Astronomy in Early Mikkyō in Japan

Astronomy in Early Mikkyō in Japan: Comparison

Please note this is a comparison between Version 2 by Jason Zhu and Version 1 by Jeffrey Kotyk.

Chinese calendrical science and observational astronomy were introduced to Japan, at the same time, there were abundant Indian materials directly or indirectly related to astronomy, astrology, and calendars also transmitted via Buddhism. Although these were arguably unimportant to earlier types of Buddhism (at least as far as the attainment of bodhi or advancement along the bodhisattva path were concerned), the situation changed considerably with the advent of Mikkyō. The scriptures and orthopraxy of Mantrayāna required an understanding of astrology and, in some cases, expertise in observational astronomy. Although such sciences were available to the saṃgha in India, the circumstances were different in China, where legal codes and even just the impracticalities of converting between Indian and Chinese systems existed. The situation became even more complex for Japanese monks who had to operate using borrowed Chinese models and only a handful of resources. Still, the available data shows that early Mikkyō monks were aware of astrology, and it was taken into consideration.

astronomy
Japanese Buddhism
calendar

1. Astrology as a Requisite Element in Mantrayāna

Unlike earlier forms of Mahāyāna, in which matters related to astronomy were strictly unnecessary for the success of a bodhisattva’s career, the situation changed considerably with the advent of Mantrayāna in the seventh century in India. Śubhakarasiṃha (Ch. Shanwuwei 善無畏; 637–735), one of the early Indian pioneers of this practice who facilitated its introduction to China, translated the Susiddhikara-sūtra (Ch. Suxidijieluo jing 蘇悉地羯羅經). Therein, it is observed the critical importance of astronomical knowledge:

During a lunar eclipse, achieve supreme activities. During a solar eclipse, do upper, middle, and lower things related to attainment. On the fifteenth, seventh, first, or thirteenth of the month, or on the third, one should carry out [things related to] attainment and all activities. If carrying out supreme attainments, one should select a time with a nakṣatra and planet that is upper [in quality]. The method for middle and lower should be understood in this sort of manner; but among the nakṣatras, Puṣya is foremost. In the case of a wrathful attainment, rely upon times suited to wrathful nakṣatras and planets.[7] (T. 893a, 18: p. 625c9-16)

Here, not only is the selection of auspicious nakṣatras held as important, but the reader is told that the foremost ritual activities are to be carried out during a lunar eclipse. Predicting a lunar eclipse in ancient times was no easy task, since it required knowledge of advanced mathematical astronomy. This way of timing specific rituals—a form of electional astrology—is unprecedented in the earlier Buddhist literature. A properly timed occasion based on astrological factors is also made necessary when creating a maṇḍala according to another major text: the Vairocanābhisaṃbodhi (Ch. Dari jing 大日經), which Śubhakarasiṃha and Yixing 一行 (673–727) translated in 724. (This text became foundational for both Shingon and Tendai in the following century.) Regarding the creation of the maṇḍala, the Vairocanābhisaṃbodhi reads as follows:

On the morning of a propitious day, having determined a day on which the time, lunar mansion [nakṣatra], and planets are all in harmony, and at a time before the [morning] meal, with an auspicious sign […]. (T. 848, 18: p. 4c4–5)8^[1]

The cleric presiding over the maṇḍala must select an auspicious time based on these factors. The maṇḍala is one element in the ritualized incorporation of the three mysteries of body, speech, and mind. The three mysteries are the means to complete awakening (i.e., buddhahood) in a single lifetime. This accelerated model does away with the need to pass through the 10 bodhisattva stages. This break from Mahāyāna ideas about a bodhisattva requiring three incalculable aeons is one of the major innovations of Mantrayāna. This potential for accelerated development—the goal in practice—is expressly affirmed in the Vairocanābhisaṃbodhi at the beginning of the scripture:

Moreover, he assumed the appearance of vajradharas and the bodhisattvas Samantabhadra, Padmapāṇi, and so on, and proclaimed everywhere in the ten directions the Dharma of the pure words of the mantra path so that [all the steps from] the initial generation of the [bodhi-]mind up to the ten stages may be progressively satisfied in this lifetime […]. (T. 848, 18: p. 1b2-4)9^[2]

