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If the moon had life and it progressed at the same rate as life on earth, including key points of technological discovery. When would we, on earth, have first detected them?

Technological discoveries I mention are things like... fire, Stone Age, Bronze Age, industrial revolution, etc... first telescopes, electricity, radio, flight, etc...

(Obviously ignoring the fact that the moon is lifeless with no atmosphere)

There is no doubt that by now we would know they are there. We would receive the radio waves they emit. We have satellites that would clearly see their cities and road infrastructure.

Before that there could be electric lights lighting up at nighttime that could be detectable by powerful telescopes.

But in the past we wouldn't know they were there. So there is a point when the first discovery was made.

What point in technological advancement would be the earliest point where we would have noticed them without knowing they were there and realised that they were a civilisation.

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    $\begingroup$ I suggest you edit your title to make it "sentient life". Detecting life would happen early on when people could see green and blue up there (since our kind of life means that somehow the moon is holding an atmosphere and has chloryphyl plants). (Whether we would have been smart enough to recognize it as life ourselves is a slightly different question!) $\endgroup$ – SRM Jan 16 '17 at 20:56
  • $\begingroup$ @SRM thanks. Edited to make the title clearer. $\endgroup$ – Fogmeister Jan 16 '17 at 21:01
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    $\begingroup$ There could be major differences between "sentient life" and life with a technology similar to ours. As for instance, sentient creatures that had developed good night vision (either natural or technical) might well think there's no good reason to expend a lot of energy on outdoor lighting. $\endgroup$ – jamesqf Jan 17 '17 at 2:22
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    $\begingroup$ Maybe they live underground? Why would they bother with the surface when they could live in lava tunnels or cooled magma chamber, using internal heat, radioactivity and pressure as energy sources. And as for detecting radio, we have only created radio emissions for a tiny bit of one percent of our sentience. What makes you sure other civilizations would have it? Or any technology? is there no life in the oceans, because of lack of radio transmissions? $\endgroup$ – Innovine Jan 17 '17 at 15:21
  • $\begingroup$ @Innovine that was the prerequisite of the question. Hypothetically if the moon was actually a moon-sized "Earth 2.0" with technological advancement at exactly the same pace. You are right about the tunnels and living underground etc... The reason I added the prerequisite was a sort of double question. i.e. they would technically be able to see us at the same rate we see them. Like Thucydides mentioned. $\endgroup$ – Fogmeister Jan 17 '17 at 15:34
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Percival Lowell speculated that there was life on Mars based on (erroneous) observations of "canals" and what he thought were changes in the Martian landscape that could be explained by seasonal changes in vegetation. This was with some of the best telescopic equipment in the late 1800's, but as we know now, his observations were coloured by wishful thinking.

Observation of the Moon would be equally limited until very late in the Industrial Revolution when not only would we have telescopes of sufficient size and quality to make out larger surface features, but also they would also have started making very noticeable changes to the Moon.

Agriculture, especially large scale agriculture like the settlement of the west and replacing grasslands and open ranges with fenced in fields and growing new crops like wheat should make visible changes, especially during seasonal changes when the fields are fallow or new crops are growing in.

The invention of electric lights and urbanization would provide clear beacons for observers on Earth, and the Lunar inhabitants might also be making active attempts to signal the Earth if they are also as sentient as we are.

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The Korean Peninsula at night from Orbit

In the early 1800's some speculative ideas for signalling the presumptive inhabitants of Mars included "drawing" massive diagrams across the Sahara desert or carving out huge diagrams in the forests of Siberia which would be visible from space, and presumably visible with a large telescope on Mars. Such structures would be visible from the Moon as well, and a Lunar engineering project of similar scope should be visible from Earth.

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  • $\begingroup$ Excellent answer. Thanks. I was actually thinking about Mars before I wrote this question. I remember a video of Patrick Moore when (I believe it was) Viking was making its approach. They were speculating that the red was caused by some sort of vegetation. It was only when we landed that we knew that life was not present (well, probably). I also like the idea of them (or us) trying to establish communication. Which makes perfect sense. Thanks :-) $\endgroup$ – Fogmeister Jan 17 '17 at 9:29
  • $\begingroup$ So, the Nazca lines were an early attempt to communicate with our lunar neighbors? $\endgroup$ – Jason K Jan 17 '17 at 17:41
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Artificial light is very visible from space and we would definitely see that eventually. In early civilizations there generally isn't too much of it though, so it wouldn't become all that visible until the invention of light bulbs or good telescopes.

A more visible effect of human habitation would be agriculture. With the naked eye it's hard to tell cultivated land from plains, but people would have started mapping the moon very early on in history, and would likely notice forest lines receding and being replaced by agriculture. This would at least imply intelligent life, but there would be debate as it could just be forests naturally dying off. Still, this should be slightly visible to the naked eye and could have been detected and theorized as early as the dawn of detailed map making (Classical period). But there would be no confirmation of this until the invention of telescopes.

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The presence of large scale civil construction like roads and canals would give hints especially if we can watch them forming, crawling across the landscape. But artificial light would be a big hint, even widespread campfires can give off significant light. All this would require the invention of telescopes on our part of course.

If there is obvious liquid water people would presuppose life, if not intelligent life, very early.

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See this question on Astronomy.SE. Earth-based telescopes cannot see small details from that distance because of fundamentals of optics. Without primary mirrors hundreds of meters across, we won’t see their roads. Large scale features due to building cities (several km across) or changes due to large scale agriculture will simply be spots of a different color with no way to tell why.

Even if there is a geometric pattern that speaks of “living growth” we can’t tell if it’s beings or invasive bugs.

Maybe we’ll get lucky and see large-scale structures based on obviously intelligent patterns like prime numbers. But in general, I think we can’t be sure there are people there until large scale artificial lighting is rolled out.

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    $\begingroup$ Citation/detail for the curious: curious.astro.cornell.edu/about-us/45-our-solar-system/the-moon/… $\endgroup$ – mattdm Jan 17 '17 at 0:47
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    $\begingroup$ Since they don't have an atmosphere, could they see our stuff easier than we could see theirs? $\endgroup$ – SRM Jan 17 '17 at 14:31
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    $\begingroup$ Yes, they would not need adaptive optics to build large telescopes. But the diffraction limit still applies. $\endgroup$ – JDługosz Jan 17 '17 at 14:32

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