Daytime Visibility of Halley's Comet in 1222

We reexamine the Goryeosa (History of the Goryeo Dynasty, A.D. 918-1392) account that Halley's Comet was seen during the daytime on September 9, 1222. To verify whether the referenced "daytime" refers to twilight or daylight (i.e., when the Sun is above horizon), we determine the absolute magnitude and heliocentric power-law exponent for Halley's Comet using observations made around the perihelion in 1986 and a formula considering the brightness enhancement by forward-scattering. We then apply the results to estimate the light curve of Halley's Comet in the 1222 event and find that the total visible magnitude could reach a maximum of -1.7 on September 8, one day before the Goryeosa's account. Therefore, we think that Halley's Comet with a coma of -1.7 mag and tail-length of about $20^{\circ}$ was actually observed during the day on September 9 because the observational conditions on that day were so good that Venus was visible in daylight. Furthermore, we think that the event might have been witnessed in the morning sky because the contents of Venus's culmination (occurred around September 9.07 TT) continue on the same day account.