It is necessary to take into consideration astrology in order to properly create the maṇḍala, since it cannot be produced at any random time. The astrological factors to consider are further explained and emphasized in the Chinese commentary to the Vairocanābhisaṃbodhi, which incorporated the oral explanations of Śubhakarasiṃha, as well as Yixing’s own notes.10^[3] It logically follows that knowledge of astrology is one requisite element in the wider set of skills and practices necessary to successfully achieve bodhi in a single lifetime according to the framework of Mantrayāna. The powers who preside over astrology (e.g., the planets, zodiac signs, and nakṣatras) also therefore have a role in enabling this process toward bodhi, and hence they also appear in the maṇḍala.11^[4]

One problem is that the commentary to the Vairocanābhisaṃbodhi only offers a basic overview of astrology. The commentary moreover defers to the “Indian calendar” (Ch. fan li 梵曆), but barely any information on Indian calendrical science was available at the time in Chinese (Kotyk 2018b, pp. 17–18). A few years earlier in 718, the Jiuzhi li 九執曆 (Calendar of the Nine Planets; Skt. *Navagraha-karaṇa) was translated by a member of the Gautama family, named either Siddhārtha or Siddha (Ch. Qutan Xida 瞿曇悉達), but this is a manual of advanced mathematical astronomy that moreover would have required previous familiarity with Indian timekeeping. Yixing might have had the opportunity to rectify this problem of a lack of authoritative materials in Chinese, which would explain the Indian calendar, given his background expertise in astronomy (he reformed the state calendar), but he died prematurely in 727. The need for an authoritative manual on astrology in Chinese remained after Yixing’s passing. In the following generation, the eminent monk Amoghavajra (Ch. Bukong 不空; 705–774) and his team compiled the first draft of the Xiuyao jing 宿曜經 (Sūtra of Lunar Stations and Planets) in 759. This version was problematic, and a revision was undertaken in 764.12^[5] The text was adequate for understanding only the mere basics of Indian astrology, but it also introduced ideas about natal astrology (e.g., predictions about a person based on their time of birth). Astrology in Mikkyō came to be almost entirely understood based on this text after it arrived in Japan.

2. Astrology in Early Mikkyō: Challenges in Defining the Stars

The importance of the Xiuyao jing (Jp. Sukuyō kyō) to Mikkyō cannot be understated, since it became the primary manual to consult when scheduling rituals and other religious activities in Mikkyō, and it was moreover also read by people outside said tradition. Minamoto no Shitagō, for instance, cites it in the Wamyō ruiju shō with a variant kanji: Sukuyō kyō 宿耀經 (maki 1, p. 3). The Xiuyao jing was first brought to Japan by Kūkai 空海 (774–835), the founder of Shingon, when he returned home in 806. The text is recorded in his catalog of items brought from China (T. 2161, 55: p. 1062a23-24). In the following generation, Tendai monks also took an interest in astrology early on. Enchin lists the Xiuyao jing in his catalog of texts and other items brought back to Japan in 857 (T. 2172, 55: 1098b8). Takada (1992, p. 40) showed that Enchin composed some works on the Xiuyao jing: Sukuyō gishū 宿曜疑集 (Compilation of Concerns on Xiuyao) and Sukuyōkyō mondō 宿曜經問答 (Questions and Answers on the Xiuyao jing).

It is clear that Kūkai took an interest in the astrology of the Xiuyao jing and furthermore facilitated its implementation. This fact is demonstrated by the oral testimony recorded in the Hino’o kuketsu 檜尾口訣 (Oral Transmission at Hino’o). This is a record of Kūkai’s instructions preserved by his disciple, Jichie 實慧 (786–847) of Tō-ji 東寺 (see discussion in Yamashita 1996, p. 297). The explanations address two problems: those of intercalary months and short months, neither of which are discussed in the Xiuyao jing. The following is recorded in the Hino’o kuketsu:

Inquiry concerning which nakṣatras to use during an intercalary month and method for assigning nakṣatras for missing days in a short month. Recorded based on oral instructions. When there is an intercalary month, the nakṣatra convergences of the true month repeat themselves in the intercalary month. Supposing the twelfth month has an intercalary month, the nakṣatra convergence of 12/1 will be Dhaniṣṭhā, the nakṣatra convergence of [12/]15 will be Maghā and the nakṣatra convergence of [12/]30 will be *Pūrvabhādrapadā. The nakṣatra convergences of an intercalary twelfth month will be identical like this. There are no differences. The preceding month is the true twelfth month. The intercalary month is the contingent twelfth month. The nakṣatra convergences and 30 days of the contingent month all use the nakṣatra convergences of the true twelfth month without any different nakṣatras. The other months and intercalary months can be understood according to this.13^[6] (T. 2465, 78: p. 30c13-c21)

