Convert Synodic Months to Sidereal Years
Convert Synodic Months (syn.mo) to Sidereal Years (a_sid) instantly and accurately.
Conversion Formula
a_sid = syn.mo × 0.08084893747
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.
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.
Quick Reference Table
| Synodic Months (syn.mo) | Sidereal Years (a_sid) |
|---|---|
| 1 syn.mo | 0.0808489 a_sid |
| 2 syn.mo | 0.161698 a_sid |
| 5 syn.mo | 0.404245 a_sid |
| 10 syn.mo | 0.808489 a_sid |
| 25 syn.mo | 2.02122 a_sid |
| 50 syn.mo | 4.04245 a_sid |
| 100 syn.mo | 8.08489 a_sid |