If there was a Counter-Earth it’s presence would eventually be detected. Counter-Earth’s gravitational influence on the other planets would cause those planets to deviate from the orbits that would be expected from a single Earth. By the 1800s at the latest there would be a strong suspicion of a massive object opposite the sun from Earth.
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From: Mr. Literal (a.k.a.: Alabama Al)
If there was a Counter-Earth it’s presence would eventually be detected. Counter-Earth’s gravitational influence on the other planets would cause those planets to deviate from the orbits that would be expected from a single Earth. By the 1800s at the latest there would be a strong suspicion of a massive object opposite the sun from Earth.