This is purely a question about terminology, one that has eluded my googling efforts.

When a satellite and its parent body are tidally locked to each other, there is (in an ideal case) a single point on the surface of the parent body and on the surface of the satellite body that remain the closest two points on the two bodes at all times.

Is there a specific name for those two points? If so, what is it? I think "antipodal points" is descriptive, but I wonder if there is a more specific term.

Thank you!

  • $\begingroup$ Both bodies don't have to be facing each other. It is the less massive body, the moon or Hot Jupiter that is facing always with one side towards the more massive body, while this one keeps turning happily ever after at its own rate. $\endgroup$ – AtmosphericPrisonEscape Jan 30 '16 at 16:43
  • $\begingroup$ @Atmospheric If I understand it correctly, it doesn't have to be the case that they are mutually tidally locked, but it can be the case, as it is with Pluto and Charon. What I'm asking is, in the case where the tidal locking is mutual, what is the name for the two closest points. Does that make sense? $\endgroup$ – Brionius Jan 30 '16 at 17:21
  • $\begingroup$ @Bironius Yes this makes perfect sense. I was just unsure whether you were aware of this. I don't know the answer however. $\endgroup$ – AtmosphericPrisonEscape Jan 30 '16 at 17:51
  • $\begingroup$ @Atmospheric I see - thanks for the clarification. $\endgroup$ – Brionius Jan 30 '16 at 20:05

For tidally locked binary stars, the two points in question are known as the substellar points.

For a tidally locked exoplanet, the point closest to the star would also be known as the substellar point. If the star was also tidally locked to the planet, then there would be a subplanetary point.

For a moon locked to a planet, the point on the moon would also be the subplanetary point. If the planet was mutually locked, it would have a sublunar point (not sure whether this latter only applies to the Earth's moon).

  • $\begingroup$ Great, that sounds like the terminology I'm looking for. Thanks! $\endgroup$ – Brionius Jan 30 '16 at 23:26

I have not heard "antipodal points" used in this way before, but it is probably fine.

For a tidally locked moon, this point is called the inner pole. It should in some sense also apply to the parent body, as that point is also facing the barycentre. The direction "down" is generally called Nadir.

For planetary coordinate systems, the constantly parent-body facing/moon facing point is used for 0,0.


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