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Under Where is the Selk crater on Titan with respect to Saturn? there is @BrendanLuke15's comment which provides a helpful hint:

Tidally locked moons have their 0° longitude defined as the 'sub-planet' point (In every instance I've seen)

The first thing I think of is libration. (though when I was much younger it was often libation).

I've included a libration GIF from Who does these mesmerizing simulations of the phases of the Moon? And how? below to show just how hard it is to try to figure out where the "'sub-planet' point" is here. This is because the Moon's eccentricity is pretty large so that while it rotates once a month steadily its rotation around Earth speeds up and slows down in angular velocity by quite a lot.

For more on Earth's moon see:

And for another tidally locked moon instance that @BL15 may or may not have seen:

Question: How tidally locked is Titan? Does it exhibit libration due to eccentricity? Have any residual oscillations not yet damped out been detected or ruled out?

By "ruled out" I mean by direct measurement: more than "we know Titan's been there a long time and made of normal materials and so it must have damped out by now".



Animation used in EarthSky.org:

the Moon's libration

Source but as pointed out in this answer by the GIF's creator, the original image is actually in Wikimedia

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    $\begingroup$ Obviously there is some libration, as the orbit has some eccentricity. As for residual wobble: that's hard question. Even for our Moon, i could not get an authoritative answer. $\endgroup$ Jan 2, 2022 at 17:22
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    $\begingroup$ @CuteKItty_pleaseStopBArking I guess someone at JPL is going have to work it out between now and 2036 so that Dragonfly can land at its target :) $\endgroup$ Jan 3, 2022 at 11:05
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    $\begingroup$ @DaveGremlin shrug. the residual wobble will be small, and with a period of hundreds to thousands of years. Just observe where it is and don't try to predict where it will be in your greatgreatgreatgrandchildren's time. $\endgroup$ Jan 3, 2022 at 11:23

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