Long ago, two ancient Eldritch Abominations ruled the planet Cyra. Kith, the giant tick-like beast that drains the thermal energy out of the environment, and Eas, who is the planet’s ocean. After centuries of war between the two, a group of super human mages decided to banish Kith to space. They succeeded, but now Eas had no competition. He reached into space and pulled the moon from orbit, plunging it into his depths.

So here is my question, how would this change the planet? Kith is now serving as the moon for the planet, but he is much smaller than the original moon. Now that the moon has been pulled into the ocean, how will the ocean level change, what will happen to the tides, what will happen to the planet in general? Assume the planet is earth-like, and the original moon is roughly half the size of earth’s.

  • 1
    $\begingroup$ Hmm, you want to have a transition from a myth to science? $\endgroup$
    – Alexander
    Feb 2, 2018 at 19:07
  • 5
    $\begingroup$ "He reached into space and pulled the moon from orbit, plunging it into his depths" - if you ask science, the gravitational energy release would have been enough to thoroughly wreck the planet, leaving Kith orbiting a ruined cinder. Your half-sized moon has an energy three orders of magnitude greater than the Chicxulub impactor (~1E+28 vs ~1E+25 J). $\endgroup$
    – LSerni
    Feb 2, 2018 at 19:31
  • $\begingroup$ Why did Eas consume the moon? $\endgroup$
    – Muuski
    Feb 2, 2018 at 20:00
  • $\begingroup$ I figured, considering he is an Eldritch Abomination whose is not strictly confined to conventional physics or biology, he could pull down the moon without wrecking it. As for why, I was thinking it would be because it would raise the sea level, putting even more of the world under Eas’ control. $\endgroup$
    – Nick
    Feb 2, 2018 at 20:08
  • $\begingroup$ So a 1000 mile across sphere of rock sticks out from the Earth without either one deforming? $\endgroup$
    – user25818
    Feb 2, 2018 at 20:14

3 Answers 3


There is a model of the Earths formation that includes a catastrophic impact that tore into the Earth and ripped loose a chunk of the Earths mantle the size of the moon. This chunk stabilized in orbit...the reason I can say it's the size of the moon is because it's what is theorized to have become the moon.

The impact being caused here reaches that scale. The majority of the ocean is either vaporized or outright ejected into space. Life on Earth is pretty much an absolute loss, what isn't destroyed by the impact itself will die soon after between the largest nuclear winter scenario or (more likely) covered in meters of rock that settles after being sprayed into the upper atmosphere from this impact. Earth itself will recover as it has billions of years to do so. Won't be recognizable as the earth we know today though...

I suspect this isn't the answer you're looking for, but it ultimately comes down to this line being a poor representation:

He reached into space and pulled the moon from orbit, plunging it into his depths.

Remember the moon is an order of magnitude larger than the earths oceans...even at half size, it plunges into the oceans depths the same way a bowling ball would plunge into a teacup.


The Chicxulub impactor is estimated to have been 6-9 miles in diameter - the size of a small city, and just one caused a mass extinction event.

A theoretical 250-mile-diameter impactor is the size of a small country, will shatter the planet's crust, utterly over-destroy the entire biosphere in several interesting ways as a by-product, and the energy released will flash-steam the oceans.

Half the moon's diameter is 1000 miles, the size of a small continent or large country. It can be broken into 6-7 world-killer impactors, or several hundred thousand mass-extinction impactors.

Either way, the sea levels will go down...way down as the liquid is converted to vapor, and much incidentally escapes into space. Eas will be looking for a new home, since the planet is now quite dead...

...unless, of course, Eas converts the moon into quintillions of meteorites small enough to burn on entry, not cause a significant impact, and sift down into the ocean as tiny nuggets and space-dust. Enough space-dust to coat the planet 2.6 miles deep (the volume of both bodies added together, then solve for the new radius). The oceans (and land) would 'rise' 2.6 miles above the prior surface.


Disclaimer: LSerni had posted a very good answer about one hour ago, but later deleted it. Only those with 10K reputation can see it now. I think it is a very good answer worthy of notice, though, so I am using most of its text here - I've done just a few modifications on it.

how will the ocean level change, what will happen to the tides, what will happen to the planet in general? Assume the planet is earth-like, and the original moon is roughly half the size of earth’s.

Slow-motion take of the whole business:

  • Eas begins pulling on the moon, destabilizing its orbit.
  • Moon distance decreased to 100,000 km: the change in angular momentum of the Cyra-Moon system shoves Eas against the continental shelves and induces massive tsunamis. The change in differential rotation starts pushing on both the Gutenberg and Mohorovicic interfaces.
  • Moon distance decreased to 10,000 km: if there has been enough time, the combination of Gutenberg-interface "ripples" and displacement on the crust triggers both colossal earthquakes and a significant increase in volcanism.
  • Somewhere between 10,000 km and 1,000 km, Eas realizes it has made a dreadful mistake. This depends both on the eldritch abomination's sanity and IQ, and on planetary composition and tectonics.
  • The Moon starts aerobraking, triggering immense, supersonic storms that scour the surface, whipping volcanic ashes, flaming debris, most trees and the greatest part of the topsoil all around the planet. Some people whimpering in armored underground bunkers with oxygen supply might still survive for a few more minutes.
  • Impact. A total energy of approximately 1028 Joules, or 500 billion Tsar Bomba's, minus the energy that powered the mega-storms, is released. Think Chicxulub, but one thousand times bigger.
  • An impactor 1000 km in radius is enough to penetrate half-way through the mantle, destroying five million square kilometers of surface instantly, and fifty million more within a few seconds. Most everything within five thousand kilometers from the impact site will die from the acceleration caused by the impact alone. Perhaps some tardigrades farther out survive.
  • The impact is also enough to cause ripples on what is left of the crust, tearing it apart. A significant portion of the surface is destroyed; vast chasms open, ejecting lava and calcinating whatever Eas happened to think of as its ass.
  • Most of the water on the surface of the planet becomes gas. A major portion of it escapes into space along with the gods know how much of the atmosphere.
  • A large part of atmospheric oxygen that remained is consumed.
  • The whole of Cyra is wrapped in red-hot dust. Tephra and ejecta reach the other side of the planet; several chunks escaping the planet give Kith the beating of its life, and the kinect energy gained from the impacts throws Kith into an escape trajectory as well.
  • The super-nuclear winter lasts anywhere from five to twenty thousand years, enough to kill off most of whatever vegetation remained.
  • Some microbial lifeforms may remain around thermal vents in the deepest parts of the remaining ocean.
  • As for Kith, he is now orbiting Cyra's star on an orbit which is dangerously close to Cyra's own. Depending on how their next approaches go, it will either impact against Cyra causing another cataclism (though on smaller scale), or get thrown onto ever higher or lower orbits until it impacts some other planet, gets captured by another body or stabilizes on another orbit. This might take billions of years.
  • $\begingroup$ I thought Kith was ejected in the third to last bullet point, why is he orbiting in the last? $\endgroup$
    – Muuski
    Feb 2, 2018 at 21:03
  • $\begingroup$ @Muuski my bad, fixing it. $\endgroup$ Feb 2, 2018 at 21:04
  • $\begingroup$ Hmm, maybe I’m looking at this the wrong way. See in the setting Eas’ entire consciousness is contained in every single part of it. So vaporize the ocean and Eas is still alive, just in vapor form. So maybe Eas is trying to pull the moon down so it can escape the planet as a massive, sentient vapor cloud and spread throughout the system. $\endgroup$
    – Nick
    Feb 2, 2018 at 21:50

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