The Hour Pillar: Double Hours and the Five Rats Rule

The hour pillar is the fourth and finest layer of the Four Pillars. The day is divided into twelve double hours, each with a fixed branch, and the stem of each hour is set by the day's stem through the five rats rule.

Twelve double hours

Traditional timekeeping split the day into twelve periods of two hours each, and each period carries one of the twelve earthly branches. The first, the 子 (Rat) hour, runs from 23:00 to 01:00 and is centred on midnight. The 丑 hour follows from 01:00 to 03:00, the 寅 hour from 03:00 to 05:00, and so on, until the 亥 hour from 21:00 to 23:00. The 午 (Horse) hour, 11:00 to 13:00, is centred on noon.

These spans refer to local solar time, the time the sun actually keeps where you were born, not to the clock on the wall. The haru engine first converts the birth time to true solar time and only then looks up the double hour. How big that correction is, and why it changes through the year, is covered on its own page, true solar time.

The five rats rule (五鼠遁)

Twelve double hours a day and sixty pairs in the cycle means the hour pairs repeat every five days. The stem of the first double hour, the 子 hour, depends on the stem of the day, and stems five apart share the same start. A 甲 day or a 己 day opens with 甲子, an 乙 or 庚 day with 丙子, a 丙 or 辛 day with 戊子, a 丁 or 壬 day with 庚子, and a 戊 or 癸 day with 壬子.

In the engine this is one line: hour stem = (day stem mod 5 x 2 + hour number) mod 10, with 甲 = 0 and the 子 hour = 0. For a 庚 day the day stem is 6: 6 mod 5 = 1, times 2 is 2, so the 子 hour has stem 2, 丙, giving 丙子, exactly as the table shows. Each later double hour moves one stem forward.

五鼠遁: hour pillars by day stem (clock spans are in local true solar time)
Day stem	子 23-01	丑 01-03	寅 03-05	卯 05-07	辰 07-09	巳 09-11	午 11-13	未 13-15	申 15-17	酉 17-19	戌 19-21	亥 21-23
甲/己	甲子	乙丑	丙寅	丁卯	戊辰	己巳	庚午	辛未	壬申	癸酉	甲戌	乙亥
乙/庚	丙子	丁丑	戊寅	己卯	庚辰	辛巳	壬午	癸未	甲申	乙酉	丙戌	丁亥
丙/辛	戊子	己丑	庚寅	辛卯	壬辰	癸巳	甲午	乙未	丙申	丁酉	戊戌	己亥
丁/壬	庚子	辛丑	壬寅	癸卯	甲辰	乙巳	丙午	丁未	戊申	己酉	庚戌	辛亥
戊/癸	壬子	癸丑	甲寅	乙卯	丙辰	丁巳	戊午	己未	庚申	辛酉	壬戌	癸亥

Two worked examples

The second example shows why the conversion matters: the clock time and the solar time are 96 minutes apart, enough to move the birth into a different double hour.

Seoul, 15 May 1990, 14:30 on the clock. True solar time is 14:01, inside the 未 hour (13:00 to 15:00). The day pillar is 庚辰, so we read the 乙/庚 row: the 未 hour is 癸未.
Seoul, 1 July 1988, 13:10 on the clock, during Korean summer time that year. True solar time is 11:34, which falls in the 午 hour rather than the 未 hour the clock suggests. The day pillar is 丁巳, so the 丁/壬 row gives 丙午.

The late rat hour

The 子 hour is the only double hour that straddles midnight, so a birth between 23:00 and 24:00 raises a question: which day does it belong to? The tradition has two customs, and the haru engine supports both.

Under the midnight rule, which is the engine's default, the date does not change until midnight. The day pillar stays on the current day, but the late rat hour (야자시) takes its stem from the next day, because the 子 hour is treated as the opening hour of the day that is about to begin. Under the 23:00 rule, the day itself turns at 23:00, so both the day pillar and the hour pillar belong to the next day.

Either way the hour pillar comes out the same; only the day pillar differs. Take a Seoul birth at 23:40 on 1 January 2000. True solar time is 23:04, so it is a late rat hour.

Seoul, 1 January 2000, 23:40 on the clock (true solar time 23:04)
Day rule	Year	Month	Day	Hour
Midnight rule (default)	己卯	丙子	戊午	甲子
23:00 rule	己卯	丙子	己未	甲子

Reading the late rat example

Under the midnight rule the day is 戊午. The next day's stem is 己, and on the 甲/己 row the 子 hour is 甲子. Under the 23:00 rule the day has already become 己未, and on the same 甲/己 row the 子 hour is again 甲子. Since the day stem is the day master, the choice of rule changes the point of view from which the tradition reads the chart, so it is worth knowing which rule a tool uses.

When the time is unknown

Many people do not know their birth time. The engine then leaves the hour pillar empty and works with six characters instead of eight. For a Seoul birth on 3 October 1975 with no time, it gives the pillars 乙卯, 乙酉 and 壬午 and no hour pillar. It is better to leave the hour out than to guess, because a guessed hour pillar would be read as if it were real.

Questions

Why does the 子 hour start at 23:00 and not at midnight?

Each double hour is centred on a point of the day, and the 子 hour is centred on midnight, so it begins one hour earlier, at 23:00, and ends at 01:00.

Is the hour pillar based on the clock time?

In the haru engine it is based on true solar time for the birth city. In Seoul it runs roughly 17 to 46 minutes behind the clock depending on the month, and more during summer time. The true solar time page explains the correction.

Which late rat rule is right?

Both are used in the tradition. The haru engine uses the midnight rule by default, and the hour pillar is the same under either rule; only the day pillar changes.

What if I only know that I was born in the morning?

Then the hour pillar cannot be fixed, since a morning covers several double hours. It is fine to read the chart with six characters.