Supernova was observed in the sky it was visible for 23 days

SN 1054 is one of eight supernova in the Milky Way that can be identified because written testimony describing the explosion has survived. In the nineteenth century, astronomers began to take an interest in the historic records. They compiled and examined the records as part of their research on recent novae, comets, and later, the supernovae.

The first people to attempt a systematic compilation of records from China were the father and son Biot. In 1843, the sinologist Édouard Biot translated for his father, the astronomer Jean-Baptiste Biot, passages from the astronomical treatise of the 348-volume Chinese encyclopaedia, the Wenxian Tongkao.

Almost 80 years later, in 1921, Knut Lundmark undertook a similar effort based on a greater number of sources. In 1942, Jan Oort, convinced that the Crab Nebula was the “guest star” of 1054 described by the Chinese, asked sinologist J.J.L. Duyvendak to help him compile new evidence on the observation of the event.

Chinese astronomy

The sky on the morning of 4 July as seen from China, when the supernova was presumably first observed (the location is indicated by the marker at the center of the image).

Stars which appeared temporarily in the sky were generically called “guest stars” (kè xīng ) by Chinese astronomers. The guest star of 1054 occurred during the reign of the Emperor Renzong of the Song dynasty (960–1279). The relevant year is recorded in Chinese documents as “the first year of the Zhihe era”. “Zhihe” was an era name used during the reign of Emperor Renzong, and corresponds to the years 1054–1056 C.E., so the first year of the Zhihe era corresponds to the year 1054 C.E.

Some of the Chinese accounts are well preserved and detailed. The oldest and most detailed accounts are from Song Huiyao and Song Shi, historiographical works of which the extant text was redacted perhaps within a few decades of the event. There are also some later records, redacted in the 13th century, which are not necessarily independent of the older ones. Three accounts are apparently related because they describe the angular distance from the guest star to Zeta Tauri as “perhaps several inches away”, but they are in apparent disagreement about the date of appearance of the star. The older two mention the day jichou 己丑, but the third, the Xu Zizhi Tongjian Changbian, the day yichou 乙丑. These terms refer to the Chinese sexagenary cycle, corresponding to numbers 26 and 2 of the cycle, which corresponds, in the context where they are cited, respectively, to 4 July and 10 June. As the redaction of the third source is of considerably later date (1280), this is easily explained as a transcription error, the historical date being jichou 己丑, 4 July.

The location of the guest star as “to the south-east of Tianguan, perhaps several inches away “ has perplexed modern astronomers, because the Crab nebula is not situated in the south-east, but to the north-west of Zeta Tauri.

The duration of visibility is explicitly mentioned in chapter 12 of Song Shi, and slightly less accurately, in the Song Huiyao. The last sighting was on 6 April 1056, after a total period of visibility of 642 days. This duration is supported by the Song Shi. According to the Song Huiyao the visibility of the guest star was for only 23 days, but this is after mentioning visibility during daylight. This period of 23 days applies in all likelihood solely to visibility during the day.