How long will it take to discover they live on a moon and not on a planet?

In my alternate reality, Earth is not a planet. It is a moon and orbits a gas giant (however it has all of Earth's characteristics, it is also full of humans and life as we know it). This is the only moon the gas giant has. This alternate Earth is tidally locked. This means that the people who live on the "outer" side of the moon have never seen the planet they orbit. And here comes the question: Assuming Astronomy develops as it did on our Earth. When will they be able to discover they don´t rotate around the sun alone?

When I say "when" I am asking which stage of astronomical development. Could Galileo and Copernicus have noticed that? Ptolomeo, perhaps? Or maybe the Greek astronomer Aristarco de Samos (310-230 BC) could have noticed that with his observations of the sky? (No, these are not multiple questions. I am just explaining the type of answer I am looking for).

Of course, as I said before, I am assuming all these inhabitants of the continent on the "outer" side of the Earthly moon have never navigated to the other side of their moon, so they have never seen the big gas giant in the sky.

posted over 5 years ago

Carlos Zamora‭

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