Question

This passage is adapted from an essay on nineteenth-century astronomical techniques and the measurement of stellar parallax.

For centuries, astronomers recognized that if Copernicus’s heliocentric model of the solar system was correct, nearby stars ought to exhibit parallax—an apparent shift in position relative to more distant background stars when observed from opposite sides of Earth’s orbit. Yet well into the early nineteenth century, no astronomer had succeeded in measuring this angle, primarily because stellar distances were so vast that the shift was smaller than a single arcsecond. The quest to detect this tiny movement demanded unprecedented mechanical precision and rigorous correction for systematic errors.

In 1838, German astronomer Friedrich Wilhelm Bessel published the first reliable measurement of stellar parallax, focusing his efforts on 61 Cygni, a binary star system in the constellation Cygnus. Bessel chose 61 Cygni not because it was particularly bright, but because it exhibited an unusually high proper motion across the night sky, suggesting it was relatively close to Earth. To perform his measurements at the Königsberg Observatory, Bessel utilized a specialized instrument known as a heliometer, constructed by the master optician Joseph von Fraunhofer. The heliometer featured an objective lens split precisely in half; by shifting one half relative to the other using a fine micrometer screw, Bessel could superimpose images of two separate stars and measure their angular separation with extraordinary exactitude.

Crucially, Bessel had to eliminate secondary sources of error that could mimic parallax shifts. Changes in ambient temperature throughout the year caused the metal frame of the telescope to expand and contract, altering the focal length. Bessel meticulously recorded thermal variations inside the dome, applying mathematical corrections to offset micrometer screw expansion. Furthermore, Earth’s atmosphere bends incoming starlight—a phenomenon called atmospheric refraction—which varies based on air temperature and barometric pressure. Bessel calculated refraction tables specifically calibrated to the atmospheric density observed during each night’s monitoring session.

While Bessel was conducting his work in Königsberg, Thomas Henderson was analyzing observations made at the Royal Observatory at the Cape of Good Hope. Henderson had focused on Alpha Centauri, recording its position between 1832 and 1833 using a mural circle, an instrument mounted directly to a stone wall to ensure stability. Although Henderson completed his observations before Bessel, he delayed publishing his results until 1839 out of concern that instrument distortion had compromised his data. Meanwhile, in Russia, Otto Struve measured the parallax of Vega using a nine-inch refractor at the Dorpat Observatory, publishing his findings shortly after Bessel.

Bessel determined the parallax of 61 Cygni to be 0.314 arcseconds, a value remarkably close to modern measurements. This achievement not only provided definitive observational proof of Earth’s motion around the Sun, but also established a quantitative baseline for calculating interstellar distances.

Based on the passage, Bessel meticulously recorded ambient temperature changes in the dome primarily because thermal variations caused which specific physical effect on his telescope?