Negative Parallax

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Stellar parallax is the apparent shift of closer stars against more distant stars due to the orbiting motion of Earth. Negative parallax, or stars going the wrong way— occurs due to measurement errors placing the nearer star as the farther one and vice versa.

Stellar parallax directly proves Earths’ motion around the sun, and flat-Earthers use negative parallaxes to dismiss it. In reality, negative parallaxes are just statistical uncertainties.

Stellar parallax is the apparent motion of stars relative to other stars, which also have parallaxes. We do not know beforehand which stars are closer than the others, and these have to be inferred using statistical analysis from the entire data. The parallaxes of distant stars should be practically zero. And because they have a statistical uncertainty, then half of these near-zero parallaxes will be negative.