The Xiuyao jing provides a system of correspondences between each day of the lunar months and a nakṣatra. The days of the 12th month, for example, run as follows: (1) 虚 Dhaniṣṭhā, (2) 危 Śatabhiṣaj, (3) 室 Pūrvabhādrapadā, (4) 壁 Uttarabhādrapadā, (5) 奎 Revatī, (6) 婁 Aśvinī, (7) 胃 Bharaṇī, (8) 昴 Kṛttikā, (9) 畢 Rohiṇī, (10) 觜 Mṛgaśīrṣa, (11) 參 Ārdrā, (12) 井 Punarvasū, (13) 鬼 Puṣya, (14) 柳 Aślesā, (15) 宿 Maghā, (16) 張 Pūrvaphālgunī, (17) 翼 Uttaraphālgunī, (18) 軫 Hasta, (19) 角 Citrā, (20) 亢 Svāti, (21) 氐 Viśākhā, (22) 房 Anurādhā, (23) 心 Jyeṣṭha, (24) 尾宿 Mūla, (25) 箕 Pūrvāṣāḍhā, (26) 斗 Uttarāṣāḍhā, (27) 女 Śravaṇa, (28) 虚 Dhaniṣṭhā, (29) 危 Śatabhiṣaj, and (30) 室 Pūrvabhādrapadā. In the case of an intercalary 12th month (or any other month), one simply reproduces this exact sequence, and then the following ordinary month resumes.

This is a reasonable solution, but the reality is that this model of assigning days to nakṣatras (which was devised by Amoghavajra and his team) is actually divorced from the precisely observed position of the Moon and would have appeared irregular from the perspective of any Indian astronomer at the time. The Xiuyao jing also interestingly describes an Indian model of 27 nakṣatras of uniform dimensions that are aligned with the zodiac signs (Wakita 1897, vol. 1, pp. 6–8). The spatial dimensions are defined according to pādas (“limbs, i.e., quarters”) rather than muhūrtas. (This new model in Sanskrit is called the navāṃśas, meaning “ninths” of a zodiac sign.) This is completely different from the earlier model described in the Śārdūlakarṇāvadāna and reflects a later reformulation of the astrometric parameters for measuring the stars in India. The resulting model is a practical system of dividing the ecliptic into manageable divisions. The described model is illustrated below.

Amoghavajra and Kūkai were certainly aware of this model, but nobody in either Chinese or Japanese Buddhism ever attempted to implement it, as far as I am aware. This model was also endorsed in the commentary to the Vairocanābhisaṃbodhi, and hence people know that Śubhakarasiṃha himself was also aware of it (T. 1796, 39: p. 618a8-13).14^[7] It simply remained as an astrometric system left unused, albeit still known, in East Asia.

The other system of assigning days of the lunar month to nakṣatras aligns easily with the East Asian calendar and, in practice, requires no knowledge of astronomy or even observation of the Moon. Adopting a new foreign system of astronomical measurements would have been impractical, even though, according to scriptures, an orthodox model of the nakṣatras (such as that of the navāṃśas) was technically necessary in order to time rituals according to Indian astrology. The reseauthor(s)rchers of the Susiddhi-kara clearly had observational astronomy in mind when their writings pointed to the prediction of eclipses. The Vairocanābhisaṃbodhi also clearly states that nakṣatras must be considered. The elite members of the Chinese saṃgha in the capital might have been able to directly consult with Indian astronomers about these matters in detail, but the Japanese saṃgha did not have this opportunity. Technical works on Indian astronomy, such as the Navagraha-karaṇa, were never brought to Japan (they were also not widely studied in China). Another obstacle to employing astronomy was the fact that unauthorized study of this topic was technically illegal according to the law codes of the Tang, although the relevant laws were enforced less and less in the later years of the dynasty (Whitfield 1998). Japanese monks such as Kūkai simply would not have had easy access to astronomy. Japanese students of astronomy as part of the Kentōshi 遣唐使 (“Missions to the Tang”) would have studied astronomy (the Chinese, not Indian, system) under the supervision of the state. Local and foreign monks did not have this opportunity, apart from the unique career of Yixing, who was a court astronomer (Kotyk 2018d). The adaptations employed in Mikkyō resulted in a form of hemerology based only vaguely on the Indian calendar, which was clearly an inevitable compromise given the circumstances. The explanation in the Hino’o kuketsu continues as follows:

