Planet Mercury is the first planet in the solar system. Considering that the path of Mercury is elliptical, it’s distance from the Sun varies from 28 million to 43 million miles. Record of observation of Mercury exist from ancient civilization such as China and Mesopotamia.
Several decades lasted theory that Mercury does not rotate on it’s axis, but only around the Sun. Therefore, astronomers thought that one side of planet is almost melted, while the other side has a temperature over -200 degrees. Developing the large telescopes, this myth was busted. According to some astronomers, the Mercury wasn’t planet, however it was satellite of Venus. It is interesting to mention that observation of Mercury lasts only 35 days in a year. The reason is that the planet is near the sun and its’s observation through the telescope would ruin the lens and electronics. As you can see in the picture below, Mercury is very similar to the Moon.For the end, I would say that Mercury has a high density, so there is question whether the theory of formation of Solar system is adequate.
NASA’s Messenger mission to Mercury