Like our Moon, Mercury can be observed going through apparent changes in its shape and size called phases. Mercury also has the most eccentric, or stretched-out, elliptical orbit.
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. Conversely, Mercury has the longest day of any planet due to its slow rotation. It also orbits around the Sun faster than any other planet, which is why it was named after the speedy Roman messenger god. Mercury has the shortest year of any planet at 88 Earth days. This is known as a transit of Mercury across the Sun and occurs when the planet comes between the Earth and the Sun. Mercury can also be seen as a small black spot moving across the Sun at intervals of seven, 13 and 33 years. With the naked eye, Mercury can only be seen at dawn or dusk, depending on the time of year (unless there is a solar eclipse). Most of what we know came from the 1975 Mariner 10 space probe’s fly-by. The Hubble Space Telescope cannot observe it, because turning that close towards the Sun would damage the telescope’s instruments. 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.
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