TL;DR: Auspicious and inauspicious day stars such as Thiên Đức, Nguyệt Đức and Thiên Cương are recorded in the Hiệp kỷ biện phương thư, a large work compiled by the Qing court in the 18th century that gathered many different thần sát sources. Each star has its own calculation based on the can chi of the day, month and year, and the books do not fully agree with one another. This is a system of conventions in traditional day selection, not scientifically verified knowledge.
What are auspicious and inauspicious day stars, and why do day-selection calendars include them?
In traditional day-selection calendars, each day is assigned a list of auspicious and inauspicious stars such as Thiên Đức, Nguyệt Đức, Thiên Cương, Nguyệt Phá and Kiếp Sát. These names were not invented by one person; they are the result of many layers of calendrical arts accumulated over centuries and then systematized in large books. Among them, the Hiệp kỷ biện phương thư (欽定協紀辨方書), completed in the 6th year of the Càn Long reign (1741), was commissioned by the Qing court to correct the verbose and self-contradictory Thông thư day-selection almanac then used by the Imperial Astronomical Bureau[1].
The book was compiled by Doãn Lộc (Prince Trang) and colleagues by imperial order, in 36 quyển: "Bản nguyên" in 2 quyển; "Nghĩa lệ" in 6 quyển (quyển 3 to 8), explaining the origin and starting rule of each star; "Lập thành" (quyển 9), which gathers those rules into tables; "Nghi kỵ" (quyển 10), which states what each star suits and forbids; "Dụng sự" (quyển 11), which regroups them by task; followed by year, month and day tables[1]. The Vietnamese translation by Vũ Hoàng and Lân Bình, edited by Mai Cốc Thành and published by Mũi Cà Mau Publishing House, was translated from the 1995 printed edition of Shanghai Classics Publishing House, with reference to the 1994 edition of Guangxi People's Publishing House[2].
How are Thiên Đức, Nguyệt Đức and Thiên Cương calculated?
Each star in the thần sát system has its own starting rule, usually based on the can chi of the month and day. Thiên Đức and Nguyệt Đức are the two auspicious stars most often mentioned in Vietnamese day-selection books, and their calculations are recorded in quyển 5, the "Nghĩa lệ" section of the Hiệp kỷ biện phương thư. The month here is the solar-term month (nguyệt kiến): the book's month table opens the second month with "驚蟄二月節", meaning the Mão month begins at the Kinh Trập solar term, not on the first day of the lunar month[1].
Specifically, Thiên Đức is determined by month. The book quotes the Kham dư kinh: "天徳者正月丁二月坤三月壬四月辛五月乾六月甲七月癸八月艮九月丙十月乙十一月巽十二月庚"[1]. So the Dần month takes the Đinh day, the Thìn month the Nhâm day, the Tỵ month the Tân day, the Mùi month the Giáp day, the Thân month the Quý day, the Tuất month the Bính day, the Hợi month the Ất day and the Sửu month the Canh day. In the four mid-season months (Mão, Ngọ, Dậu, Tý), Thiên Đức sits at the trigrams Khôn, Càn, Cấn and Tốn, which are directions rather than heavenly stems or earthly branches: the book's table for the second month only says "天徳在坤", and the book states "四仲之月天徳居四維故無合也", meaning those four months have no Thiên Đức Hợp[1]. The widely circulated table that gives the Mão month a Thân day, the Ngọ month a Hợi day, the Dậu month a Dần day and the Tý month a Tỵ day is the Bát Tự mnemonic for the Thiên Đức quý nhân, not the Hiệp kỷ biện phương thư's rule. Nguyệt Đức is calculated by the tam hợp of the month: the Dần, Ngọ and Tuất months take the Bính day; the Thân, Tý and Thìn months take the Nhâm day; the Tỵ, Dậu and Sửu months take the Canh day; the Hợi, Mão and Mùi months take the Giáp day[1].
Thiên Cương is an inauspicious star, also counted by month but tied to the twelve trực. Quyển 4 of the Hiệp kỷ biện phương thư quotes Tang Đạo Mậu: "天罡河魁者月内凶神也所值之日百事宜避", and the Lịch lệ: "陽建之月前三辰為天罡後三辰為河魁隂建之月反是"[1]. In a yang month (Dần, Thìn, Ngọ, Thân, Tuất, Tý), Thiên Cương is the branch three places ahead of the month branch and coincides with the trực Bình day; in a yin month (Mão, Tỵ, Mùi, Dậu, Hợi, Sửu), it is the branch three places behind and coincides with the trực Thu day. For example, in the Dần month Thiên Cương falls on the Tỵ day; in the Mão month on the Tý day. Vạn sự bất cầu nhân also lists Thiên Cương among the inauspicious stars of certain days, together with Nguyệt Hoa, Độc Hỏa and Thổ Cấm[3].
Why does one book mark a day as auspicious while another marks it as inauspicious?
An important feature of the thần sát system is its inconsistency between books. The opening pages of the translation of the Hiệp kỷ biện phương thư edited by Mai Cốc Thành remark that the art of day selection is a complex system, one might say mixed, because it is composed of many sources, built by many schools, creating many branches, and thus contains many contradictions[2]. This explains why the same day may be marked as auspicious in one calendar and inauspicious in another.
