text: Before the 4th century BC , Greek astronomers believed the planet to be two separate objects : one visible only at sunrise , which they called Apollo ; the other visible only at sunset , which they called Hermes .
simplified: Before the 4th century BC , Greek astronomers thought that Mercury was two different objects : one able to be seen only at sunrise , which they called Apollo ; the other that was only able to be seen at sunset , which they called Hermes .