Details of Tibetan Astrology 2: Heavenly Bodies and Periods of Time

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Twenty-Seven Lunar Constellations

Let us go on with some further general features. For most calculations, the belt of the zodiac is divided into 27 lunar constellations (rgyu-skar, Skt. nakṣatra) or lunar mansions, each divided into 60 degrees. Just as 12 prominent constellations can be noted in this belt, so can 27. Thus, there are 1620 degrees in the zodiac in this system, rather than 360. Each of the 12 signs covers 2 ¼ lunar constellations. 

This scheme is not found in the ancient Greek or modern European systems but was shared in common with the classical Hindu ones. Sometimes 28 lunar constellations are specified, but while in the Hindu system the zodiac would then be divided into 28 equal portions, in the Tibetan system one of the 27 equal portions, namely the 21st, would be divided into two. 

A system of 28 lunar constellations is also found in ancient Chinese astronomy, however there the approach is different. In the Indian and Tibetan systems, these clusters of stars, like those of the constellations of the 12 signs, are located around the zodiac which, as was explained, is the belt through which the sun, moon and planets rotate around the earth in a geocentric scheme. The Chinese are not particularly concerned with the belt of the zodiac or with the ecliptic or apparent orbit of the sun. Instead, the Chinese system emphasizes the pole star, Polaris, which is likened to the emperor. The lunar mansions consist of slightly different clusters of stars located along the stellar equator and are likened to the imperial ministers and feudal territories. In other words, the pole star and lunar constellations in the heavens are like the north pole and equator on the earth and, in keeping with the Confucian worldview, the constellations revolve around the pole star in the way the ministers and territories revolve around the emperor at the center of the traditional Chinese imperial court. 

Furthermore, the 28 Chinese lunar mansions do not make an equal division of the sky. The portion of the heavens that each covers varies widely, so that the moon takes less than two hours to cover some and more than two days to cover others. There are 4 quarters or palaces of the sky, each with 7 lunar mansions, starting with the east and going counterclockwise, with a central palace in the middle. The quarters are likewise of irregular size. In later times, the Chinese divided the circle of the sky into 365 ¼ degrees, with the sun conjuncting one each day of the solar year. They never had a system of either 360 or 1620 degrees.

A system of 28 lunar mansions is also found later in Arabian astronomy, from which undoubtedly the reference in Chaucer is derived. It is unclear from where the Arabian system of 28 derives.

The Ten Heavenly Bodies

Ten heavenly bodies are treated in the Kalachakra system, all of which are called “planets” (gza’). These are, firstly,

  • The sun (nyi-ma)
  • The moon (zla-ba)
  • Mars (mig-dmar)
  • Mercury (lhag-pa)
  • Jupiter (phur-bu)
  • Venus (pa-sangs)
  • Saturn (spen-pa
  • Comet (mjug-ring). 

Although the calculations exist for the position of this comet, it is not treated extensively and is not used in horoscopes. The two remaining heavenly bodies can be referred to as the planets of the north and south nodes of the moon. 

The orbits of the sun and the moon, although both in the belt of the zodiac, are not exactly parallel, but rather slightly askew. In other words, they crisscross each other. The two points of their intersection are known as the north and south nodes of the moon. At each new moon, the sun and the moon are roughly conjunct each other, in other words at the same spot. But it is only when this conjunction occurs at either the north or south node, where their orbits intersect, that the conjunction is exact and a solar eclipse occurs. At full moon, the sun and the moon are in opposition. When this coincides with one being conjunct the north node and the other the south, the opposition is exact, and a lunar eclipse occurs.

