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Suppose on the moons of Jupiter there is a growing civilization of intelligent life. Assuming their technology timeline is approximately similar to our own, except lagging behind us, then...

What level of technology (e.g. in terms of an era or century on Earth) would a Jupiter-lunar civilization need in order to detect/deduce civilization on Earth as it exists today?

Maybe it's radio waves that gives us away first, or some kind of thermal signature, or chemical signature, or night lighting, or even our deep space probes. I'm not sure. Regardless of which of our own developments gives us away, what does the other civilization need in order to catch onto us?

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    $\begingroup$ Telescopes (renaissance era, but could be earlier). As soon as they point a telescope to the night side of Earth, they will see that there is something very unnatural there. $\endgroup$
    – Alexander
    May 14, 2018 at 18:58
  • $\begingroup$ The easiest was would be to look at Earth at night but I'm not sure what level of tech they need to see this clearly enough to know it is not a natural phenomenon : linky $\endgroup$
    – Vincent
    May 14, 2018 at 18:58
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    $\begingroup$ Telescope sounds plausible, with one caveat: Earth is closer to sun than Jupiter, so they might have hard time catching earth against a dark sky, just like we can only see Venice during sunrise or sunset. We need an astronomer to check this. Even if viewing opportunities exists, Jupiter might be in the way half the time. Their best chance to see night-side of the Earth might be when Jupiter blocks the sun, but not the Earth $\endgroup$
    – Bald Bear
    May 15, 2018 at 1:58

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They might be able to see Earth from their home moon with spyglasses a la Galileo, but they wouldn't see Earth in detail until they launched probes towards us.

If they advance their technology as fast as we do, then decades before developing probes they will be listening to radio.

If both us and them develop radio at around the same time, both species would be able to communicate. There would just be a few minutes of lag between messages :)

Otherwise, if one species takes longer than the other to develop radio, and neither has a SETI program, they might not listen to each other's broadcasts. When TV was invented we broadcast signals in all directions, and each station was sending signals at the Kw range. Nowadays a satellite can beam a signal pointed at Earth and using less than 80w. Even less power is used for digital transmissions. Our signal is increasingly fainter. In such a scenario, the first contact would be through probes.

And I think they would be more likely to come to us first. From the point of view of someone who is outside our system, Earth is much easier to "find" and send probes than one moon among many circling around a gas giant.

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    $\begingroup$ 30x magnification of Galileo's telescope is indeed insufficient to see night time illumination in any detail. But Keplerian telescope (also early XVII century) would give those aliens a very big reason to wonder about Earthlings. $\endgroup$
    – Alexander
    May 14, 2018 at 20:25
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A telescope similar to the one used by Schiaparelli (1893) to discover the channel on Mars should provide enough resolution to spot the light of the major Earth cities at night.

Also spectroscopic analysis of the reflected spectrum would point to the presence of Oxygen, water and even chlorophyll. That can be achieved with the same telescope.

Radio was invented after that time, so it will be used afterwards to strengthen the suspects.

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