 21, 2019 Related Program: Our Island Universe ShareTweetEmail Credit www.nationalgeographic.org A year is how long it takes the Earth to orbit the Sun. A day is how long it takes the Earth to spin once on its axis. Why are there 7 days in a week?! #OurIslandUniverse Listen Listening... / 2:40 ShareTweetEmailView the discussion thread.