On the Accuracy of Galileo's Observations

TL;DR

Galileo's precise observations were limited by optical diffraction, meaning his measurements of stellar sizes reflected diffraction artifacts rather than actual physical dimensions.

Contribution

This paper clarifies that Galileo's measurements were affected by diffraction, highlighting the limitations of his observational accuracy due to the lack of wave optics knowledge.

Findings

Galileo achieved measurement accuracy of 2 arcseconds or better.

His measurements of stellar sizes were influenced by diffraction artifacts.

Galileo's observations did not directly measure physical sizes of stars.

Abstract

Galileo Galilei had sufficient skill as an observer and instrument builder to be able to measure the positions and apparent sizes of objects seen through his telescopes to an accuracy of 2" or better. However, Galileo had no knowledge of wave optics, so when he was measuring stellar apparent sizes he was producing very accurate measurements of diffraction artifacts and not physical bodies.