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Why was the lack of observed stellar parallax a challenge for Copernicus’s theory?

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Nicolaus Copernicus Portrait of Nicolaus Copernicus, 1580, from the Town Hall in Toruń, Poland; in the collection of Muzeum Okręgowe w Toruniu (Regional Museum in Toruń). In his book *De revolutionibus*, he proposed that the Sun was the center of the solar system and that the planets circle the Sun.

The lack of observed stellar parallax posed a significant challenge to the Copernican system because it contradicted the expected observational consequences of a moving Earth.1

If Earth truly revolved around the Sun, astronomers reasoned that the positions of nearby stars should appear to shift relative to more distant background stars throughout the year—an effect known as annual parallax. Because observers at the time could not detect this shift, critics argued that Earth must be stationary.213

In reality, this failure to observe parallax was due to the immense distance of the stars, which makes the angular shift far too small to have been detected by the technology available during Nicolaus Copernicus’s lifetime. The effect was not successfully measured until 1838, when Friedrich Wilhelm Bessel used precise instrumentation to demonstrate the parallax of the star 61 Cygni.21