Because Mercury is one of the five visible planets to the naked eye, its original discovery is indeterminable. It was known during prehistoric times and was mentioned in the second millennium B.C. by Sumerians.
The five planets that were visible to Greeks and Romans were Jupiter, Mars, Mercury, Saturn and Venus. The movement of these planets across the sky became part of mythology and ancient astronomy. Galileo studied Mercury in the 17th century with an early model of the telescope.
Scientists are still making discoveries about Mercury as of 2014. In the 1960s, astronomers concluded after years of debate that Mercury did in fact rotate. The rotation measures 59 days for a complete revolution. In 1974, the Mariner 10 was sent to perform flybys for imaging. The mission came back incomplete and was finally finished in 2008 by a NASA spacecraft.
A transit occurs when Mercury passes directly in front of the sun. This rare event draws worldwide attention and was broadcast through the Internet in 2004. This highly visible planet has been part of humanity's dialogue and continues to merit study. Scientists study ways in which the atmosphere of Mercury is able to help regulate temperature given the extreme proximity of Mercury to the Sun.