Method for selecting the nakṣatra convergence on the missing thirtieth day in a short month. Suppose the first month lacks the thirtieth day. Even without that thirtieth day, that day still has a nakṣatra convergence, hence the first half of the first day of the following second month is assigned the nakṣatra convergence of the missing day in the first month, while the second half is that day’s nakṣatra convergence. Thus, even if there are fluctuations with the days, in the case of a short month lacking a day, the nakṣatra convergence will always apply to a day. The actual nakṣatra of the first day in the following month does not change.15^[8] (T. 2465, 78: p. 30c21-c26)

This explanation further illustrates that the calendrical day, rather than the observed position of the Moon, is of primary importance. In the case of short months, one half of a day is assigned to one nakṣatra. This would be irregular if the position of the Moon were involved since, in practice, it would only account for half of the Moon’s time in a given nakṣatra. This problem, however, does not arise, because the assigned nakṣatra is a nominal designation. This system devised by Amoghavajra attempts to align the days of the lunar month so that the Full Moons transit through the nakṣatra from which a given month derives its name in Sanskrit (e.g., the Full Moon, i.e., the 15th day, should be aligned with Kṛttikā 昴宿 in the 9th month), but the resulting model is unwieldy and not based on observation.

On this point, wpeople might ask how Mikkyō understood nakṣatras as asterisms (i.e., as actual constellations visible in the sky). There was an awareness that the asterisms connected to the Chinese lunar stations only vaguely correspond to nakṣatras. In other words, the constellations which are used for spatial identification are, in fact, different between the Indian and Chinese models. Unlike the typeset editions of the Xiuyao jing, the manuscript of the Xiuyao jing from 1322, held at Dōshisha University (148.8|F9632), reproduces the illustrations of asterisms. The text explicitly states that the nakṣatras are different from what is used in China and provides illustrations:

In the astronomy of the country of Tang, Mao is seven stars. Now according to the explanation in this sūtra, the stars are not the same as those of the Great Tang, thus we rely on the astronomy of the Great Tang. Each are illustrated following the corresponding lunar station.16^[9]

Early Indic sources such as the Nakṣatrakalpa of the Atharvaveda, Śārdūlakarṇāvadāna, and Gargasaṃhitā all define Kṛttikā as comprising six determinative stars, called yogatārās (Pingree and Morrissey 1986, p. 102). The Mātaṅga-sūtra in Chinese also gives six stars (T. 1300, 21: p. 404c10-11).

The Japanese side did not necessarily always favor the Chinese definition, and in some instances, the Indian number is cited. For instance, the section on astronomy in the Jiu Tang shu 舊唐書 (Old Book of Tang), the dynastic history of the Tang finished in 945 by Liu Xu 劉昫 (887–946), clearly states in its section on astronomy that “Mao is seven stars 昴七星 (Jiu Tang shu 35.1300).” In contrast, the Wamyō ruiju shō (maki 1, p. 3) reads “The Xiuyao jing states that the asterism Subaru is six stars and of the fire god [Agni] 宿耀經云昴星六星火神也.” It is significant that the Xiuyao jing was cited in this dictionary, since it demonstrates that in the 10th century, people outside the immediate Buddhist fold indeed read the Xiuyao jing, but more important to our present discussion, it can be observed that the official star counts of state sources from China did not necessarily take precedence in Japan.

The ambiguity concerning star counts is also reflected in the iconographical record. It is through illustrations that people can also see how different models of uranography (the illustration and mapping of the heavens) were present in Japan. This points to the fact that Mikkyō possessed astronomical materials originating from diverse sources. For example, the famous Kara zu 火羅圖 (Hora Diagram) for Mao/Subaru displays seven stars (the Chinese number) alongside a bodhisattva or Buddha-like figure.

The illustration of the asterism in the Kara zu is noteworthy, in light of seven stars being illustrated. people can point to other variants in the literature available to early Mikkyō. There are unique icons in the Qiyao xingchen bie xingfa 七曜星辰別行法 (Special Practices for the Seven Planets and Stars; T. 1309). This is an illustrated manual that documents spirits who affect people on specific days of the month. It is nominally attributed to Yixing based on the fantastical story told in the text about him summoning down the stars (Kotyk 2018d, pp. 23–27). Researchers know this text was brought to Japan by Eun 惠運 (798–869) in 847 as it appears in his catalog of items (T. 2168A, 55: p. 1088b11). The spirits, apart from a few, are all associated with lunar stations, but these depictions, primarily anthropomorphic and zoomorphic, are unlike anything seen in contemporary Indic or Chinese sources.

