Bo\v{s}kovi\'{c}'s Spherical Trigonometric Solution for Determining the Axis and Rate of Solar Rotation by Observing Sunspots in 1777

TL;DR

This paper revisits Bošković's 18th-century spherical trigonometric method for determining the Sun's rotation axis and rate from sunspot observations, deriving modern equations and confirming his original results.

Contribution

It derives explicit equations for Bošković's spherical trigonometric method, enabling modern computation of solar rotation parameters from historical sunspot data.

Findings

Confirmed Bošković's original calculations using new equations.

Demonstrated the efficiency of the spherical trigonometric approach.

Compared different methods for determining solar rotation elements.

Abstract

In September 1777 Ru\dj er Bo\v{s}kovi\'{c} observed and measured the sun-spot positions to determine the solar rotation elements. In 1785, among other methods, he described a trigonometric spherical solution for the determination of the position of the axis and rate of the solar rotation using three sunspot positions, but without equations. For the first time, we derive the equations that are applicable to modern computers for calculating the solar rotation elements, as they were described by Bo\v{s}kovi\'{c}. We recalculated Bo\v{s}kovi\'{c}'s original example using his measurements of sunspot positions from 1777 and the equations developed here, confirming his results from 1785. Bo\v{s}kovi\'{c}'s methodology of arithmetic means determines $i$, $\Omega$, and sidereal period $T'$ separately, while the planar trigonometric solution determines $i$ and $\Omega$ together. His spherical trigonometric solution calculates $i$, $\Omega$, and the sidereal period $T'$ in a single procedure. Keywords: Ru\dj er Bo\v{s}kovi\'{c}, Sunspots, Solar rotation, Spherical trigonometry