Accuracy
The Burmese calendar uses lunar months but tries to keep pace with the solar year. The present Thandeikta system's solar year is about 23 minutes 51.43 seconds ahead of the actual mean tropical year of 365.241289 days. The older Makaranta system was actually slightly more accurate, with 23 minutes 50.87 seconds ahead of the actual year. The table below shows how Thandeikta purports to achieve a narrower difference (hence better accuracy) over Makaranta.
| Makaranta | Thandeikta | |
|---|---|---|
| 19 solar years | 6939.91625 days | 6939.9163731466 days |
| 235 lunations | 6939.687005 days | 6939.68816731 days |
| Difference | 0.229245 day | 0.2282058366 day |
The gain in accuracy is 0.0010391634 day (89.78371776 seconds) over 19 years, or about 4.72546 seconds a year. However, this gain is illusory as Thandeikta achieves the gain by redefining the mean lunar month (lunation) and the solar year, which unfortunately are less accurate. The table below shows the solar years of both systems in comparison with the actual mean tropical year. Thandeikta is 0.56 second a year less accurate than Makaranta.
| Makaranta | Thandeikta | |
|---|---|---|
| 19 solar years per own definition | 6939.91625 days | 6939.9163731466 days |
| Actual 19 tropical years | 6939.601591 days | 6939.601591 days |
| Difference over 19 years | 0.314659 day | 0.3147821466 day |
| Difference over per year | 23.84784 minutes (1430.8704 seconds) |
23.85717322 minutes (1431.430393 seconds) |
In sum, both systems are about 24 minutes per year ahead of the actual tropical year; the systems' methods of intercalation fixes only their internal error; and Thandeikta slightly accelerates the annual drift. The accumulating error means the new year's day which used to fall near the vernal equinox at its launch in 638 now falls on 17 April in 2013—a drift of 15 days, (after adjusting to the Julian and Gregorian calendar switch). Burmese calendarists have dealt with the issue by using apparent reckoning and periodically modifying the intercalation schedule in the Metonic cycle. One major downside of this approach is that it is not possible to publish future calendars more than a few years (often even a year) ahead.
Read more about this topic: Traditional Burmese Calendar
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