In both the classical Hindu and Kalachakra systems, the north and south nodes of the moon are conceived as planets, whereas in the ancient Greek they are not. In both these Indian systems, eclipses are explained as conjunctions of the sun and moon with these nodal planets. Since these planets are round, the shape of their passage across the sun or moon during an eclipse is curved. Aristotle, on the other hand, in the mid-4th century BCE in ancient Greece, explained that an eclipse of the moon was caused by the earth’s coming between the sun and the moon when both luminaries are located at the moon’s nodal points. He went on to explain that it is earth’s shadow on the moon that is obscuring it during an eclipse, and the fact that this shadow is always round demonstrates that the earth is spherical. Even when the view of the earth as spherical later appeared in Hindu cosmology, still the nodes were conceived as planets. In the Kalachakra system, a spherical earth was never postulated. Its view of the earth and the motion of heavenly bodies will be discussed later.

In the Kalachakra system, the north node planet is called Rahu (sgra-gcan), literally “growler,” or the “head star” (gdon-skar), and the south node one either Kalagni, which means “fire of time,” or the “tail star” (mjug-skar). In the Hindu systems, although the former is also called Rahu, the latter is called Ketu (mjug-ring), literally “long tail.” According to pan-Indic mythology, Rahu was an asura, an anti-god, who drank some of the nectar of immortality of the gods. Alerted by the sun and the moon, Mohini, an avatar of Vishnu, cut off Rahu’s head before the nectar could pass through his throat. As a result, Rahu’s head became immortal. It stayed in the heavens and, from time to time, swallows the sun, causing a solar eclipse as the sun enters his mouth and leaves through his throat. The body turned into Ketu and went to the opposite side of the heavens and, from time to time, swallows the moon causing a lunar eclipse. In drawings, Rahu who is often depicted as the head of a serpent and Ketu as the rest of the body and tail, from which the names “head star” and “tail star” are derive. 

In the Kalachakra system, Ketu is the name given to the tenth planet, the comet, which is not included in the classical Hindu or Greek systems, which treat only nine or 7 heavenly bodies respectively. Actually, there are 4 comets discussed in the Buddhist texts, but this is the most prominent, since it has an orbit cycle around the sun of 3 years and 3 phases of the moon. A moon phase (phyogs) is either from new to full or full to new moons. 3 years and 3 phases of the moon is a significant period in the Kalachakra system because of the number of so-called “deep awareness” (ye-shes) breaths then pass into the central channel during that period and is the source for citing this period as the shortest time required for attaining enlightenment through anuttarayoga tantra methods.

The classical Chinese system did not include any mention of the north and south nodes of the moon. Astronomers of the Han Dynasty, from roughly the 2nd century BCE to the 2nd CE, for instance, did not believe that solar eclipses could occur only at the new moon. The Chinese speak only of the sun, moon, Mercury, Venus, Mars, Jupiter and Saturn. In later times, when the concepts of the north and south nodes of the moon appeared in Chinese astronomy, they were referred to as the dragon’s head and tail, clearly indicating their Indian origin. They were not, however, taken as planets.

In the Kalachakra system, all the heavenly bodies are conceived as revolving around a static Mount Meru, around which the various regions of our earth and other human realms are spread. The positions of Mercury and Venus, however, as in the Greek and Hindu systems, are calculated in terms of their seemingly revolving also around the sun, since from the perspective of the earth they appear to act like moons circling the sun. The analysis and description of the motion of Mercury and Venus, actually, are more complicated than that, but no need to go into further detail here.  

Days of the Week

Another common feature with the ancient Greek and Hindu systems is the naming of the days of the week after the planets: 

  • Sunday – the sun
  • Monday – the moon
  • Tuesday – Mars
  • Wednesday – Mercury
  • Thursday – Jupiter
  • Friday – Venus 
  • Saturday – Saturn. 

Because of this, the Tibetan word for weekday (gza’) is the same as that for planet. There are some differences, however, in the explanations of how and in what order these planets arose and how they became associated with the days of the week. In the Kalachakra system, each planet arose at a different time and conjunct to a different so-called “birth sign” (skyes-khyim) and birth constellation (skyes-skar) position. The names of the days of the week are given in accordance with the sequence in which the first 7 planets arose as the sun made several circuits around Mount Meru. In the Hindu systems, on the other hand, all the heavenly bodies were created at the same time, with all of them arising at the same point in the zodiac with a universal conjunction. 

