Possible Reforms of the Tibetan Lunisolar Calendar
Tsogtgerel Gantumur
Abstract
The family of Tibetan lunisolar calendars operates on a shared arithmetic axiom (67 lunar months = 65 solar months) that provides a rigid structure but causes observable seasonal drift. This study deconstructs the calendar through a progressive analytical sequence, first presenting it as an explicit computational procedure, then isolating its structural core of incidence rules and mean-motion models. This separation distinguishes structurally forced features from tradition-dependent ones, allowing inaccuracies to be rigorously decomposed into internal arithmetic drift, sidereal misalignment, and anomaly-phase defects. Crucially, computational analysis also reveals remarkable historical robustness: the discrete arithmetic of traditional day rules renders boundary tie-cases operationally absent, while large internal temporal buffers and the multi-hour inaccuracy of the classical lunar model insulated the calendar against geographic variation. On this basis, the paper develops a stratified reform space rather than a single replacement proposal. The resulting standards range from conservative rational repairs preserving traditional arithmetic to explicit astronomical reconstructions culminating in fully dynamical models of true solar and lunar motion. The guiding question is how far astronomical correction can be carried without discarding the Tibetan calendrical identity embodied in the structural rules for month and day labeling. Finally, calendric reform requires more than new formulas and constants; it demands precise numerical semantics. The proposed standards are thus formulated not merely as abstract models, but as executable, reproducible specifications suitable for implementation, validation, and long-term transmission across computational environments.