About This Project

A digital preservation and educational initiative bringing traditional Tibetan astronomical calculations to the modern web.

Project Origins

This web application was created by Thupten Chakrishar, a Tibetan technologist and cultural preservationist. The goal: to make traditional Tibetan astronomical calculations accessible to anyone with a web browser, while providing comprehensive educational content about this remarkable system.

The mathematical engine is based on Edward Henning's TCG (Tibetan Calendar Generator), a command-line C program that implements traditional grub rtsis (siddhānta calculations) as described in the Kālacakra Tantra. Henning's scholarly work, particularly his book Kālacakra and the Tibetan Calendar, provided the foundation for translating these ancient algorithms into code. His decision to release TCG under the MIT license made this web adaptation possible.

By porting Henning's C code to TypeScript and wrapping it in an educational web interface, we hope to serve both the Tibetan diaspora community seeking to connect with their cultural heritage, and scholars or curious learners worldwide who wish to understand this sophisticated tradition.

Two Kinds of Truth: Religion & Astronomy

One cannot discuss the Tibetan calendar without addressing a fascinating tension at its heart: the system is religiously and traditionally authoritative, yet astronomically approximate. Understanding this duality is essential to appreciating what the calendar is—and what it isn't.

A Living Religious Tradition

For Tibetan Buddhists, the calendar is not merely a timekeeping tool but a sacred framework that determines:

  • When to observe Buddha's birth, enlightenment, and parinirvāṇa
  • Auspicious dates for empowerments, retreats, and ceremonies
  • Monthly practice days (new moon, full moon, 10th and 25th days)
  • Annual festivals and commemorations of great masters
  • Personal astrological readings and life decisions

In this context, the calendar's authority comes from its roots in the Kālacakra Tantra—a text believed to have been taught by the Buddha himself at the request of King Suchandra of Shambhala. The calculations have been transmitted through an unbroken lineage of masters for over a millennium. When a Tibetan Buddhist asks "what day is it?", the answer carries spiritual weight that transcends astronomical measurement.

Astronomical Approximations

From a modern astronomical perspective, however, the Tibetan calendar contains known inaccuracies:

  • Sidereal vs. Tropical: The system uses sidereal zodiac positions (fixed stars) rather than the tropical zodiac (seasons), and the assumed rate of precession differs from observed values.
  • Simplified Models: The Sun and Moon's motions are modeled with single-epicycle approximations that were state-of-the-art in classical India but lack the precision of modern ephemerides.
  • Fixed Parameters: Values like the mean lunar month length (29.5305... days) were fixed centuries ago; actual lunar months vary.
  • Eclipse Predictions: The Rahu calculations can predict eclipse seasons but not exact times or visibility.

As a result, the calendar's full moon may not coincide with the astronomical full moon. The solar longitude at a given moment differs from what modern software would calculate. Eclipses may be predicted on days when none occurs, or missed when one does.

Not a Flaw, But a Feature

It would be a mistake, however, to view these discrepancies as "errors" that need correction. Consider:

The calendar serves its intended purpose excellently. It synchronizes a global community of practitioners. When Tibetans around the world observe Saga Dawa (the fourth month), they do so together—not because the Moon is in a particular astronomical position, but because the tradition says it is Saga Dawa. The shared observance creates community and continuity across generations.

Historical corrections have been made. The existence of multiple calculation traditions (Phugpa, Tsurphu, 'khrul sel, etc.) shows that Tibetan astronomers were aware of accumulating errors and developed refined parameters. The "Error Correction" ('khrul sel) system is explicitly named for this purpose.

Perfect accuracy was never the goal. The Kālacakra system is fundamentally a correlative cosmology—it maps correspondences between celestial movements, the human body, and spiritual practice. Whether the Sun is "really" at 15° Aries matters less than the fact that the system provides a coherent framework for relating outer, inner, and secret dimensions of reality.

A Parallel Worth Considering

The Gregorian calendar we use daily is also "wrong" in certain ways—it doesn't track lunar phases, its months are arbitrary lengths, and it required papal intervention to correct drift. Yet no one suggests we abandon it. Calendars are social technologies, and their value lies in shared use, not astronomical perfection.

The Tibetan calendar, then, is best understood as a traditional astronomical system—internally consistent, practically useful, spiritually meaningful, and deserving of study on its own terms. This project aims to present it authentically, neither dismissing its approximations nor pretending they don't exist.

Five Calculation Traditions

The software supports five distinct calculation systems, each with different epoch dates and parameters:

  1. Generalised Phugpa (Epoch: -1000 CE) — The most widely used, standardized by Phugpa Lhundrup Gyatso
  2. Generalised Tsurphu (Epoch: -1000 CE) — Used by the Karma Kagyu school
  3. Error Correction ('khrul sel) (Epoch: -2000 CE) — A refined system addressing accumulated drift
  4. mkhas pa'i snying nor (Epoch: 1796 CE) — An 18th-century scholarly tradition
  5. New Ganden Calculations (Epoch: 1747 CE) — Modern Gelug refinements

These variations occasionally produce different dates for the same event—a reminder that even within the tradition, "correctness" is not singular.

Technical Implementation

The web application is built with:

  • Astro — Static site generator for fast, accessible pages
  • TypeScript — Type-safe port of the calendar algorithms
  • Vanilla CSS — Custom design inspired by Tibetan manuscripts

The core challenge was porting Henning's C code, which uses custom BCD (Binary Coded Decimal) arithmetic for precision over millennia-spanning calculations. We replaced this with JavaScript's native BigInt where necessary while preserving the original algorithm's logic and KTC page references in comments.

Licensing & Attribution

Edward Henning's original TCG software is released under the MIT License (Copyright © 2009-2013). This web implementation is also MIT-licensed. We are grateful to Henning for his decades of scholarly work and his generosity in making the source code freely available.

Contact & Contributions

Questions, corrections, or contributions are welcome. We particularly value input from:

  • Tibetan calendar practitioners and astrologers
  • Scholars of Tibetan astronomy and the Kālacakra
  • Native Tibetan speakers for terminology review
  • Developers interested in cultural preservation

Reach out through the project's GitHub repository.

References & Further Reading

  • Henning, Edward. Kālacakra and the Tibetan Calendar. Columbia University Press, 2007.
  • Schuh, Dieter. Untersuchungen zur Geschichte der tibetischen Kalenderrechnung. Wiesbaden: Franz Steiner Verlag, 1973.
  • Janson, Svante. "Tibetan Calendar Mathematics." Unpublished manuscript, 2014.
  • The Kālacakra Tantra and Vimalaprabhā commentary (primary sources)