The names of the days of the week and their sequence are explained in the same manner by the Hindus and the Greeks as follows. First, the 7 planets are listed in the sequence of the reverse order of the proximity they are considered from the earth, namely Saturn, Jupiter, Mars, the sun, Venus, Mercury and the moon. This is in accordance with the explanation of Ptolemy, the 2nd century CE Alexandrian Greek astronomer, that the static, spherical earth is surrounded by increasingly larger concentric spheres upon which these heavenly bodies move. The outermost sphere is the location of the fixed stars. Starting with the sun, and proceeding in order, each rules a successive hour of a 24-hour day. The sun rules hour one of the first day, Venus hour two and so on. Thus, the ruler of the 25th hour, or the first hour of the second day, is the moon, and the ruler of the 49th hour, or the first hour of the third day, is Mars. The days of the week are then named in sequence according to the ruling planet of their first hour. 

Actually, it was the Babylonians who first had the 7-day week, since to them the number 7 was sacred. It passed from them into the Hebrew, and then the ancient Greek and classical Roman calendars. In the case of the Jews, this fit in well with the Biblical description of the creation having taken 7 days. 

The concept of a 7-day week undoubtedly came into India from the Greco-Roman world. During the Gupta Dynasty, starting in the early 4th century CE, strictly solar calendars with 7-day weeks began to appear in India. Prior to this, there were only solar-lunar calendars, that is lunar calendars corrected so as to correlate with the motion of the sun and regular seasons of the year. Lunar months were divided in half, with the days of the waning and waxing halves merely being numbered from 1 to 15. Two solar calendars developed, the Vikrami and Shaka. The Vikrami, also known as the Sambat calendar, was backdated to begin with the founding of the Vikrama Era of counting years from 58 BCE and the rule of Vikramaditya. It is used mostly in North India. The Shaka was made to begin in 78 CE with the Shaka Era of the Kushan Dynasty. It spread to the Deccan plateau in south-central India and then to Southeast Asia. Certain features of these two calendar systems will be explained later.

Comparison with Chinese Days of the Week

The Chinese traditionally had a 10-day week and only started using a 7-day one, beginning in the 7th century CE, due to the influence of the Nestorian Christian communities of Persians and Sogdians living in China. The Chinese refer to the days of the week by their number, however, and not by the names of the planets. This is the case despite the fact that the 28 Chinese lunar constellations are correlated with the days of the week upon which the moon will roughly conjunct them during a four-week cycle, as well as with the sun, moon and five planets in the order in which they are usually associated with the days of the week. This is found by the 10th century. This needs to be explained in a little more detail.

As was noted, the 28 Chinese lunar mansions do not provide an equal division of the skies. It takes the moon anywhere from two hours to more than two days to pass through one of these portions. For astrological almanac purposes, a system of 28 “theoretical” ideal lunar constellations was devised that symmetrically divides the equator of the heavens such that the moon conjuncts one of these each day, despite the fact that the moon orbits through the ecliptic and not the heavenly equator. 28 days is four weeks of seven days each, and thus each theoretical constellation is conjuncted only on one specific day of the week. The days of the week, although not called by the names of the sun, moon and five planets in Chinese, are nevertheless correlated with them in exactly the same manner as with the Indians and Greeks. The sequence begins with the first theoretical constellation being conjuncted on Thursday, correlated with Jupiter. The reason for beginning with Thursday and Jupiter may be because Jupiter is called the wood-planet in Chinese, and the Chinese element of wood is associated with spring, which is considered the first season of the Chinese year.


