Although we’ve been observing Mercury from Earth for thousands of years, its close proximity to the Sun - about 58 million kilometres on average - has made it difficult for astronomers to learn much about the planet. The Hubble Space Telescope is unable to observe the tiny planet because turning that close towards the Sun would damage the telescope’s instruments. Most of what we know came from the Mariner 10 space probe’s flybys in 1974 and 1975 and from NASA’s MESSENGER, which orbited Mercury between 2011 and 2015. Europe’s BepiColombo spacecraft will gather more data on the planet when it arrives in 2025.
With the naked eye Mercury can only be seen at dawn or dusk - depending on the time of year. Mercury can also be seen as a small black spot moving across the Sun at intervals of seven, 13 and 33 years. This is known as a transit of Mercury, and occurs when the planet passes directly between Earth and the Sun.
Mercury has the shortest year of any planet at 88 Earth days. It orbits around the Sun faster than any other planet, which is why it was named after the speedy Roman messenger god. Conversely, Mercury has the longest day of any planet due to its slow rotation. Because it revolves so quickly around the Sun, yet only rotates on its axis once every 59 Earth days, the time between sunrises on Mercury lasts 176 Earth days. Mercury is also the planet that has the most eccentric orbit. Like our Moon Mercury can be observed going through apparent changes in its shape and size called phases.