We shall briefly describe the Tibetan calendar in this short article. From ancient time there were some phenomenal calendars. In the 7th century, Princess Wen-Cheng brought Tang's calendar to Tibet, later Princess Jin-Cheng did the same thing. Apparently, they did not bring with them the theoretical Astronomy which the calendars were based on. Those Tibetan students who were sent to Tang to study Arithmetics did not learn the computations to make calendars. After the down fall of the Tibetan Dynasty, Tibet became chaotic, and the transmissions of the Han calendars stopped, Tibetan were left without a scientific calendar until 11th century.
Around 9th century, Islam extended to India, many Indian Buddhists escaped to Tibet with `aru-yoga' of the Esoteric (Tantric) Buddhism. Its part of `Kala (time) Wheel' was a part of the Indian Astronomy, and not up to the early Tang's achievement. Tibetan fused the Indian Astronomy, the Tibetan creativities and the Han calendars (rgya-rtsis) to form their own calendars, later they adopted more from the Han calendars to form the present Tibetan calendars.
The main purposes of the Tibetan calendars were (1) providing a working seasonal schedule for farmers, shepherds, etc. (2) dating the enlightening date of Buddha, for Buddha propheted that Buddhism would exist for 5,000 years (different schools might have different figures). If we knew the precise date of the enlightening of Buddha, then the end of the world of Buddhism would be open to us. The important clue was that there was a complete moon eclipse on the enlightening day of Buddha. (3) computing the moon and sun eclipses. The complete moon eclipse on the date of the enlightening of Buddha indicated the importance of moon or sun eclipses which should increase one's intelligence 1 million times. Therefore, it was important to predict the timing of them for a believer to meditate or study.
II: The Tibetan and the Han calendar are Solar-Lunar calendars.
Basically, there are three kinds calendars in the world: (1) the Solar calendar. Ancient Egyptian, Gregorian, etc. (2) the Lunar calendar. Islamic, Judaic, etc. (3) the Solar-Lunar calendar. Han, Tibetan, etc.
Many people mistakenly classify the Solar-Lunar calendar as a Lunar calendar. In fact, in the Solar-Lunar calendar, the month is counted by the position of the moon, and the year is counted by the position of the sun. For 12 lunar months (a lunar year), there are 354 or 355 days, which is 11 days short of the solar year. To compensate the discrepancy, a `leap' month is added every few years. The proper way to add the leap month was intelligently arranged in Tang Dynasty in the Han calendars, the Tibetan calendars followed them (see below).
III: Leap Month.
The way is to take a solar year and divide it into 24 notes, the odd ones (the first, third,...) are called `jie', the even ones (the second, fourth, ...) are called `zhong-qi'. The distances between successive `zhong-qi's are always bigger than any lunar month. Tang astronomers simply used the 12 `zhong-qi' as the indicators of months. If there is no `zhong-qi' for a particular month, then there is no indicator for the month, hence the month become nameless, and therefore become a `leap month' (or rather, it takes over the next month's indicator, the next month becomes a `leap' month). Tibetan followed it.
IV: Length and Name of a Month.
The way to compute the length of a month is peculiar in the Tibetan calendars. As in the Indian Astronomy, a hypothetic `Lunar Day' is created for the following computations. A Lunar Day is the time the moon covered an apparent angle of 12 degrees (1/30 of the total degrees of a circle). Note that a Lunar Day varies and maybe longer or shorter than a Solar Day which also varies. We use the ends of the Lunar Days as indicators of the Solar Days. If two ends fall inside one solar day, the solar day will have two numerals. We shall assign the first numeral to that day, and the second will not be used any more. It is called `skipped date' (`zhag chad'). Or if there is no end of Lunar Days for a solar day, then that particular solar day should use the numeral of the previous day. Therefore the same numeral will be used twice, the second one is called `repeated date' (`zhag lhag'). In this way, a lunar month is determined. It turned out that there were either 30 or 29 days in a lunar month. Note that although Tibetan adopt the Han way of placing `leap month', due to the different lengths of month, the positions of `leap month' were different which will affect the dates of the New Years' Day in this two different calendars (see below).
At the beginning, the Tibetan adopted the Indian way of naming the month according to the `moon station' (`nakshatra', in Indian, `hsui', in Han) of the position of the full moon. Moreover, the beginning date of the month was fixed to be the date of the full moon. Later, Tibetan moved to the Han customs of naming month numerically and starting the month with the new crescent, with the full moon fixed as the 15th day of a month. Note that the full moon could be the 15th or 16th day of a month in the Han calendar.
V: Sexagenary Cycles.
Now let us discuss the year-count system. Tibetan used the period of sixty years (rab-byung i.e., fire-rabbit) as in the Han tradition. Either it came from Indian or it was a later adoption from Han people. After (yin) fire-rabbit, came (yang) earth-dragon, (yin) earth-snake, (yang) iron-horse, (yin) iron-goat, (yang) water-monkey, (yin) water-rooster, (yang) wood-dog, (yin) wood-pig, (yang) fire-mouse, etc. Note that the signs `Yang', `Yin' were irrelevant, and hence were commonly omitted. The 12 animals plus the 5 elements (with iron replacing metal) would form a cycle of 60. For larger number, one used the number of cycles. For instance, 1996 AD would be 17th rab-byung cycle and fire-mouse in he following computation:
It went without saying that 0 meant 1026 AD, 10 meant fire-mouse. The first month of a year was fixed by the position of the third `zhong-qi' (rain-water, or yu-shui) counted from the Winter Solstice as in the Han calendar. However, the animal sign changed at the third month (the original Indian calendar used the third month as the first month) which caused some confusions.
VI: Some Results.
The Indian theory used which was inferior to the Han theory and the modern theory caused some problems in computations. For instance, the procession of the equinoxes, which was discovered in the Han Astronomy around 4th century AD, was not observed. The basic data is not good enough for the long term predictions of eclipses. Using Astronomical computations, the date of the nirvana of Buddha was given to be 2134 BC, 1027 BC, 544 BC by the different Tibetan schools.
The annual Tibetan almanacs are published by several (medical) institutes or hospitals. These almanacs not only forecast the coming year's general climate, they also forecast droughts, blizzards and earthquakers. The authors usually claim that with sufficient staff and instruments, they could have forecast with perfect accuracy the epicenter and the time of an earthquake. However, they refuse to use computers, because the computers would destroy the secret formulas handed down by their forebears.
Due to the difference in setting the lengths of months, the New Year Day in the Tibetan and the Han calendars are either identical, or the Tibetan one is one day late or, the Tibetan one is one month late or the Tibetan one is one month and one day late.
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