Mao/Subaru shows six stars, which correspond to the classical Indian number. Nü/Urukiboshi, however, displays what appears to be 2 asterisms with 11 stars in total. The Chinese number is four stars according to the Jiu Tang shu (Jiu Tang shu 35.1299). The corresponding nakṣatra, Śravaṇa, is three stars in the ancient Indian texts. people might compare these asterisms to three other sources: the Dōshisha manuscript of the Xiuyao jing, the “Dunhuang Star Chart” (British Museum Or.8210/S.3326), which is the oldest extant complete star chart from East Asia, and a later but quite valuable resource in the history of astronomy, the Gujin lüli kao 古今律曆考 (Study of Ancient and Present Tunes and Calendrical Sciences), which is a compendium with rich information on Chinese astronomy compiled by Xing Yunlu 邢雲路 (1549–?). In the last item, people see a detailed uranographical illustration titled Yaodian zhong xingtu 堯典中星圖 (Star Chart from the Classic of Yao). This displays the classical constellations connected to the Chinese lunar stations. When comparing these sources, it is evident that the respective illustrators were drawing from various sources. people might imagine that a scribe might have mistakenly merged the constellations of Nü and Dou 斗 in the case of Nü in T. 1309. A Mikkyō monk in the late ninth century with access to the Xiuyao jing and T. 1309 would have been looking at often dissimilar asterisms, which would have furthermore differed from “canonical” or official Chinese sources.

The depicted anthropomorphic figures in T. 1309 are also anomalous and require a brief explanation. Mao/Kṛttikā is associated with Agni in Buddhist and more broadly Indic lore. Nü/Śravaṇa is associated with Viṣṇu (Yano 2003, p. 380). The depictions display nothing conceivably connected to Vedic deities. The clue to identifying the source of these figures is the illustrated nails driven into them. The text tells uspoeple their function as follows:

It was instructed that should someone see the forms of the spirits with their eyes, then nail the top and bottom of the forms of the spirits and the following day bind them. The victim [of the spirit] will recover that day. When the nails are removed, the spirit will gain release and dare not come before one’s gate.17^[10] (T. 1309, 21: 456c29-457a2)

I believe these spirits are jinn (djinn) based on their visual representations, often with bestial and zoomorphic features, and the fact that nails are used to bind them.18^[11] The latter in particular is a key element in jinn lore. Omidsalar (2000) explains that “the jinn are especially afraid of iron, and anyone who manages to insert an iron needle in their bodies or clothes, gains control of them because their great fear of iron prevents them from pulling the needle out of their persons or attire.” If these spirits are in fact derived from jinn lore, then it is plausible that variant uranographical material also might have entered into the wider body of the astrological literature in East Asia, although this is only a speculative possibility at present. In any case, people can see that Mikkyō possessed variable illustrations of asterisms, but there was not an evident grasp on the canonical Indian asterisms linked to the nakṣatras.

A similar observation can be made with respect to the zodiac signs. The zodiac signs appear in the Taizō mandara 胎藏曼荼羅, which is the maṇḍala of the Vairocanābhisaṃbodhi as preserved in Japan. There are even seed syllables assigned to each of the zodiac signs (Somekawa 2013). Yet, people might ask, did anyone in early Mikkyō know about the original constellations associated with the zodiac signs? How were the zodiac signs understood? Again, people can look back to the commentary to the Vairocanābhisaṃbodhi, where people read what is likely Yixing’s voice:

The ecliptic is divided into twelve chambers like the twelve [Jupiter] stations here [in China]. Each station has nine quarters [pāda].19 ^[12](T. 1796, 39: p. 618a8-9.)20^[13]

Here, the zodiac signs are simply equated to Jupiter stations. The latter are an ancient Chinese concept that demarcates the celestial equator into 12 spaces based on the orbital period of Jupiter. This period is approximately 12 years, or more precisely, the sidereal orbital period is 11.86 years. A zodiac sign, in contrast, is one of 12 30° segments of the ecliptic (not the celestial equator). One of the tables in the first fascicle of the Xiuyao jing illustrates how Amoghavajra understood the zodiac signs as equivalent to Jupiter stations. For example, Scorpio (Ch. Xie gong 蝎宮) is positioned among three lunar stations—Xin 心, Fang 房, and Di 氐—but no spatial degrees are indicated. Scorpio is equated to “Great Fire” (Ch. Da huo 大火), one of the Jupiter stations. Below this, “fire” (Ch. huo 火) denotes Mars, since Scorpio is the domicile of Mars in classical astrology (Libra is the domicile of Venus). The doctrine of domiciles is a concept which was originally a part of Hellenistic astrology that was adopted into Indian astrology.21^[14]

