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In my story, Earth follows an alternate history where this civilization has reached the same level of development as us, today. Assuming that this civilization can lift a second moon half the size of our current one into orbit around Earth and they had the funds to do it, why would they want to?

I have my premise down, but I still have no rationale for why they would do something so expensive for some unseen benefit.

Assume that the civilization is trying to lift this moon into orbit for the overall benefit of humanity, not just their own country. (for example, the dual moon system might allow for greater protection against asteroids, the dual moon might favorably impact tides for fishermen, etc.) Assume that the moon is lifted into orbit exactly across from our current moon. Also, assume that the gathering enough matter to create the moon has already been dealt with through some frantic handwaving (so there's no need to siphon off part of Earth's crust to make the moon).

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    $\begingroup$ Your first two sentences seem contradictory: if their level of development is comparable to ours, then they are quite firmly incapable of lifting a second moon half the size of our current one into orbit. Such a feat is so, so far beyond our abilities that we might not even be able to conceive of the motivations of a civilization so advanced, or of the lifeforms that constitute it. $\endgroup$
    – ruakh
    Commented Oct 21, 2016 at 6:05
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    $\begingroup$ You couldn't 'lift' such a thing into orbit, as that implies that it would be sitting on the surface of the Earth. But if it did, it would slump into the Earth's crust and mantle, destroying itself and most of the Earth's surface. (For a parallel, see the Biblical Book of Revelations, in which Heaven - a cube 1500 miles on a side, made mostly of gold - is supposed to land on the Earth.) You might be able to move a large asteroid, like Ceres, or one of the Jovian moons, into orbit around the Earth. $\endgroup$
    – jamesqf
    Commented Oct 21, 2016 at 6:07

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If they can create another moon then clearly they are far more advanced then us and are just showing off.

As this moon is far larger than asteroids try moving the asteroids to protect earth. Given the mass of the moon and earths oceans, may I suggest removing the oceans, scooping fish off the sea floor, and returning the oceans? (If you want to help fishermen get them a new boat or something.) Power on this scale could be easily used to fix major problems, it seems to be being squandered on something with no particular purpose.

The only scenario where earth gets another moon is if a large collection of minerals (asteroids, mars rock ...) are being placed near earth as a stockpile or slag heap.

In other huge actions done for the sake of tiny benefits, try setting off a nuke across town to melt the snow off your car.

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  • $\begingroup$ "try setting off a nuke across town to melt the snow off your car"? That is far too energy efficient and I want to boil a cup of tea. Please launch a manned vessel towards the sun carrying a single cup worth of water, then bring that water as close to the sun as its steal container can handle and bring the gas back to earth. The temperature should be fine until they arrive at my place. $\endgroup$
    – hajef
    Commented Jul 6, 2020 at 8:43
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Reasons to Want a Second Moon

  • Some other event might be getting rid of the existing moon or negating its effects, which humanity needs. Scientists could put the replacement on the sun-side until the natural moon eventually got destroyed or removed.
  • The moon we have helps keep the Earth's axial tilt stable. Another moon would guarantee stability and keep seasons at a reasonable length.
  • Your society might either value eclipses or it might need them for some task

Reasons to Not Want a Second Moon

  • Our tides work well as they are. By adding a second moon, you would double the strength of tides, meaning devastating high tides and environmentally costly low tides.
  • Any miscalculation in the way we put it into orbit could lead to a collision between both moons or a collision between the Earth and either moon.
  • Conversely, miscalculations could "slingshot" our existing moon into space, or send the Earth out of orbit - crashing us into the sun, the asteroid belt, another planet, or into the freezing black abyss. Not fun.
  • A moon during the day would lead to many more, if not daily, eclipses, and would at the very least block a substantial amount of sunlight needed for agriculture.
  • The combined gravity on opposing sides of the Earth could slowly tear our planet apart
  • In the long term, this would increase the length of the day - not noticeable now, but problematic for people in the far future
  • Developing and indigenous cultures without contact with the outside world (many of which do exist) would potentially change completely if they saw a new astronomical object every day. The development of their religion, calendars, and other things would never be the same.
  • The phases of the moon we already have would change as the new moon obsucred sunlight that would otherwise hit it. This would in turn affect nocturnal organisms with moonlight-sensitive eyes
  • Expenses
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    $\begingroup$ Re: "A moon during the day would lead to many more, if not daily, eclipses": Note that the Moon orbits the Earth much faster than the Earth orbits the Sun, so the Moon is actually "during the day" half the time as it is. Since the second moon is to be "exactly across from our current moon", it should only double the frequency of eclipses (or in fact a bit less, since it's to be much smaller than the Moon; so it would cause no eclipse in some cases where a Moon-sized moon would cause a partial eclipse). $\endgroup$
    – ruakh
    Commented Oct 21, 2016 at 5:57
  • $\begingroup$ Also: an object can only "slingshot" off an object far more heavy than itself. There is incomprehensible much energy in Earth's orbit and Earth - is and still would be - far heavier than either of the moons. Even the original moon would never change its orbit in a significant way unless there is an impact. But any impact of the new moon with wither either Earth or the original Moon would be apocalyptic. $\endgroup$
    – hajef
    Commented Jul 6, 2020 at 8:32
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Zyxrra's answer has covered some very plausible suggested reasons. Great, that leaves an implausible one for me.

