TL;DR: Nhị thập bát tú trực nhật is the method of assigning one of the 28 mansions to each day of the can chi cycle, based on a 420 day cycle made of seven Giáp Tý cycles. Those seven cycles begin at Hư, Khuê, Tất, Quỷ, Dực, Đê and Cơ, and Hiệp kỷ biện phương thư traces the method to the Tú diệu kinh of the Western Regions and criticises the untraceable origin point, yet holds that it should not be abandoned.
What Is Nhị thập bát tú trực nhật?
Nhị thập bát tú trực nhật is the method of assigning one of the 28 mansions to each day of the can chi cycle, so that the mansion of that day can be used to judge good or bad fortune. This pairing does not follow months or solar terms; it follows a separate cycle called the seven Giáp Tý cycles, 420 days long. Hiệp kỷ biện phương thư, the day selection compendium compiled by order of the Qianlong emperor (the Tứ khố edition names Doãn Lộc as compiler by imperial order in 1741, the Vietnamese translation names Mai Cốc Thành as chief editor), records this method in the section Nhị thập bát tú phối nhật (二十八宿配日)[1]. The book both criticises its weak points and concludes that it should not be abandoned.
Two different things must be distinguished. One is the 28 mansions as a system of stars used to locate heaven and earth, present on the feng shui compass in rings such as Nhị thập bát tú phân độ, Nhị thập bát tú phân độ cát hung and Nhị thập bát tú giới hạn. The other is the 28 mansions trực nhật, that is, assigning mansions to days as a spirit star of the day selection calendar. This article deals with the second.
Why Is the Cycle 420 Days Long?
The number 420 is not arbitrary. Hiệp kỷ biện phương thư cites a source explanation that days number sixty and mansions number twenty eight, and that four hundred and twenty is the least common multiple of sixty and twenty eight, so 420 days are taken as one cycle. Within those 420 days, the can chi cycle runs exactly seven rounds of 60 days, and the 28 mansion cycle runs exactly fifteen rounds of 28 days. Those seven can chi rounds are the seven Giáp Tý cycles.
The method exploits the fact that 60 and 28 have 420 as their least common multiple, creating a closed paired calendar: every seventh Giáp Tý day the mansion cycle returns to its original place. Without that divisibility, the two cycles would drift apart forever and no fixed table could be established.
Which Mansions Do the Seven Giáp Tý Cycles Begin At?
The core point of this method is that the seven Giáp Tý days in a 420 day cycle do not all begin at the same mansion, but begin in turn at seven different mansions. Hiệp kỷ biện phương thư records that sequence as follows[1]:
| Cycle | Giáp Tý day begins at mansion |
|---|---|
| First cycle | Hư |
| Second cycle | Khuê |
| Third cycle | Tất |
| Fourth cycle | Quỷ |
| Fifth cycle | Dực |
| Sixth cycle | Đê |
| Seventh cycle | Cơ |
After the seven cycles, the Giáp Tý day begins again at Hư, and the new cycle returns to the start. The table above is the whole original rule: one only needs to know which cycle the day under consideration falls in, and how many days it is from the Giáp Tý that opens the cycle, to derive the mansion ruling that day. For example, if a Giáp Tý day begins at Khuê, then the next day Ất Sửu is Lâu, the day Bính Dần is Vị, and so on accumulating in the exact order of the 28 mansions.
Where Does the Method Come From, and What Is the Middle Character in a Mansion's Name?
Hiệp kỷ relates that a search through the books found no origin until the Tú diệu kinh of the Western Regions explained it: those countries did not use can chi but recorded days by the 28 mansions, attaching the seven luminaries (Nhật, Nguyệt, Hỏa, Thủy, Mộc, Kim, Thổ) to the 28 mansions as stems are attached to branches, not because the seven luminaries actually pass through those mansions[1]. The book lists four mansions for each luminary: Hư, Mão (昴), Tinh and Phòng belong to Nhật; Nguy, Tất, Trương and Tâm to Nguyệt; Thất, Chủy, Dực and Vĩ to Hỏa; Bích, Sâm, Chẩn and Cơ to Thủy; Khuê, Tỉnh, Giác and Đẩu to Mộc; Lâu, Quỷ, Cang and Ngưu to Kim; Vị, Liễu, Đê and Nữ to Thổ[1]. So the middle character in three-character names such as Giác Mộc Giao or Phòng Nhật Thố is one of the seven luminaries, not one of the five phases, and the book notes that the rotation of the seven Giáp Tý cycles matches that system exactly.
How Does Hiệp kỷ biện phương thư Criticise This Method?
This is the most noteworthy part, because the book records the method and then criticises it. Hiệp kỷ biện phương thư does not hesitate to say that the origin of the first cycle cannot be traced to any year, month or day[1]. That is, those seven cycles have a starting point, but that starting point cannot be traced back to a definite moment in history.
