That Mercury is rotation locked to the Sun is actually an old misconception that has existed since the late 18th century. It wasn't until the early Space Age in the 60ths when radar-based temperature scans confirmed that to not be the case.
In reality Mercury has a particular 3:2 spin-orbit resonance that's kind of tricky to explain, but let's start with this animation for visually demonstration:
About the numbers to the bottom left, let's ignore their decimals for now. It's only the integers that are interesting, since for every time they change, a full cycle of something has occured. Clarification follows:
Time = A Mercurian year (when the planet makes a full orbit around the Sun)
SidT = Sideral period (when Mercury makes a full 360 degrees rotation around its own axis)
SolT = Synodic period (An actual Mercurian day)
Imagine the red point on Mercury is where you are standing. Notice on the planet's first full orbit (a Mercurian year), the red marker is on the opposite side where it originally was. Once Mercury has made its second orbit, the red marker is back to its original orientation.
At that exact point, notice how the numbers has accordingly changed to:
Time = 2.000 (Two years on Mercury has passed)
SidT = 3.000 (Mercury has rotated 3 times around its own axis)
SolT = 1.000 (A full day-night cycle has occurred on Mercury)
Feel free to study the animation's logic some more.
But to not ignore the point of this topic, let's assume there would be a planet that's tidally locket to its parent star. I don't mind the concept of visiting separate zones of a celestial body, albeit it's beyond my expectations.
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