A day is the length of time it takes a planet to rotate once on its axis.
A year is the length of time it takes a planet to orbit a star.
On Earth, we're so used to the fact that a year is a lot longer than a day here, we are apt to sometimes forget that there's no reason why this has to be the case.
On Mercury, a day is 176 Earth days long. Even more strangely, a Mercury day is 2 Mercury years. In other words, a year is longer than a day there. If you'd grown up on Mercury, your default concept of time ordering would be minute, hour, year, day.
Huh.
Via the always interesting Astronomy Picture of the Day.
Update: The Dogg makes the excellent point that the interaction of day and year would mean than sunset and sunrise would be a really weird combination of the day and year effects. As would the path of the sun in the sky, for that matter. I demand that NASA commission a simulation and post it in a youtube clip to satisfy my idle curiosity!
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