The rotation of Mercury is very strange. For hundreds of years, it was thought to rotate synchronously, so that it always kept one face to the Sun, just as the Moon always keeps one face to the Earth. Its actual rotation, however, causes it to turn exactly one and a half times each time it goes around the Sun, so that it turns one side toward the Sun in one orbit, and the other side toward the Sun in the next orbit, making the day on Mercury twice as long as the year. In addition, at perihelion, the motion of the planet around the Sun is faster than its rotation, so that the Sun actually seems to stop its normal (westward) motion, and move the other way for a little while!
How The Sun Moves In Mercury's Sky
Mercury rotates once every 58.647 days, which is exactly 2/3 of its orbital period of 87.970 days, so it turns on its axis exactly 1 1/2 times during one of its years, causing the stars to move 1 1/2 times around the sky, each year. During that time, it moves once around the Sun, causing the Sun to appear to move backwards relative to the stars, one full turn. As a result, it has only half a day during one year (1 1/2 forward rotations by the stars, less 1 backward motion by the Sun), so that it takes two years (175.942 Earth days) for one day, three times as long as its rotation period. This seems very strange to us, but it is actually normal for a planet with a slow rotation to have a rotation period very different from its day length (refer to Rotation Period and Day Length for a detailed discussion of how rotation periods are related to day length).
The reason that Mercury rotates this way is related to its orbital motion. On the average, Mercury moves four degrees around the Sun each (Earth) day, while rotating six degrees. The westward motion of the stars, due to its rotation, is opposite the eastward motion of the Sun, caused by its orbital motion, so the Sun's net westward motion is only two degrees per (Earth) day, which is three times slower than the star's six degrees per (Earth) day; hence, the day ought to be three times longer than the rotation, as it is.
If the orbit of Mercury was nearly circular, this would be all we'd need to say about its rotation; but since it is quite eccentric, things end up very differently at different times of the year.
At aphelion, Mercury is over 20% further from the Sun than normal, and as a result, moves about 17% slower than normal, in its orbit around the Sun; and, being 20% further away than usual, even if it were moving at the same speed as usual, it would LOOK like it was moving slower (that is, it would move at a slower angular velocity), as seen from the Sun. The net result is that the orbital motion of four degrees per day, on the average, slows to only 2/3 of that, or less than three degrees per day.
The ROTATIONAL speed of Mercury on its axis, however, does NOT change. Rotational motions can change over long periods of time, as a result of tidal forces, but such changes are very slow, and in Mercury's case, nonexistent, as its rotation is locked to its orbital period (as explained below). So, at aphelion, the westward motion of the stars is still six degrees per day, and the Sun's motion, which is that six degrees minus the orbital motion, is a little over three degrees per day to the west, or more than 50% faster than normal .
At perihelion, on the other hand, Mercury is over 20% closer to the Sun than normal, and as a result, moves about 25% faster than normal, in its orbit; and being 20% closer than usual, even if it were moving at the same speed as usual, it would look like it was moving faster (the exact opposite of the situation described above), at least in terms of its angular velocity around the Sun. These two factors result in an increase of the orbital motion to six degrees per day, or 3/2 of the normal value (note that this is the inverse of the slowing, of 2/3 of the normal value, at aphelion).
As at aphelion, the rotational velocity is unchanged, so the stars still move westward at six degrees per day, and subtracting the orbital motion, which makes the Sun move eastward at six degrees per day, we are left with -- NOTHING! In other words, at perihelion, Mercury's orbital motion around the Sun is so fast that the Sun completely stops its westward motion. In fact, if the calculations are done accurately, instead of the approximate way just described, it turns out that at perihelion, the orbital motion actually EXCEEDS the rotational motion, so that the Sun slows, stops moving to the west, and starts slowly moving to the east.
This eastward motion only lasts for a few days, but during that time, the Sun moves a little more than one diameter (which, since it is three times closer to Mercury than to the Earth, is about a degree and a half). Then, as the planet moves past perihelion, and moving away from the Sun, starts to experience a slowing of its orbital motion, the Sun stops its eastward movement, and starts slowly moving to the west again.
