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Some time ago, aliens placed the Moon around Earth with every intent to deceive us about its authenticity, except for the glaring synchronization that allows for total solar eclipses, as something like a practical joke.

What indirect evidence do we have that places the Moon's age as far back as possible? "Indirect" as in data points not originating from the Moon itself, e.g., radiometric dating of Moon rocks.

The aliens have never tampered with Earth, only everything about the Moon (its make-up, surface topography, orbital path, etc.) meaning any lunar influences found on Earth are legit. The aliens ceased all tampering 2 million years ago.
The Moon is as it appears to be: a big old rock — just made to fool. There's a greater lore reason, it's just not important here.

Essentially, I'm trying to place the Moon's insertion on a timeline in my world (which helps inform the period of other events), and because nothing about the Moon can be trusted as a chronological indicator, only evidence of the Moon's presence found on Earth (or elsewhere in the solar system, too, I suppose, if any exist) can be credited.

All this of course means that the protoplanetary collision with Theia didn't happen, and everything that hypothetical event precipitated is explained elsehow. (Unless it can't be, in which case that counts as evidence of the Moon (aw shucks), but then you'd've also proven the Theia hypothesis beyond doubt so what are you even doing here?)

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    $\begingroup$ For reference, Mutineers' Moon (1991) by David Weber, first book of the Empire from the Ashes trilogy. $\endgroup$
    – AlexP
    Commented Mar 28, 2023 at 0:42
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    $\begingroup$ Does your premise contradict itself? If the aliens put the moon there recently, then why would there be any indirect evidence of it being older? All such evidence would not be there because in your story it's not older. It reads more like you're arguing that aliens did in fact put the moon there (in this, the real universe) and asking for evidence disproving it. But that's not worldbuilding. $\endgroup$
    – causative
    Commented Mar 28, 2023 at 0:46
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    $\begingroup$ Don't know enough about the subject to write a full answer, but I know that geologists have found tidal rhythmites at least 300 million years old. They are one of the data points which enable the reconstruction of the number of days in a lunar month and in a year. $\endgroup$
    – AlexP
    Commented Mar 28, 2023 at 0:56
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    $\begingroup$ Any moon rocks found on Earth are part of the hoax, since the Moon...as every child knows...is made of cheese, not rock. $\endgroup$
    – user535733
    Commented Mar 28, 2023 at 1:39
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    $\begingroup$ @RobbieGoodwin scifi is normally based on the real world. If evidence for the Moon pushes past the period I'm comfortable with then yeah I'd BS it (and that's what it's looking like). If not then I might have had an interesting argument to make the case to the reader. "Well, it was strange that rhythmites only started appearing on Earth X million years ago..." I'm not well-informed in all the disciplines where such evidence might crop up, (geology, Earth science, biology, astrophysics) which is why this site exists, and this question. Wouldn't have known unless I asked. $\endgroup$
    – BMF
    Commented Apr 1, 2023 at 0:43

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If you want to get really weird (and in fact avoid using any material that came from the Moon), you can try sediment analysis. I went to a talk on this a few months ago, where the speaker discussed how studying the history of sediment stratification can actually allow us to map changes in the Moon's orbit over time (implicitly showing that the Moon existed far in the past).

The logic is that the Moon plays a role in a couple periodic changes in Earth's orbit that 1) play out over timescales of tens and hundreds of thousands of years, and 2) are stable over hundreds of millions of years. Through complex processes, these changes affect sediment deposition rates; by measuring changes in those rates, you can see the fingerprints of the Moon's influence. One of the more recent papers on this is Meyers & Malinverno 2018, who studied the 1.4-billion-year-old Xiamaling Formation. The most extreme studies I'm aware of actually probed as far back as ~2.5 billion years ago (Walker & Zahnle 1986, Williams 2000.

You could argue that perhaps the Moon isn't responsible for these changes. Perhaps the scientists could get adequate fits to their data if they used models involving no influence from the Moon whatsoever. On the other hand, the only folks who would know this for sure are the teams who did the work (but if you ask them, let me know!).

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  • $\begingroup$ I thought it might be something in the geological strata, or maybe a thing more weird like something in the fossil record pointing to lunar cycles followed by ancient life. Also, my question makes no sense! "Moon rocks found on Earth are fair game?" I'm glad you saw past that mistake though to what I was really interested in. $\endgroup$
    – BMF
    Commented Mar 28, 2023 at 10:24
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    $\begingroup$ @BMF I suspect (but haven't confirmed) that the team which found & dated the moon rocks on Earth did so by comparing their composition with sample-return missions to the Moon... so I'd still class this evidence as derived from the lunar surface. You might be able to get similar constraints with spectrometry of the light reflected from the lunar surface, but probably with much less precision, and the observable-from-Earth lunar density places another restriction on composition, but it seems hard to imagine definitively placing meteorites origin on the Moon because of that. $\endgroup$
    – Sarah Messer
    Commented Mar 28, 2023 at 13:23
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    $\begingroup$ However, @hde-226868 's description of the Moon's effect on sediment deposition is fantastic. $\endgroup$
    – Sarah Messer
    Commented Mar 28, 2023 at 13:25
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    $\begingroup$ @BMF Yeah, I'm totally fine with you changing that -- I agree it's a weird exception to carve out. (I do appreciate you asking to change it, by the way!). $\endgroup$
    – HDE 226868 ♦
    Commented Mar 28, 2023 at 13:39
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    $\begingroup$ @ChristopherJamesHuff Especially since tidal forces push the moon gradually outward (the dinosaurs did not enjoy this cosmic coincidence) $\endgroup$
    – No Name
    Commented Mar 28, 2023 at 19:05

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