The voyage of the Greek navigator Pytheas of Massalia is a particularly notable example of a very long, early voyage. A competent astronomer and geographer, Pytheas ventured from Greece through the strait of Gibraltar to Western Europe and the British Isles. Pytheas is the first known person to describe the Midnight Sun, polar ice, Germanic tribes and possibly Stonehenge. Pytheas also introduced the idea of distant "Thule" to the geographic imagination and his account is the earliest to state that the moon is the cause of the tides.