Disguise - maybe there are robot doomsday ships cruising the universe destroying all planets capable of bearing intelligent life, something like the ones that appear in Fred Saberhagen's Beserker stories. For this setting you have to assume the truth of the theory that a single large moon is necessary for life, particularly intelligent life, to evolve on a planet. This might be due to the stabilizing effects of a moon on a planet's axial tilt or due to the beneficent effect of tides on heat circulation and developing ecosystems. In fact, recent research suggests this theory may not be quite correct but, come to think of it, all that the scenario requires is that whoever programmed the doomsday ships believed it to be true. One giant moon > chance of horrible, disgusting organic life being present now or in the future > slag the planet.

But two big moons? Then all Zyxrra's excellent reasons for not wanting two moons act to keep you safe, not to harm you. The robot destructor ship scans the solar system, thinks life could never have evolved there because those two moons would have long since collided with each other or the planet, and leaves your planet alone.

You probably have to put a lot of thought into keeping your planet safe from the very real dangers of having two large moons but it is still safer than the alternative.

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    $\begingroup$ Creative and unique, but just having a second moon would not hide all the light, radio waves, space junk and other clear signs of life $\endgroup$
    – Zxyrra
    Commented Oct 20, 2016 at 19:41
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I have another implausible one!

Extra space for people to live — Maybe you're running out of room? Plop a man made moon up there, and live in it. "Why not the original moon?" I hear you ask! Because, been there, done that! Already colonized it! Or maybe the Moon People just don't like humans. They're picky people. All these worlds joke

Hell, maybe you decided to mine out the moon, and accidentally destroyed it! Humans do lots of VERY stupid things.

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Energy: Collection of solar wind would create resources. If the moons were critical for solar energy production, it would be convenient to have two, because earth gets periodically on the way of the another of the moons. If there is no atmosphere, the solar panels produce more energy.

Livable: The moon does not have to be like our moon. It could have an atmosphere. At least used to test terraforming, relating to the point of Nasty stuff, if that were to fail.

Food: There could be plants or bacteria that does not require atmosphere. With stronger UV-radiation they would be more efficient over there, provided that they had nutrients.

Nasty stuff: You can make the moon the test lab for all kind of dangerous stuff. Especially if the human were to utilize the another moon in living or food way, some really bad stuff could be done on the another moon.

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  • $\begingroup$ Lunar eclipses are very rare and only last a few minutes. I think that would be in the noise compared to the panels simply being dark half the month. $\endgroup$
    – JDługosz
    Commented Dec 21, 2016 at 14:59
  • $\begingroup$ Lunar eclipse is when moon blocks sun light from the earth. If the solar energy would be produced on the moon. Earth block a while longer the sun from moon due to its size. If the solar energy were critical, then a huge drop in production could cause problems. $\endgroup$
    – user3644640
    Commented Dec 22, 2016 at 7:08
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If there was a massive threat and a defensive weapon which both required a substantial mass to place it on (potentially using the gravity well itself or maybe just to absorb the recoil) and could not be used inside the atmosphere then you would have a good reason to place another moon on the exact opposite side of earth from our current one so that you could get complete coverage.

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Earth already has a second Moon. It's just not as big as the one we observe with our naked eyes ;)

You wouldn't lift to orbit, you'd construct in-situ from asteroidial/cometary material.

If it were mostly ice, you could be using it as a fuel/volatile reserve for your space industry.

--

https://en.wikipedia.org/wiki/2020_CD3

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  • $\begingroup$ According to the linked Wikipedia article, 2020 CD3 was not known to exist in October 2016 (Discovered in February 2020), and probably returned to solar orbit in May 2020. $\endgroup$
    – notovny
    Commented Jul 6, 2020 at 3:47
  • $\begingroup$ Correct, at that time there was a prior moon. :) $\endgroup$
    – anonymouse
    Commented Jul 6, 2020 at 3:48
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Let's be as far-fetching as possible. Reasons to deploy Mini:

  • Moon has been mined and lost enough mass to negatively affect Earth. Since the country that wants to save Earth has no rights on Moon (all owned by evil Corp., Bad-State, or similar), they create Mini as a second orbiter. Benefits: Earth's rotational axe stays stable; tides stay about the same.
  • Moon is drifting too far away to keep Earth stable and returning Moon is not an option for some reason. Benefits: Earth's rotational axe stays stable; tides stay about the same.
  • Moon is about to be traded off to space people that have the means to take it with them. Benefits: Mini keeps earth's rotation and tides somewhat stable while Moon is away; Foundation for the creation of an orbiter of Moon's size.
  • Brag to other civilizations (on Erath or others) as part of a Cold War. Benefits: Display of abundant energy supply; no one wants to fire the first shot; inevitable war does not begin before one side has already won; entire peoples if not planets live longer if not survive for good.
  • Brag to other civilizations (on other planets) to gain prestige. Benefits: Aid in human initiation to the galaxy's cool kid's club; possibly skyrocket human development in any and all ways.
  • Segregation of global hazards requires two outsourcings. Let's assume we harvest nuclear power on Moon and develop/ research potentially apocalyptic organisms on Mini. Benefits: If a potentially apocalyptic event happens (e.g. a nuclear plant leaks tons of radioactive material or biological weapons go rogue), only one of the orbiters is affected if at all - either way, we are safe at Earth while doing stuff that needs doing but can get out of control, endangering the entire celestial body where it happens; we abide interstellar safety regulations; we can do more dangerous research with less actual danger to ourself.
  • We know a cloud of radiation is coming and do not have the time to find and bring a celestial body that is large enough and has enough metal to intercept while small enough to not screw our tides, rotational axe and Moon's orbit entirely. Benefits: We don't all become Fantastic Four.
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