Mercury is one of the five planets visible to the naked eye. Because it is so close to the Sun however, it is only visible for about one hour after sunset or for the same length of time prior to sunrise. Even though it can become quite bright, it never looks as impressive as some of the stars blazing forth in a darker sky later at night. It is also difficult to observe since it lies so close to the horizon; any object can block it from view. For that reason, most observers, including Nicholas Copernicus, have never seen it. In addition, it is difficult to observe through a telescope.

The first observations after the invention of the telescope were mainly attempts to see if the planet showed phases like the moon, as Copernicus predicted. Johannes Hevelius (1611-1687) saw the phases in 1644, but it was not...