Daylight observation of 1P/Halley in 1222

It has been questioned whether comet 1P/Halley that appeared on 1222 September 9 was bright enough to be observed in daylight, as recorded in the Korean account. We investigate the light curve of 1P/Halley in the 1222 apparition considering brightness enhancement of the comet by forward-scattering, in order to verify the Korean account. We first analyze the Korean accounts of 1P/Halley observed in 1222, and of Venus and Jupiter appearing in daylight generally. We then determine the absolute magnitude (m0) and heliocentric power-law exponent (n) using observations made around the perihelion in 1986 and a formula considering brightness enhancement by forward-scattering. We apply the results to estimate the light curve of 1P/Halley in the 1222 event. According to the accounts, 1P/Halley underwent a sudden change in brightness and the conditions were good on the day the daylight observation was recorded. We also find that Jupiter could have been visible in daylight at about −2.1 mag. Regarding the light curve, we find that if the m0 and n values are similar to those in a previous study, 1P/Halley could have been reached a maximum near mag −2 in 1222. In conclusion, there is a strong possibility that 1P/Halley was actually observed in daylight, inferring from observations and the predicted total visual magnitude at that time.