Mercury rotation characteristic - notice the ellipitical orbit

Mercury rotates slowly.  One rotation takes nearly 59 Earth days to complete.  However due to an orbital-rotational resonance ratio of 3:2, a fictitious observer on Mercury would see that a solar day from noon to noon would take about 176 Earth days to complete.  (This is assuming an observer is not at one of the poles.  Mercury does not tilt on its axis, therefore the Sun would constantly be situated on the horizon at the poles).  The image above shows how Mercury completes three rotations around the Sun for every two orbits.  In this example, our fictitious observer is shown as the small circle "attached" to Mercury standing on or near the equator. 

Starting at the top of the image in location number 1 at perihelion (28.6 million miles from the Sun), we can consider this "noon" for the observer.  Our "observer" is denoted by the small dots; black for the first rotation, green for the second, and purple for the third rotation.  Two-thirds of the way around the orbit, location 5 shows the first rotation complete and nightfall beginning.  It took 58.6 Earth days to get from position 1 to 5, with the Sun being visible to our observer right up until just before position number 5.  After baking in temperatures near 800 degrees F., our observer would probably like to cool off.  He/she gets their wish as a long night settles in.  Location 7 shows the time of day is now midnight, and Mercury has completed one orbit around the Sun.  Position 10 shows the second rotation complete, and our observer is likely starting to see some "morning" twilight just before sunrise.  It has now taken 117.2 Earth days to get from our observers noontime placement on position 1, to just before sunrise at number 10.  Our person of interest has now just endured a long night of temperatures falling to between -300 and -350 degrees F.  After another 58.6 Earth days, location 15 shows the third rotation of Mercury completed and the planet having just revolved around the Sun twice.  Notice how we are finally back to our noontime position where we started, directly at perihelion.  175.8 Earth days has elapsed for our observer.

You may have noticed something about Mercury's revolution around the Sun.  Observe how slow Mercury's precession is for our observer around the Sun close to perihelion.   It is much quicker around aphelion (position 4) which is 43.4 million miles from the Sun.  Such is the case with a highly elliptical orbit and an orbital-rotational resonance ratio other than 1:1.  As stated on the main Mercury page, depending on latitude an observer would see the Sun do some very strange things.


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