The 24-hour day was an Egyptian invention, from which it went to the Jews, Greeks and Romans. For all of them, however, the day and night were each divided into 12 hours, from dawn to dusk, so that the length of an hour, being one-twelfth of the day or night, varied according to the season and length of daylight, and whether it was an hour of the day or the night. A standard-length hour was not adopted in Western Europe until the end of the 13th century with the invention of mechanical clocks. It seems to have been adopted quite late, like this, into the Hebrew, Muslim, Indian, Tibetan and Chinese systems as well.

The Babylonians had equal-length hours the entire year-round but used 12 rather than 24. As was seen in the discussion of astrological periods of the day in connection with the ascendant and 12 houses, both the Indian Hindu and Buddhist systems use 12 such periods, rather than 24, in this context, and only in later times did these 12 become standard in length. The same seems to be the case with the Chinese division of the day into 12 periods, not 24, as will be discussed later. 

Fixed Star Zodiac 

Another feature of the Kalachakra system held in common with the classical Hindu, but not the Greek systems, is the use of a fixed star or sidereal zodiac. Zero degrees Aries, or zero degrees of the first of the 27 constellations, Upper Aries, always refers to when the sun is conjunct the actual position of the beginning of the constellation Aries. 

In the ancient Greek and modern European systems, which use the tropical zodiac, whenever the sun is at the vernal equinox in the northern hemisphere, this position is called zero degrees Aries regardless of where the actual constellation Aries is in the sky. Each year this position shifts slightly counterclockwise, so that now it occurs in Pisces, the sign before Aries. 

This phenomenon is known as “the precession of the equinox,” in other words, its moving backwards. When it shifts into the next sign back, Aquarius, in about four centuries from now, the so-called “New Age of Aquarius” will technically begin. In common discussion, when people speak of the Age of Aquarius starting very soon, they are undoubtedly confusing this with the Christian notion that the change of a millennium marks a new golden age. This precession phenomenon also explains why in the Vedic listing, in about 1500 BCE, of the 28 constellations, and in the most ancient Chinese one as well, the order begins with the present second constellation. In those days, at the vernal equinox, the sun would have been further along in Aries, approaching Taurus. 

Although many of the classical Hindu astro systems knew of calculations for the precession of the equinox in connection with determining the solar position at the time of the vernal equinox, they were not concerned with this phenomenon in calculating the planetary positions in general since they used a fixed star zodiac. However, the positions calculated from the traditional mathematical models were slightly inaccurate. 

During the Moghul period, particularly from the 18th century onwards, when observations of planetary positions became widespread through continuing Arabian astro influences and contact was made with European astronomy, particularly with the work of Kepler, many of the Hindu systems discarded the traditional mathematical models. They saw that the European models gave more accurate results that could be confirmed through telescopes and the various heavenly measuring devices that the Moghuls built in their observatories. Many therefore adapted the new technique of subtracting a standard precession value uniformly from the European-derived tropical zodiac positions of all the planets in order to derive their positions in the fixed star zodiac. Each of the Hindu lineages adapted a slightly different precession value as its conversion factor. The most commonly used one is 23 degrees 6 minutes. 

Some Hindu astrologers claim, however, that the traditionally calculated planetary positions give more accurate astrological information. This is a very important point, because Tibetan astrology is now at the stage at which Hindu astrology was in the 18th century when it came into contact with European astronomy. The positions of the planets as derived from the mathematical models of the Kalachakra system also do not correspond exactly to what is scientifically observed. Whether or not, however, it will be necessary to follow the Hindu example of discarding tradition and using the European values modified by a precession factor is yet to be decided. 

One could argue that it does not really matter what the actual observed positions of the planets are, because the Tibetan Buddhist astro system was never intended for sending a rocket to the moon or navigating a ship. The astronomical data is calculated for astrological purposes, and if the astrological information is empirically accurate and helpful, that is all that matters.

Tibetan astrology is intended to allow one to know one’s basic karmic situation in life so that one can work with it in order to overcome all one’s limitations and realize all one’s potentials so as to be of best benefit to others. It is within this Buddhist context that Tibetan astro studies must be viewed. It would seem irrelevant to judge and alter it on the basis of its astronomical data not corresponding to observed planetary positions.

