The strangest calendar
A Day on Mercury Lasts Longer Than Its Year
Mercury goes round the Sun twice for every three times it turns. The result is a very odd sort of day.
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Step 1 of 5
A day and a year are two different journeys.
On Earth the two never get tangled. On Mercury they do. A year is one lap around the Sun. A day is one sunrise to the next. On Earth we lap the Sun once for every 365 spins, so the two feel completely unrelated.
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Step 2 of 5
Mercury's lap is very short and its spin is very slow.
Being closest in, it races around the Sun in 88 days. But it turns on its axis only three times for every two laps — so the two journeys end up entangled.
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Step 3 of 5
So the Sun crawls across its sky.
Because the planet is swinging around the Sun while it turns, sunrise to sunrise takes 176 Earth days — two full Mercury years. The day is literally longer than the year.
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Step 4 of 5
With no air, nothing carries the heat around.
430°C in the sunlight, −180°C in the shade, a few metres apart. Mercury has essentially no atmosphere. Nothing moves warmth from the lit side to the dark side, so the difference between them is more extreme than anywhere else with solid ground.
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Step 5 of 5
Which is why there is ice on the planet nearest the Sun.
Some craters at the poles have floors that sunlight never reaches. With no air to warm them, they stay permanently frozen — and radar has found water ice sitting in them.
It has no moons, no rings and almost no air, and until 2011 no spacecraft had ever orbited it.
The short version
Mercury turns three times for every two laps of the Sun, so one day lasts two of its years — and with no air to spread heat, it holds ice at its poles.
Try it yourself
Walk slowly around a lamp while turning on the spot three times for every two laps. Watch how long it takes the lamp to come back round in front of you.