Convert Sidereal Years to Synodic Months
Convert Sidereal Years (a_sid) to Synodic Months (syn.mo) instantly and accurately.
Conversion Formula
syn.mo = a_sid × 12.36874635
About Sidereal Years
The sidereal year = Earth's orbital period relative to the stars = 365.25636 days = 31,558,149.8 s - about 20 min 24 s longer than the tropical year. The gap arises from precession of the equinoxes (~50 arcsec/year westward drift), which causes the tropical year to end slightly before the sidereal orbit completes. JPL DE440 ephemerides and J2000.0 epoch are anchored to the sidereal frame. 1 a_sid = 365.25636 d.
About Synodic Months
A synodic (lunar) month = new moon to new moon = 29.5306 days = 2,551,442.9 s. Longer than the sidereal month (27.32 d) because Earth advances ~27° in its orbit during that time; the Moon must travel further to realign with the Sun. Islamic Hijri, Hebrew lunisolar, and Chinese calendars are built on synodic months. Eclipses repeat via the Saros cycle (18 yr 11 d = 223 synodic months). 1 syn.mo ≈ 29.5306 d.
Quick Reference Table
| Sidereal Years (a_sid) | Synodic Months (syn.mo) |
|---|---|
| 1 a_sid | 12.3687 syn.mo |
| 2 a_sid | 24.7375 syn.mo |
| 5 a_sid | 61.8437 syn.mo |
| 10 a_sid | 123.687 syn.mo |
| 25 a_sid | 309.219 syn.mo |
| 50 a_sid | 618.437 syn.mo |
| 100 a_sid | 1236.87 syn.mo |