Due to the moon's gravitational pull, objects at Earth's surface directly under the moon are slightly lighter. This tiny lessening of gravity would also take effect if the moon were on the exact opposite side of the Earth. The Earth and the moon orbit a common point that is not the dead center of the Earth. That point ends up being somewhere in the mantle on the side of the core directly under the moon.

That means that, while someone standing directly under the moon would be very slightly lighter due to the attraction of the moon's gravity, someone standing on the exact opposite side of the Earth would be the same fraction lighter due to being further away from the center of gravity in the Earth-Moon system.

Source: http://ryanmarciniak.com/archives/518

I make that point to ask a question premised upon it. We could increase this by moving the moon closer to Earth. However, eventually, you would reach the Roche limit and tidal stresses would tear the moon apart. However, a black hole wouldn't be torn apart by tidal stresses and could hypothetically orbit within that limit. Could a black hole of the correct mass in geostationary orbit create dual areas of 1G on an otherwise high-gravity world?

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    $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. I've downvoted your question - an action I'm happy to reverse - due to a lack of research. That link has all the math. Using the equations for the Jupiter/Io example, you only need plug in values representing the black hole and your super earth to work out what their relative mass and radii must be. You haven't done that and, frankly, should. (*continued*) $\endgroup$ – JBH May 15 '19 at 13:26
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    $\begingroup$ You might find that you should be asking a different question. Please take the time to work through the mathematics yourself, then provide the numbers you came up with. Then I'll reverse my vote. $\endgroup$ – JBH May 15 '19 at 13:27


Tidal locking near a black-hole (which you'd need for your effect to remain in one place on your planet) is impossible because of:

Magneto-rotational instability

Black holes exist in an extremely hot, turbulent, and magnetically charged surrounding, IE it's not just gravity tearing you apart that you'd need to worry about.

Near a black-hole Velikhov-Chandrasekhar instability (or Balbus-Hawley instability) tears everything apart:

fluid instability that causes an accretion disk orbiting a massive central object to become turbulent. It arises when the angular velocity of a conducting fluid in a magnetic field decreases as the distance from the rotation center increases.

  • You may think "What if nothing on the planet is magnetic?"

Even water is diamagnetic (repels magnetic fields, loosely IE is pushed away by them). All (that can be measured) the elements on the periodic table posses some magnetic property or other.

Basically unless you develop some kind of handwavium to shield from the magnetic eddies, you're in big trouble.

  • $\begingroup$ Also, before I get banned, I appreciate your answer. Take care. $\endgroup$ – Pinkunz May 24 '19 at 17:36
  • $\begingroup$ @Pinkunz Thank you, but that's not a good plan on the whole. Best to stick around and learn. That's what I did. As Winston Churchill used to often remark during the second world war - "KBO. - Keep buggering on!". (I wasn't there, just watch a lot of films and TV). $\endgroup$ – Tantalus' touch. May 24 '19 at 17:41
  • $\begingroup$ People choose communities as much for their culture as they do for the benefits afforded them. Your loss as much as mine. $\endgroup$ – Pinkunz May 24 '19 at 17:45
  • $\begingroup$ Understood, perhaps see you on another stack - believe me if you compare Veganism & Vegetarianism, this one and say Mi Yodeya, the cultures are totally different. So many to explore..... $\endgroup$ – Tantalus' touch. May 24 '19 at 18:05

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