Mercury can be seen with the naked eye. Like Venus, it sometimes appears in the morning before the Sun rises, and sometimes in the evening, after the Sun sets. For this reason, Mercury was thought to be two stars. Eventually, it was observed that when the evening star was visible, the morning star never appeared, and vice versa. With this information came the realization the two stars were one.
1610 - Galileo Galilei makes the first telescopic observation of Mercury.