The book also compares the method with the real motion of heaven: the sun, moon and five planets pass through the 28 xá at unequal speeds, their degrees of motion all differ, they interweave unevenly, sometimes holding back and slowing, sometimes reversing. Therefore, although from the initial calendar one can derive years, months, days and hours, after only a short while the moon will exceed its degree. Assigning mansions to days by a rigid numerical cycle does not match the real movement of the stars.
The book also criticises the Phục đoạn method separately, a method using the four metal mansions Cang, Ngưu, Lâu and Quỷ as Phục đoạn hours, and remarks that although that method speaks of seven cycles, its meaning is not coherent and its numerical order does not match, so at present it is not used in hour selection[4]. That comment shows that even within the day selection tradition, some rejected this rigid pairing.
Even so, the book does not discard the method. Hiệp kỷ notes that the annual almanac still annotates the mansions beside the sixty Giáp Tý days, that among the spirit stars only Phục đoạn and Ám kim are derived from it, and that because the Western Regions record days by the 28 mansions, keeping the method lets people in distant lands agree on which Giáp Tý a day is; so the book concludes that it is truly useful and must not be abolished (甚為有益，良不可廢)[1].
What Do Cầm tinh dị kiến and Diễn cầm phú Say?
Cầm tinh dị kiến is a book of diễn cầm by Trì Bản Lý of Cám Châu in the Ming. The Đề yếu Tứ khố says the book takes only the seven Giáp Tý cycle layout and uses the method of turning birds and reversing generals to infer good and bad days and hours; some claimed the method began with Trương Lương as a secret of military men, but the Đề yếu calls that a later embellishment and says the book divines every gain and loss in human affairs, mentioning hours for camping and building fortifications only occasionally (特間一及之)[2]. The Đề yếu also notes one point where the book departs from other astrologers: it takes Đẩu Mộc as the crab (蟹), not the mythical giải trãi (獬)[2].
The Diễn cầm phú printed with the book opens by saying that to divine human affairs one counts the 28 mansions on the palm (以二十八宿于掌演之), then divides the 28 mansions into four groups of seven by the day branch: Hư, Khuê, Tất, Quỷ, Dực, Đê and Cơ are the stars of Thân, Tý and Thìn days; Nguy, Lâu, Chủy, Liễu, Chẩn, Phòng and Đẩu of Tị, Dậu and Sửu days; Thất, Vị, Sâm, Tinh, Giác, Tâm and Ngưu of Dần, Ngọ and Tuất days; Bích, Mão (昴), Tỉnh, Trương, Cang, Vĩ and Nữ of Hợi, Mão and Mùi days. The seven stars of the first group are exactly the seven mansions that open the seven Giáp Tý cycles.
How Does This Method Differ from the Mansion Rings on the Compass?
Three different layers must be separated, because beginners often confuse them.
The first layer is the 28 mansions as a system of stars dividing the sky, linked to the four images Thanh Long, Bạch Hổ, Chu Tước and Huyền Vũ, each image with seven mansions. This is the ancient astronomical layer, used for locating.
The second layer is the mansion rings on the compass. The book Sử dụng la bàn by Thanh Thủy lists many rings related to the 28 mansions on different types of compass, such as the Nhị thập bát tú phân độ ring, the Nhị thập bát tú phân độ cát hung ring, the Nhị thập bát tú giới hạn ring, and on the 36 ring compass there is also the Nhị thập bát tú phân dã ring. This is the layer used to measure direction, not to assign mansions to days.
The third layer is nhị thập bát tú trực nhật, assigning mansions to days by the seven Giáp Tý cycles. This layer belongs to the day selection calendar, not to the compass. The three layers share one list of 28 mansions, but their operating rules are entirely different, and one layer should not be used to explain another.
Nhật Nguyệt's Suggestions: How Should This Method Be Used?
First, nhị thập bát tú trực nhật should be regarded as a convention of the day selection book, not an astronomical fact. Hiệp kỷ biện phương thư itself states clearly that the origin point cannot be traced and that the real motion of the stars does not match the rigid cycle. Therefore, if someone presents this method as if it reflected the true positions of the stars in the sky, the book's own criticism should be reread.
Second, the origin point cannot be traced, but current calendars do not have to guess it: the 28 mansion cycle is locked to the seven day week through the seven luminaries, so the four Nhật mansions (Phòng, Hư, Mão, Tinh) always fall on Sunday and the four Nguyệt mansions on Monday. An origin off by one day would shift the whole week, so the weekday is enough to check a lookup table.
Third, this method should only be a thin reference layer in day selection, not the sole basis for deciding important matters.
In Summary
Nhị thập bát tú trực nhật is the method of pairing the 28 mansions with days according to a 420 day cycle made of seven Giáp Tý cycles, with the seven cycles beginning in turn at Hư, Khuê, Tất, Quỷ, Dực, Đê and Cơ. Hiệp kỷ biện phương thư traces the method to the Tú diệu kinh of the Western Regions, criticises it because the origin point cannot be traced and the method does not match the real motion of the stars, yet holds that it should not be abandoned. It is therefore a convention of the day selection books, to be used for reference with the convention clearly stated, not an astronomical fact nor a sole basis for deciding important matters.
