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A modern attempt at reform of the Tibetan calendar
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There have for many centuries been obvious differences between the results derived from the calculation of Tibetan calendars and the position of the Sun, Moon and planets in the sky, and the timing of celestial events. This is one reason behind the variety of calendars that have been created. The Phugpa is the most popular today, and some still use the Tsurphu system, but there have been many others. The main problem has always been the need to reconcile the Tibetans' understanding of the calculations given in the Kālacakra system and the motions of heavenly bodies as observed by the Tibetans. In general, the Tibetans have struggled as a result of not understanding the nature of precession – not even acknowledging its existence. As a result they have effectively assumed a sidereal zodiac, not realising that such an approach is in direct contradiction to the methods given in the Kālacakra literature.
Indeed, they have tried to follow those methods, and have taken solstice measurements in order to determine the longitude of the Sun, but have done so with a mistaken understanding that the zodiac is fixed in the visible stars. These are ultimately irreconcilable, and the Tibetans will continue to have problems with their calendars and introduce errors into them until they accept the reality of precession.
There have therefore been many attempts since the first calendars based on Kālacakra were introduced into Tibet to improve on the calculations and reform the calendar. Ironically, the only one that actually used the method of the Kālacakra without twisting it to suit mistaken views on the nature of the zodiac was Zhonnu Pal's, back in the 15th century. As far as I am aware, there has been no similar attempt since that time. But reforms have kept coming.
One recent development has been a reformed "Tantra method" (rgyud lugs) calendar produced at Sherab Ling, in Himachal Pradesh, northern India. This calendar has been the brain child of Kojo Tsewang Namgyal (go 'jo tshe dbang rnam rgyal, Tsenam), formerly of the Tibetan Medical and Astrological Institute in Dharamsala, and currently resident at Sherab Ling. [Update: Tsenam unfortunately passed away a few months after this page was written.]
Tsenam had been investigating the errors in the calendar for many years, and for about 30 years conducted his own solstice determinations using a gnomon and midday shadow measurement, as described in the Kālacakra literature. He also encouraged his students to take an interest in the sky and planets, leading them out onto the monastery rooftops to observe the stars and planets. However, he denied the existence of precession, and claimed that the seasonal points such as the spring equinox and the winter solstice are in the same positions relative to the stars now as they occupied at the time of the Buddha. As a consequence, his calendar contains similar errors to other Tibetan calendars.
There are three important points where his calendar differs from the standard Phugpa system. He has reset the solstice solar longitude in the light of his solstice determinations, but instead of setting it to 20;15,0 (0° Capricorn) as specified in the Vimalaprabhā, he has set it to 18;31,30 (7° Sagittarius). This is the same figure that was used when the Phugpa system was set up in the 15th century, and seems to have been chosen in a mistaken attempt to match the apparent positions of the planets with the visible stars. In doing this, Tsenam has effectively removed the accumulating calculation errors that have accrued in the Phugpa system during the last nearly six hundred years.
The second point is rather curious. The Kālacakra Tantra and Vimalaprabhā contain a set of inconsistent figures for determining the time of the change of year (lo 'pho). These figures yield a figure for the length of the solar year that is different from both the original karaṇa calculations as given in the Tantra, and the siddhānta calculations derived from the Vimalaprabhā by the Tibetans. It is the latter that is used in all other Tibetan calendars. As it happens, the year-change solar year length is more accurate than the siddhānta figure. Tsenam has told me that he uses this figure because of the way in which the year-change is highlighted in the Vimalaprabhā, although it is not used there for the main calculations.
Finally, Tsenam has set the Tibetan months properly in line with the Indian months. This has been a glaring error in Tibetan calendars for several hundred years. It was pointed out as long ago as the 13th century by Kālacakra experts that in India and Kashmir the months named Caitra and so forth occurred one month before they did in the Tibetan calendars. This advice was noted by the Tibetans but ignored by just about all of them. Zhonnu Pal corrected this in his calendar, but since his time (15th century) Tsenam's is the first other calendar that I have come across that does the same. In Tsenam's calendar the first month of the year is Phālguna, rather than Māgha, as in the Phugpa and other systems. So, his calendar still contains errors, but represents a major step in the right direction. It is a great pity that he died before his work could be completed and properly documented. Open source software for his calendar is available here.
New almanac from CIHTS
A variant on Tsenam's calendar has started to be published from 2006 by the Jyotish Department of the Central Institute for Higher Tibetan Studies (now the Central University for Tibetan Studies) in Sarnath, India. This uses Tsenam's calculations, but styles the almanac more along the lines of a general Indian calendar, with daily information being printed in concise tables rather than boxes. The main difference with Tsenam's calendar lies in the definition of the months.
A lunar month can be defined in two different ways, either running from new Moon to new Moon or from full Moon to full Moon. The first of these is called waxing-first (yar ngo sngon 'gro), meaning that the waxing fortnight comes before the waning fortnight. In this system, the waxing fortnight commences the day following new Moon, and culminates with the day of full Moon. The next day starts the waning fortnight which concludes on the next new Moon day. This is how the months are structured in normal Tibetan calendars, following the system defined in the Kālacakra Tantra and Vimalaprabhā. However the literature tells us that the original Kālacakra Mūlatantra used the waning-first (mar ngo sngon 'gro) in which a month starts with the waning fortnight immediately following full Moon, and is followed by the waxing fortnight, ending with the next full Moon. It is said that this was the original Kālacakra system, but this was changed when the shorter Laghu Tantra was composed in order to help enable Hindu followers to accept the Kālacakra methods. Both types of month have been in use for many hundreds of years in India, and if there is some genuine historical background to this tale, one can presumably assume that the waxing-first system predominated in the area in which these original Kālacakra materials were composed.
The CIHTS calendar also styles itself as following the Kālacakra system – but by this is implied the system of the Kālacakra Mūlatantra. The effect of using this waning-first month definition is that the months start a fortnight earlier in this system than in others. For example, in 2006, the third month Vaiśākha lasted from 14th April until 13th May in the Sarnath calendar. In the Sherab Ling calendar, Vaiśākha ran from 28th April until 27th May.