Because Mercury is the closest planet to the sun, the sun's extreme gravitational pull swings it around very fast -- like a ball on a very short string. It travels in its orbit at more than 107,000 miles per hour, circling the sun every 88 days.
But Mercury turns slowly on its axis, so one of its days is equal to 59 of our Earth days. Its slow rotation rate combined with its nearness to the sun also means that Mercury is about 800 degrees hot on the side facing the sun and 300 degrees below zero on the side facing away.
Mercury can only be seen for 35 days a year because it is so close to the sun's glare. During those days, it appears either at sundown or sunup, like Venus.
The first recorded sighting of Mercury was in 265 BC by the Greeks, who thought it was two gods. (To the Greeks, what we know as planets were "known" to be gods.) When it appeared in the morning the Greeks thought it was Apollo. When it showed up in the evening it was called Hermes. Once they realized it was a single "god," they dropped Apollo. Since it was such a fast planet, they stuck with the name Hermes, the fleet-footed messenger of the gods.
Later, the Romans borrowed the Greek gods and gave them Roman names. The Roman name for Hermes is Mercury, and since the Romans ruled the world for so long, the name Mercury took hold.
Not much was known about the planet before Galileo invented the telescope in 1609. Shortly thereafter, in 1639, astronomers found that Mercury has phases just like our moon's half moon and crescent moon phases. In 1974-75 the Mariner 10 spacecraft took detailed pictures of Mercury that show it has a surface pitted with crater holes very much like the moon.