In order to learn and benefit from each other’s systems, both Europeans and Tibetans must respect the integrity of each other’s corpuses of knowledge and wisdom. There can be a sharing of ideas and indications gained for new areas of research, but one must not uncritically toss out traditional approaches and adopt foreign ones. As can be seen from the histories of both Tibetan medicine and astrology, ideas from foreign cultures were not blindly copied. They stimulated the Tibetans to work out a unique system of their own, based on their own research and experience, in which foreign ideas took on a new form. This is the way that progress occurs, for the benefit of all. 

Zodiac, Solar and Lunar Days 

A further feature in common with the classical Hindu systems is the presentation of three types of days: zodiac, solar and lunar. 

Zodiac Days

A zodiac or sidereal day (khyim-zhag) is the time it takes for the sun to progress one out of 360 degrees of the zodiac. If a year is taken to be the period of time it takes for the sun to return to its same position in the zodiac, there are 360 such days in a year and this is the longest type of day. 

Solar Days

A solar day (nyin-zhag) is the period from dawn to dawn and there are 365 such days per zodiac year. This is the type of day used in Europe. The issue of what constitutes a solar day and its length is actually quite complex. In both the Indian Hindu and Buddhist systems, as well as the Greek, the solar day begins at dawn, before the actual sunrise. In the Babylonian system and the Hebrew, which is mostly derived from it, and later in the Muslim one, the solar day begins at sunset. For the Jews, this is in keeping with the Biblical creation description of darkness preceding light. In all these systems, however, there is a complication. As the year progresses, sunrise and sunset will occur at different times, so that the length of the solar day, if it is reckoned from dawn to dawn, will increase slightly each day as the sun rises later and decrease as it rises earlier. This is a different, though related question to the one discussed before about the length of an hour when an hour is taken to be one-twelfth the time of daylight or of darkness. Also, obviously, the solar day will start at different times throughout the year if it begins at dawn or sunset. 

One solution is to take the midpoint between sunrise and sunset, namely midnight, and call that the beginning of the solar day. With such a system, found first among the Egyptians, the solar day becomes of constant length and always starts at the same time. With the Chinese as well, the solar day begins with the two-hour period that occurs at the middle of the night. The reason for this will be explained later.

In those calendar systems in which the solar day begins at dawn or sunset, a division was often made between the start of the day for religious purposes and for civil, calendar ones. This is found in the Tibetan Buddhist system, in which for rituals that are to begin at dawn, the actual hour of dawn is used, but for calendar and horoscope purposes, dawn is set at 5 o’clock throughout the year, in keeping with a system of wristwatch hours of equal length, as discussed before. Parallel to this, in the Hebrew system the calendar day begins at six in the evening, although religious observances commence at the actual sunset.

Lunar Date Days

The third type of day found in both the Tibetan and Hindu systems is the lunar date day (tshes-zhag), which is associated with the phases of the moon during a lunar month. Such a day is the period of time it takes for the moon to travel one-thirtieth the distance between new moon positions in each successive sign of the zodiac. As already mentioned, new moon is when the sun and moon are conjunct each other, which means at the same spot, for instance in Aries. To reach the next new moon or conjunction with the sun, the moon must travel not only the 360 degrees of the zodiac back to where it was before in Aries, but must progress further into the next sign, Taurus, to catch up with the advance of the sun. Thus, the moon travels approximately 390 degrees between the new moon in Aries and that in Taurus. If one divides that distance into 30 equal portions, then the period of time it takes for the moon to travel one of them, or approximately 13 degrees, is called a lunar date day. There are 375 such days in a zodiac year, and this is the shortest type of day.

The length of a lunar date day is not standard but varies. This is rather complicated to calculate because the sun and moon move at different speeds at different parts of the zodiac. This is congruent with the European description of elliptical orbits. A planet in an elliptical orbit also does not have a constant speed. This variation in the length of this type of day is very important for explaining why, in the Tibetan and Hindu calendars, certain days are omitted and others are doubled, which will be discussed toward the end of this lecture.

