Space-themed music is any music, from any genre or style, with lyrics or titles relating to outer space or space travel. Songs or other musical forms influenced by the concept of outer space have appeared in music throughout history, both in instrumental and vocal pieces with lyrics. As early as Ancient Greece, Pythagoras believed in something called the "harmony of the spheres". He believed that since planets and the stars all moved in the universe according to mathematical equations that these mathematical equations could be translated into musical notes and thus produce a symphony. This idea was explored further throughout Western history under the theories of Musica universalis. Some more recent and widely different examples are The Planets by Gustav Holst, and the song "Space Oddity"