How long is a Day on Mercury?
One day on Mercury is about 176 Earth days long. This is called a a "Mercurian day".
One year on Mercury is actually shorter—about 88 Earth days. This means that a day on Mercury day is twice as long as a Mercurian year.
Mercury's day is unique in the Solar System, in that three days is exactly equal to two years. This is due to the planet being in what is technically called a "tidally locked 3:2 spin–orbit resonance". In other words, Mercury rotates on its axis three times for every two time it orbits the Sun.
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Author: Dave Owen