Precision of measurements and the biases due to the geomagnetic declination in the Korean observations of the comet C/1723 T1

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Abstract Positions of the comet C/1723 T1 recorded in Korean historical resources are analysed to estimate the precision of measurements and the bias due to the geomagnetic declination at Seoul. We calculate the ephemeris of the comet by adopting the orbital elements determined by Gustav Spörer in 1843 based upon the Bradley’s telescopic observations in 1723. We confirm that the Bradley’s observations have a measurement error of 25″. The positions of the comet recorded in the Korean manuscripts are compared with the modern ephemeris. We find that there are misalignments in the instrument’s polar axis with Earth’s rotation axis: if the measurements were rounded up, there are alignment errors of 0.°0 ± 0.°2 in elevation and $-{0{_{.}^{\circ}}3}\pm {0{_{.}^{\circ}}3}$ in azimuth (to 1σ); if the measurements were rounded down, there are alignment errors of −0.°5 ± 0.°4 in elevation and +0.°5 ± 0.°6 in azimuth (to 1σ). Assuming that the instrument’s polar axis was aligned with a magnetic compass, we can regard the misalignment of the polar axis in azimuth as the geomagnetic declination at Seoul in 1723. We find that the geomagnetic declination obtained is in agreement with the recent models of the historical geomagnetic fields. We also obtain the one-time measurement error of ${0{_{.}^{\circ}}25}$, which is not different from the one-time measurement error of ${0{_{.}^{\circ}}3}$ obtained from the observational data of Mars in 1491.