Scientists once believed that the same side of Mercury always faced the Sun.
However, in 1965 astronomers discovered that the planet rotates three times during every two orbits it makes.
The animation above showed the apparent motion of the Sun across the Mercurian sky at a location where the Sun is on the meridian (halfway between rising and setting) at perihelion. It was set for 45 degrees north latitude, so it does not correspond exactly to the Caloris Basin or its antipodes, which are on the Mercurian Equator, and have the Sun overhead at perihelion. Unfortunately, there are several problems with the animation, such as (1) it does not bother to show the constant westward motion of the stars, which is three times faster than the Sun's average westward motion, (2) it shows the Sun larger than its actual apparent size, so that the backward motion does not appear to be larger than its diameter, and (3) neither it nor the NASA site it came from bothered to explain the details of the various "dials" shown in the diagram. If and when I can produce my own version, I will make it more accurate, and include a detailed discussion of everything it shows.
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, instead of at perihelion. 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 just as slowly rise again as the Sun stopped its eastward motion 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 larger 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 (or at least until the next day, two Mercurian years later).
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, instead of at perihelion. 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 just as slowly rise again as the Sun stopped its eastward motion 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 larger 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 (or at least until the next day, two Mercurian years later).
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