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In my world, I need to change the earth's orbit so it is tidally locked to the sun. How can I do this? No matter how unrealistic the solution—whether it be magic or what not—anything that can explain this phenomenon would be helpful.

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    $\begingroup$ Your title is about how to get a tidally locked earth, but the body of your question has nothing to do with that. Which is it? $\endgroup$ Commented Sep 8, 2023 at 11:28
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    $\begingroup$ To extend on @BarbaudJulien, "tidally-locked" means that the planet always shows the same side/face towards the sun. You want an unstable orbit if the sun gets closer and closer :). $\endgroup$ Commented Sep 8, 2023 at 11:35
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    $\begingroup$ Not whilst it has a moon. $\endgroup$ Commented Sep 8, 2023 at 11:47
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    $\begingroup$ If you're willing to include magic, I don't see your problem! Can't you just say that magic was used to have the Earth be tidally locked, and make up some mystical sounding phenomenon? Once you include magic you can explain anything you like! $\endgroup$
    – komodosp
    Commented Sep 8, 2023 at 14:18
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    $\begingroup$ Well, the first thing to do is to get rid of the Moon. As natural satellites go, the Moon is huge with respect to Earth, so huge that the Earth-Moon system behaves almost as a double planet, with much too much rotational momentum to ever allow Earth to become tidally locked to the Sun. So, my point is, should we assume that you have somehow gotten rid of the Moon? $\endgroup$
    – AlexP
    Commented Sep 8, 2023 at 15:46

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Tidal locking normally happens because tidal forces gradient moving through the planet dissipates momentum and slows down the planet rotation until the gradient stops moving.

This process takes a long, long time.

To tidal lock earth to sun, you have two paths:

  1. slow down earth rotation to the point where from sunrise to sunrise takes the same as today's 365 days
  2. lower earth orbit to the point where a complete orbit around the sun is completed in 24 current hours

From what you describe in your question (Earth getting closer to the sun), it sounds like you are pursuing option 2.

Mercury orbits the sun in just 88 days. You are aiming for 88 times less than that.

Based on this calculator, Earth would need to be at $2.927\cdot10^6$ km from the Sun to orbit it in 1 day.

That would require lowering the energy of the sun-earth system from $-2.6\cdot10^{33} J$ to $-135\cdot10^{33} J$, or in other words you would need to shed $133 \cdot10^{33} J$ of energy.

For a reference, that's about the same amount as

  • 100 days worth of the total energy output of our sun
  • 10 times the gravitational binding energy of earth
  • 10000 billion times the electric energy we as mankind consumed in 2008

By comparing the last two bullets I can safely state that, as of today we can't make anything close to the needed amount of energy to make what you want "realistic".

It is a feat available for a type II civilization.

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    $\begingroup$ Also note that at that radius from the Sun, Earth will be entirely uninhabitable. $\endgroup$ Commented Sep 8, 2023 at 12:23
  • $\begingroup$ Also note that Earth will be half the distance from the Sun as the Parker Solar Probe at closest approach -- the surface is going to be molten, and probably slowly evaporating. $\endgroup$
    – Mark
    Commented Sep 9, 2023 at 1:40
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There are quite a few natural examples of tidally locked planets in somewhat habitable zones of their stars.

What they all have in common is that all of these planets orbit a red dwarf star.

It goes like this: The habitable zone distance depends on the star luminosity and the luminosity goes down much faster than the mass goes down.

Just get a half-mass Sun (it will be about 1/15 of the luminosity) and get your planet where it is hot enough.

Bonus: your star will live much longer than out Sun.

Bonus (for your plot, not in the real life): somewhat unstable luminosity and orders of magnitude more powerful stellar storms.

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