The difference lies in the fact that each day-selection school has a different starting calculation for the same star, or uses stars that other schools do not. For example, in one month of the book Vạn sự bất cầu nhân, the Dần day is recorded as having the auspicious stars Kim Đường, Ngũ Phú and Lục Hợp, but also inauspicious stars such as Băng Tiêu, Ngõa Hãm, Mộc Mã Sát, Hà Khôi and Bệnh Phù[3]. Meanwhile, quyển 11 (Dụng sự) of the Hiệp kỷ biện phương thư lists, task by task, the stars to use; for marriage, for instance, it names Thiên Đức, Nguyệt Đức, Thiên Đức Hợp, Nguyệt Đức Hợp, Thiên Xá, Thiên Nguyện, Tam Hợp, Thiên Hỷ, Lục Hợp and Bất Tương[1].
| Star | Type | Calculation (by solar-term month) | Suits and forbids (according to Hiệp kỷ biện phương thư) |
|---|---|---|---|
| Thiên Đức | Auspicious | Dần: Đinh; Mão: direction Khôn; Thìn: Nhâm; Tỵ: Tân; Ngọ: direction Càn; Mùi: Giáp; Thân: Quý; Dậu: direction Cấn; Tuất: Bính; Hợi: Ất; Tý: direction Tốn; Sửu: Canh | Suits almost all major tasks; forbids only hunting and fishing |
| Nguyệt Đức | Auspicious | Dần, Ngọ, Tuất: Bính; Thân, Tý, Thìn: Nhâm; Tỵ, Dậu, Sửu: Canh; Hợi, Mão, Mùi: Giáp | Suits almost all major tasks; forbids only hunting and fishing |
| Thiên Cương | Inauspicious | Yang month: trực Bình day; yin month: trực Thu day | Forbids marriage, moving house, building, groundbreaking, burial |
| Nguyệt Phá | Inauspicious | Day clashing with the month branch (trực Phá) | Forbids marriage, building, groundbreaking, opening a business, burial |
| Kiếp Sát | Inauspicious | Dần, Ngọ, Tuất: Hợi; Thân, Tý, Thìn: Tỵ; Tỵ, Dậu, Sửu: Dần; Hợi, Mão, Mùi: Thân | Forbids marriage, moving house, building, groundbreaking, burial |
The disagreement between books appears at the very start of the Hiệp kỷ biện phương thư: the memorial printed before the text says of the Thông thư almanac "子月巳月誤載天徳宜改正", that is, it recorded Thiên Đức wrongly for the Tý and Tỵ months and must be corrected[1].
Is the Hiệp kỷ biện phương thư the only source?
No. The Hiệp kỷ biện phương thư is the largest and most systematic collection, but before and after it there are many other books that also record thần sát. The opening of the Nghĩa lệ section itself says that the day-selection thần sát came from several schools (Kiến trừ, Kham dư, Tùng thần), that their meaning was not fully transmitted and that their starting rules were often copied wrongly: "選擇神煞古有建除堪輿叢辰諸家顧其義不盡傳起例尤多襲誤"[1].
In addition, the Lục Nhâm đại toàn has a section "Trục nguyệt thần sát" (逐月神煞) listing stars month by month and attaching each star to an earthly branch: in the first month, Nguyệt Đức is at Tỵ, Tử Khí at Ngọ and Thiên Đức at Mùi[4]. The Hiệp kỷ biện phương thư records the first-month Thiên Đức and Nguyệt Đức with the heavenly stems Đinh and Bính, so the same star names are recorded in two different ways by the two systems.
Should one trust auspicious and inauspicious stars when choosing a day?
This is a question often asked by those who consult day-selection calendars. Two aspects need to be distinguished: the cultural value of the system and its practical value. Culturally, the thần sát system is part of folk knowledge about time, reflecting how people in the past conceived of harmony between humans and nature. It has a history of thousands of years and is recorded in many important texts.
Practically, no scientific study has proven that choosing a day according to these stars affects the outcome of work. Even among researchers of calendrical arts, some opinions hold that one should not be overly bound by it. The book Tìm hiểu Cổ dịch Huyền không học by Hồ Kinh Quốc and colleagues, when discussing day selection, writes: "In fact, it is only necessary to clearly determine the purpose of day selection, which is to avoid inauspicious qi, especially the inauspiciousness of the year and month. As for the inauspiciousness of days and hours, it can be disregarded. If one had to act according to rules every day and every hour, people would have no way to work"[5].
How are auspicious and inauspicious stars applied in Vietnamese day-selection books?
Vietnamese day-selection books such as Vạn sự bất cầu nhân usually present each day with two lists: auspicious stars and inauspicious stars. For example, in one month the Sửu day is recorded as having the auspicious stars Thiên Thanh, Thiên Phú, Ngọc Đường, Hoàng Đạo and Lộc Khố; and the inauspicious stars Thiên Cầu, Nguyệt Yếm, Đại Họa, Thiên Tác, Phù Liêm, Đại Sát, Thổ Ôn, Quả Tú and Địa Hốc[3]. This presentation helps readers quickly grasp the information, but it can also cause confusion if one does not clearly understand the origin and calculation of each star.
It is noteworthy that many stars in Vietnamese books are not in the Hiệp kỷ biện phương thư, or have different names. This reflects a process of cultural acculturation: when introduced to Vietnam, the thần sát system was localized, with additions, omissions and changes to suit local conditions and customs. Therefore, when consulting, readers should know which system they are using, and should not be surprised if two calendars give different results.
Nhật Nguyệt's suggestion: When choosing a day for important work, one should consider this as one reference factor among other practical factors such as weather, work and health, and should not let auspicious or inauspicious stars decide completely. If a day is marked as inauspicious but is truly convenient for you and your family, you can still proceed; conversely, if a day is marked as auspicious but conditions do not allow, you should not insist.