60-Year Cycles

Another common feature of the Hindu and Kalachakra calendars is the use of a 60-year Jupiter cycle for the naming of periods of years. The heliacal setting of a planet is when, at sunset, it sets into the sun as the sun itself sets, so that the planet no longer appears behind the sun as an evening star. It is followed by a heliacal rising, when the planet first appears to re-emerge from the sun as a morning star rising ahead of the sun. The period between heliacal settings of Jupiter is approximately one year, and each year when this happens, the sun has moved ahead one sign of the zodiac. Thus, in approximately 12 years, the heliacal setting of Jupiter will recur in the same sign of the zodiac and Jupiter will have completed one orbit. 

The ancient Indians counted five of these together, since in their calendars an extra doubled month was added once every five years to bring the solar and lunar calendars into harmony. This is the derivation of the Indian 60-year Jupiter (Skt. Bṛhaspati) cycle. It is found from the 5th century CE among the Hindu astronomers, but never featured prominently in any of the classical Hindu calendar systems. Each year is given a name and these 60 names are used in common in the Kalachakra system. 

In Kalachakra, this 60-year system is called after the name of the first of these 60 years, “prominent” (rab-’byung). It is quite important in Kalachakra astronomy since the calculations for the positions of the planets are made in terms of the distance each travels during 60 years, based on a daily motion constant for each and taking into account the leftover position of where each was at the end of the last cycle. 

In Hindu astronomy, this 60-year cycle was never used in this manner. There the equivalent calculations are made in terms of the distance each planet has traveled from the initial universal conjunction mentioned before, when all the planets arose simultaneously. Consequently, the figures in the Hindu calculations are enormous in comparison with their more manageable size in the Kalachakra system. This will be discussed in more detail later.

Although a 60-year cycle of years is found also in the classical Chinese system, its theoretical basis and functioning are very different. It is based on the 10 heavenly stems (gnam-gyi rtsa-ba, Chin. tiangan 天干) and 12 earthly branches (sa’i yan-lag, Chin. dizhi 地支) and later the 5 elements and 12 animal-signs. This will be explained in a moment. 

Numerical Notation

One last feature in common between the Tibetan and Hindu systems that should be mentioned first, however, is the manner of referring to numbers used in astronomical calculations. When describing mathematical formulas and operations, the texts never call the numbers by their numerical names such as “one,” “two,” etc. Rather, each number has several code names taken from common pan-Indic mythology. For instance, “fire” means 3, “ocean” 4, “arrow” 5 and so on, because there are 3 fires, 4 oceans and 5 arrows in the mythology known to everyone. Corresponding European examples would be the use of “little pig” for 3, “dwarf” for 7 and “reindeer” for 8, since everyone knows from fairy tales about the 3 little pigs, the 7 dwarves and the 8 reindeer. 

The Tibetan Buddhist calculations use “space” to mean zero. This usage of a zero is in common with Indian Hindu mathematics, as is the system of place notation. Both appear in India by the end of the 6th century CE, if not earlier. Before that, like the Greeks, Romans, Hebrews and Chinese, there were separate symbols for tens and hundreds, similar to the Roman numeral X for 10. The Europeans seemed to have learned about zero and place notation from India. But, unlike the European mode of referring to numbers in words from their higher units down to their lower, for instance four hundred fifty-three, it is the reverse in the Tibetan and Hindu systems. There this same number would be called the equivalent of “three, fifty, four hundred,” namely “fire-arrow-ocean.”

Both the Indian Hindus and the Greeks had a value for pi used in calculating the circumference of a circle, with the Indian value at the end of the 5th century CE being more accurate than that of the Greeks. The Kalachakra system and Tibetan astronomical calculations in general do not have the concept of pi and have only very rough estimates for the circumference of circles as being triple the diameter.