The zodiac signs were part of the canonical lore of Mikkyō, but they only exercised a symbolic role. As deities of the Taizō mandara, they appear in the Taizō zuzō in forms mostly recognizable to anyone familiar with the classical zodiac. Here, people even see Sagittarius as a centaur-like or satyr-like bowman, one of the rare instances in which this type of icon is found in premodern East Asia.22^[15] Although Kūkai and Enchin would have known about Leo and Sagittarius, did they know which constellations were associated with the zodiac signs? Unlike the lunar stations, I am unaware of any uranographical depictions of zodiac signs in the Mikkyō corpus or any East Asian materials from the period in question. The zodiac signs in the Kara zu are each associated with one of the months, so in that way, they were connected to the passage of time, but this is different from a precisely defined tropical or sidereal zodiac. In this sense, in Mikkyō, the zodiac signs were even more symbolic in function than the nakṣatras. This is noteworthy because in Indian astrology and astronomy, the zodiac signs were important in various ways, whether it be for making astrological predictions or positioning the planets.

There is one example from the ninth century in which the zodiac signs were loosely connected to the degrees of lunar stations. This is found in the Qiyao rangzai jue 七曜攘災決 (Secrets of the Seven-Planet Apotropaism). This is a manual of horoscopy compiled sometime between 806 and 865 which includes ephemerides (astronomical tables) and enough information to calculate the positions of the planets, in addition to a number of spells and mantras to ward off the prognosticated baneful effects of the planets. Shūei 宗叡 (809–884) brought it to Japan in 865. It is recorded in his catalog of items brought from China (T. 2174A, 55: p. 1111b20-21). In this instance, the approximate numbers of degrees for each lunar station are listed under the zodiac signs, which seems to imply that a zodiac sign includes all the degrees of any lunar station subsumed under them. This results in uneven allocations; Aries is 27, and Taurus is 27, but Gemini is 48, although Zi 觜 ought to be 1, and therefore Gemini would be 39 (cf. table in Yano 1986, p. 30). The zodiac signs are not of significance in the text, so here they appear to only be loosely defined. Each zodiac sign is associated with an earthly branch, which again denotes Jupiter stations (i.e., Taurus is you 酉, which corresponds to “Great Bridge” (Ch. Da liang 大梁). Again, this is only a vague way of defining the zodiac signs.

6. The Seven-Day Week in Mikkyō

The above discussion demonstrates that astronomy, timekeeping, and uranography from India (and possibly elsewhere to some extent) were largely left unimplemented or even unstudied in Mikkyō. However, the seven-day week was, in fact, implemented from the very beginning. The concept of the week was easily implemented, since one need only keep track of the passage of days. The main source of information in Mikkyō about the days of the week was evidently the Xiuyao jing. The seven-day week was used primarily as a kind of schedule for religious activities rather than for timekeeping.

The earliest wpeople can trace the seven-day week in the case of Japan is to Kūkai’s return in 806, when he brought with him a copy of the Xiuyao jing. Shingon traditionally credits Kūkai with introducing the concept of Sunday to Japan. Shōken 聖賢 (1083–1147) in 1118 wrote the following:

Before the Daidō reign era [806–810], calendar experts did not know of Sunday. This is why there was confusion about astrological auspiciousness and inauspiciousness. People often violated this. After the Great Master [Kūkai] returned to court, he transmitted this practice.23 (^[16]Kōya Daishi go kōden, p. 661b14–17)

The essential concept here is that some days are more auspicious than others for certain activities. This is all explained in detail in the Xiuyao jing. The existing civil calendar, based on the Chinese system, divided the month into 3 segments of 10 days each (jun 旬). The seven-day week did not supplant this earlier system. During the ninth century, people also see Sunday cited alongside the conventional date. For example, the Ono rokuchō 小野六帖 by the Shingon monk Ningai 仁海 (951–1046) records the following date and highlights that it was a Sunday:

12/13 in year 10 of Jōwa, Sunday, at Tō-ji [6 January 844] (T. 2473, 78: p. 76b19).24^[17]

