Orbit determination just from historical observations? Test case: The comet of AD 760 is identified as 1P/Halley

Abstract

Recent advances in techniques of critical close reading of historical texts can now be applied to records of pre-telescopic celestial observations – allowing significant progress for analyzing and solving orbits of past comets: here, as a blueprint test case, we exemplify our method by solving the orbit of the comet in AD 760 only with historical observations and then identify it with 1P/Halley. A detailed eyewitness record with drawing of a comet in AD 760 in the Syriac Chronicle of Zuqnīn (finished AD 775/6) was not yet included in the study of its orbit – the Chinese reports alone do not yield a sufficient number of dated positions. We analyze the Syriac and Chinese sources with critical methods for quantitative astronomical usage, we also consider a few further records from the Mediterranean and West Asian area. With our conservatively derived dated positions we can determine the best fitting Keplerian orbital solution by least squares fitting yielding the orbital elements (χred2 < 2 based on 1 million runs); the parameter ranges for non-periodic solutions and highly eccentric periodic solutions are consistent with each other. The allowed parameter ranges for perihelion distance and inclination are sufficiently small to identify the comet with 1P/Halley. Although 1P/Halley is the only comet, where the telescopic orbit is credibly linked to pre-telescopic returns, e.g. to AD 760, our identification confirms claims from extrapolating telescopic observations backward in time – here independently based on historical data. In particular, we obtained a precise perihelion time (760 May 19.1 ± 1.7). The inferior conjunction between comet and Sun as on the previously published orbit (760 May 31.9, Yeomans and Kiang, 1981) is shifted by about one day compared to our new orbit (June 1.8), only the new one is consistent with the last observation (June 1.0) before conjunction as reported in the Chronicle of Zuqnīn. Such a precision would be most critical for studying non-gravitational forces on comets. By studying the comet's brightness evolution, we also compute its absolute brightness and activity parameter for AD 760 and found indications that the comet was quite dusty that year. As the last return before a close encounter with Earth in AD 837, the AD 760 perihelion is particularly important for extrapolation further back in time (at AD 837 and AD 800, Yeomans and Kiang, 1981 had to introduce corrections in their standard orbit). Our improved methods developed in a multidisciplinary cooperation offer possibilities also to solve more orbits and to identify more comets from the rich and widespread pre-telescopic transmissions.