Not the Earth, but its orbit: Andre Tacquet and the question of star sizes in a heliocentric universe

TL;DR

This paper translates and analyzes Andre Tacquet's 1668 discussion on star sizes in a heliocentric universe, highlighting his argument that lack of observable parallax implies stars are extremely large, comparable to Earth's orbit.

Contribution

It provides the first detailed translation and analysis of Tacquet's critique of heliocentric star size arguments, which has been largely overlooked in history of science.

Findings

Tacquet argued star sizes must be enormous due to lack of parallax.

His reasoning links apparent star sizes to the scale of Earth's orbit.

The paper clarifies historical debates on star sizes in early heliocentric theory.

Abstract

This paper consists of a translation of Andre Tacquet's discussion of the question of sizes of stars in a heliocentric universe, as published in his posthumous Opera Mathematica of 1668, along with introductory material and analysis. While Robert Hooke mentions Tacquet as one of the "great Anti-copernicans", who argued the question of star sizes against the heliocentric theory with "great vehemency and insulting", Tacquet's discussion has received only scant attention. The kernel of Tacquet's argument is that the absence of any detectable parallax in the stars, combined with the measured apparent sizes of the stars, means that, in a heliocentric universe, the sizes of stars compare to the size of Earth's orbit via the same proportion that they compare to the size of the Earth in a geocentric universe. The translated material presents this argument in a straightforward manner, insulting absent.