It is clear that Tendai also adopted the seven-day week early on. The biography of Ennin, the Jigaku Daishi den 慈覺大師傳 (Biography of Great Master Jigaku) by Minamoto no Fusaakira 源英明 (d. 939), states that in the year 849, Ennin had to identify an auspicious day on which to create the Vajradhātu-maṇḍala (Kongōkai mandara 金剛界曼荼羅). He found a suitable day using sukuyō 宿曜, which at this point seems to have primarily denoted hemerology based on the Xiuyao jing rather than astrology in the sense of prognosticating the fate of an individual. Ennin identified a kanro nichi 甘露日 or “Day of Amṛta” (8-2: 691b5–8). This is a concept from the Xiuyao jing, in which a “Day of Amṛta” (especially favorable for religious rites) becomes constituted when there is a specific configuration of a nakṣatra and day of the week (Wakita 1897, vol. 1, p. 33). In this case, Ennin found that lunar 5/8, which is associated with Hasta (Ch. Zhen 軫), fell on a Sunday, and thus this auspicious day was identified (Yamashita 1996, p. 297). The Sanskrit name underlying this concept is not given in any East Asian sources of which I am aware, but this concept is attested in Indic sources and even still in modern India as amṛtasiddhiyōga.25^[18] It is indeed noteworthy that Mikkyō and Indian astrology both continue to observe this practice.

The seven-day week, with each day associated with a planet, clearly became an important element within Mikkyō, but it must be pointed out that, in reality, this is not strictly connected to astronomy. The positions of the planets are unrelated to the days of the week. The original system was Greco-Egyptian in origin, in which each of the 24 h of a day was associated with a planet in the order of Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon (the Chaldean ordering, running from the slowest to fastest moving planets from a geocentric perspective). The ruler of the first hour of a day becomes the ruler of that day. This results in the ordering of Sunday (Sun), Monday (Moon), Tuesday (Mars), etc. (Yano 2003, p. 383). The planetary hours are unknown in East Asian sources, but the days of the week were adopted from the eighth century in China and then the ninth in Japan. The recurring cycle of seven days could be easily tracked alongside the progression of the sexagenary cycle without any need for observation of the sky. The week therefore was, strictly speaking, another form of hemerology, rather than an element of astrology requiring astronomical knowledge.

The iconographical data from early Mikkyō suggests that the seven planets were more associated with the days of the week, rather than being wandering stars in the sky to be carefully tracked. In this sense, the planets take on a more abstract form rooted in the passage of time (meaning they rule over days) rather than the movement of visible planetary bodies above, much in the same way that the nakṣatras were not generally treated as asterisms or spatial divisions based on astrometrics in Mikkyō. Researchers can infer that this was, in fact, the reality, based on the Kuyō hiryaku 九曜秘暦 (Secret Calendar of the Nine Planets). This text includes rich lore about each of the seven planets as days of the week, as well as the pseudo-planets Rāhu and Ketu, although comments about these two are limited. The well-preserved manuscript at the NYC Metropolitan Museum is a reproduction by Sōkan 宗觀 in the year 1125, but the postscript states that the original manuscript was an earlier copy produced in Tengyō 天慶 3 (940), although it is uncertain whether the text was originally a Chinese or Japanese composition.26 ^[19]This earlier copy was apparently a reproduction of an earlier manuscript, so researchers might assume the original text was composed sometime before 940, perhaps in the ninth century. The text provides mantras and even some spells to counter the influences of the planets. Illustrated icons of the planets in anthropomorphic and even zoomorphic forms are provided, a fact that highlights their deified quality. Several of these are entirely unlike earlier Indian precedents, but they share common features with Arabic and Persian codices of a somewhat later period (e.g., Venus as a lute player and Mercury as a scribe). An Iranian heritage has been proposed (Kotyk 2017b).

