Convert Tropical Years to Sidereal Months
Convert Tropical Years (a_trop) to Sidereal Months (sid.mo) instantly and accurately.
Conversion Formula
sid.mo = a_trop × 13.36822753
About Tropical Years
The tropical year = time from March equinox to the next = 365.24219 days = 31,556,925.2 s - the year that governs the seasons. The Gregorian 97/400 leap rule approximates it as 365.2425 days, leaving a residual drift of only ~26.8 s/year. The tropical year shortens ~0.53 s per century due to tidal deceleration. Persian Solar Hijri and Mayan Haab' calendars also approximate it. 1 a_trop ≈ 365.24219 d.
About Sidereal Months
The sidereal month = Moon's orbital period relative to the fixed stars = 27.3217 days = 2,360,591.6 s - the shortest lunar month. It is ~2.21 days shorter than the synodic month (29.53 d) because Earth advances ~27° along its orbit in the same period; the Moon must travel further to re-align with the Sun. Used in GPS orbit design, tidal modelling, and lunar laser ranging. 1 sid.mo = 27.3217 d.
Quick Reference Table
| Tropical Years (a_trop) | Sidereal Months (sid.mo) |
|---|---|
| 1 a_trop | 13.3682 sid.mo |
| 2 a_trop | 26.7365 sid.mo |
| 5 a_trop | 66.8411 sid.mo |
| 10 a_trop | 133.682 sid.mo |
| 25 a_trop | 334.206 sid.mo |
| 50 a_trop | 668.411 sid.mo |
| 100 a_trop | 1336.82 sid.mo |