To summarize, during one of Mercury's years, the AVERAGE motion of the Sun is two degrees per day to the west, or one-third of the stars' motion of six degrees per day, making the day three times longer than the rotation period; but at various times of the year, the motion can be very different. Near aphelion, the orbital motion is slower, and the net westward motion of the Sun is more than half again its normal angular velocity, or more than three degrees per day; but as the planet approaches perihelion, the Sun slows, stops moving westward, moves a little over one diameter to the east, then turns around and starts slowly moving westward again, faster and faster, as Mercury moves away from the Sun, until the Sun is going more than three degrees per day to the west, at the next aphelion.
At the same time that the Sun is moving slower, and then faster, it gets larger, and then smaller, because its apparent size depends upon how far away it is.
A Very Strange Day
Now, let's suppose that we were at a place on Mercury, such as the Caloris Basin or its antipodes, where the Sun is overhead at perihelion. At such a place, as shown in the animation below, we would see the Sun rise in the east as a ball a little over one degree in diameter (twice the size as seen from the Earth, because the distance is half our distance), at a rate of more than three degrees per day, or a little over three diameters per (Earth) day. During the next half-orbital period (44 Earth days), the Sun would gradually grow larger, and move more and more slowly, so that as it approached the meridian, and its highest point in the sky, it would be more than a degree and a half in diameter, and hardly moving at all. Then, as it passed the meridian, it would stop moving westward, slowly move eastward, until it was a little over one diameter to the east of where it stopped moving westward, then slowly start moving to the west again. Over the next half-orbital period, the Sun would move downward in the sky, gradually moving faster and faster, and shrinking in size, until, as it set in the west, it was once again only a degree in size, and moving more than three degrees per (Earth) day.
(This is a Shockwave Flash Player animation. If you don't have Shockwave Flash Player, go here to get it. If you do, click the image to activate the Player, then push the PLAY button.)
The animation above shows the apparent motion of the Sun across the Mercurian sky at a location, such as the Caloris Basin (or its antipodes), where the Sun is overhead at perihelion. It does not show the apparent motion of the stars, which is three times faster than the Sun's average motion. (NASA, Mercury Messenger, Carnegie Institution, Applied Physics Laboratory, University of Montana)
Now, let's imagine moving a quarter of the way around the planet, until we are at a longitude where the Sun is overhead at aphelion. Then we would see a sight unlike anything seen anywhere else in the solar system; for as the Sun rises on the eastern horizon, Mercury would be at perihelion, and the degree and a half diameter disc of the Sun would take several days to slowly rise, then just as slowly set again as it stopped its westward motion, turned to the east, then stopped and turned westward again.
After this "double dawn", the Sun would rise in the sky, gradually shrinking as it rose, moving upward faster and faster, until it passed the meridian with a motion of more than three degrees per day. Then it would sink in the west, gradually growing as it moved lower and lower, slower and slower, as Mercury approached perihelion. Finally, as it neared perihelion, the Sun would very slowly set in the west, then rise again, as it turned to the east at perihelion, before finally setting, for good.
Why Does Mercury Rotate This Way?
The Sun's back-and-forth dance in Mercury's sky is unique, as well as amazing; but the reason for it is quite common -- the tidal forces which act between any two bodies which have significant size, compared to the distance between them. These forces cause not only the tides that we are familiar with, but also precession, the synchronous rotation of the Moon (and many other moons), the tidal slowing of the Earth and other planets, and the synchronous rotation of Pluto, relative to its large moon, Charon.
(discussion to follow: how the Sun's tidal force on Mercury, tries to force Mercury to always keep one face to the Sun; and how, save for an interaction between its varying orbital velocity, the eccentricity of its orbit, and its rotation rate, it would have ended up with the synchronous rotation we once thought it had, within a billion years or so of its formation.)
(The tidal force is so much stronger at perihelion than at aphelion, that the rotation does not have to be synchronous to be stable; it just has to be a half-whole multiple of the orbital revolution, and the orbital angular velocity at perihelion has to be the same half-whole multiple of the average orbital angular velocity. In this case, the orbital period is 3/2 of the rotation period, and the angular velocity at perihelion is approximately 3/2 of the average orbital angular velocity, so the criteria are satisfied.)