References

8
遇良日晨, 定日時分宿直諸執皆悉相應, 於食前時値吉祥相者 […]. English translation by Giebel (2005, p. 19).
又現執金剛, 普賢, 蓮華手菩薩等像貌, 普於十方宣説真言道清淨句法. 所謂初發心乃至十地次第此生滿足 […]. English translation by Giebel (2005, p. 4).
See translation and discussion of this section of the commentary in Kotyk (2018b).
Asterisms and planets were understood as deities in Mantrayāna, perhaps as a result of their importance within astrology. The maṇḍalas of not only the Vairocanābhisaṃbodhi but also that of the Vajraśekhara-sūtra (Jp. Kongōchōkyō 金剛頂經) include various celestial deities. The Taizō zuzō 胎藏圖象 and Taizō kuzuyō 胎藏舊圖樣 are illustrated manuscripts which were brought to Japan by the Tendai monk Enchin 圓珍 (814–891) and then recopied in later centuries. The icons therein are the prototypical forms as they were drawn in China based on Indian precedents (Mammitzsch 1990, pp. 30–32). There also existed star maṇḍalas in Japan, which are important, but postdate the period under present investigation. See Takeda (1995).
Yano (2013, pp. 226–64) has importantly shown that there exist two major recensions of the Xiuyao jing. He denotes them as Mainland and Japanese. The former is reproduced in the Taishō canon (T. 1299) but significantly postdates Amoghavajra and moreover includes a number of editorial revisions. The latter recension was first reproduced as a typeset edition by Wakita (1897), although the alternate Mainland recension was instead incorporated into the Taishō canon. The Japanese recensions are arguably more faithful to the original version created by Amoghavajra, as Yano shows.
勘宿曜經取潤月之宿及小月之闕日分宿法. 依口訣記. 若有閏月時, 其正月直宿即亦重直閏月, 謂假令十二月有閏月, 而其十二月一日直宿是虚宿. 十五日直宿是星宿, 乃至三十日直宿是星宿. 如是閏十二月直宿亦同之, 更無異也. 先月是正十二月, 閏月是傍十二月也. 故傍月直宿三十日皆用正十二月直宿, 更不異宿也. 餘月聞月准之知耳. Reading shō 星 (Maghā) as shitsu 室 (Pūrvabhādrapadā).
See the translation in Kotyk (2018b, pp. 15–17).
取小月闕第三十日直宿之法. 假令正月小闕第三十日, 雖無其第三十日, 而彼日分直宿猶有故, 次二月初一日半已上者正月闕日之分宿直也. 半已下者即彼當日宿直, 故雖大小異日有増減, 而小月闕日直宿無日不得. 次月初一日真宿無改代也.
唐國天文昴七星. 今案此經説, 星不與大唐同, 故依大唐天文, 各圖於當宿之下.
教人眼見諸鬼形, 於鬼形上下釘手, 明日收禁之. 患人當日差訖. 與出却其釘, 鬼既得放更不敢到於門焉. Read shou 手 as zhi 之.
Jinn (Persian parī) are regarded as a class of spirits. They are common in Islamic cultures and are believed to regularly interact with the human world. See Omidsalar (2000) for details.
分周天作十二房, 猶如此間十二次, 每次有九足.
See the translation in Kotyk (2018b, p. 15).
For an outline of the domiciles in Hellenistic astrology, see Greenbaum (2015, pp. 407–9).
I must thank Sonja Brentjes (24 March 2022) for pointing out that this icon more closely resembles a satyr than a centaur. This point was made at my talk hosted by the Max Planck Institute (Berlin, Germany), Dept. III.
大同以往, 曆家無知密日, 是故日辰吉凶雜亂, 人多犯之, 大師歸朝之後傳此事. This account is in the Kōya Daishi go kōden 高野大師御廣傳 (Extensive Biography of the Kōya Great Master). See Yamashita (1990, p. 488). The translation of this line is from Kotyk (2018c, p. 42). The use of the word mitsu 密 for Sunday is significant because this is a transcription of the Sogdian word Myr (also written as Jp. mitsu 蜜) for Sun (which also denotes Sunday). There was clearly an Iranian input into East Asian astrology during the Tang, a point taken up by Kotyk (2017a). The Xiuyao jing lists the names of the weekdays in Persian, Sogdian and Sanskrit. We might expect that the Sanskrit transcription āditya (Jp. aniteiya 阿儞底耶) would have been preferred amongst Buddhists, but the Sogdian readings were apparently more popular in the Tang when naming the days of the week. See the table in Yano (2013, p. 110).
承和十年十二月十三日密日東寺. For converting premodern East Asian dates, see Academia Sinica’s helpful tool: https://sinocal.sinica.edu.tw/ (accessed on 14 May 2022).
Molesworth’s Marathi-English dictionary (Molesworth 1857, p. 41) provides the following definition: “अमृतसिद्धियोग amṛtasiddhiyōga m S A common term for certain periods in astrology, viz. the days of the week, beginning with Sunday, on which occur respectively the following nakshatras, हस्त, श्रवण, अश्र्विनी, अनुराधा, पुष्य, रेवती, रोहिणी, [Skt. Hasta, Śravaṇa, Aśvinī, Anurādhā, Puṣya, Revatī, Rohiṇī]—the day having its proper nakshatra, or the occurrence of the proper nakshatra on the proper day.”