Boskovi\'c's Spherical Trigonometric Solution for Determining the Axis and Rate of Solar Rotation by Observing Sunspots in 1777
Abstract
In September 1777 Ru er Boskovi\'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 Boskovi\'c. We recalculated Boskovi\'c's original example using his measurements of sunspot positions from 1777 and the equations developed here, confirming his results from 1785. Boskovi\'c's methodology of arithmetic means determines i, , and sidereal period T' separately, while the planar trigonometric solution determines i and together. His spherical trigonometric solution calculates i, , and the sidereal period T' in a single procedure. Keywords: Ru er Boskovi\'c, Sunspots, Solar rotation, Spherical trigonometry
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