Mercury has been known since at least the time of the Sumerians (3rd millennium BC), who called it Ubu-idim-gud-ud. The earliest recorded detailed observations were made by the Babylonians, who called it gu-ad or gu-utu. It was given two names by the ancient Greeks, Apollo when visible in the morning sky and Hermes when visible in the evening, but Greek astronomers came to understand that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbited the Sun, not the Earth.