A lunisolar calendar calibrated to the eclipse of 632 CE, built with real astronomical algorithms
This site is an astronomical computing tool that converts Gregorian dates into a lunisolar calendar calibrated to a documented historical astronomical event, and compares it with the official Hijri calendar. The calendar combines two properties: following the Moon for months, while keeping those months fixed to the solar seasons.
It is intended for enthusiasts, students, and researchers in astronomy, history, and calendar studies. It is not offered as a substitute for any adopted calendar and carries no religious or official academic authority.
We chose the eclipse of 27 January 632 CE as our starting point for precise scientific reasons: a solar eclipse can only occur at the moment of astronomical conjunction — it is therefore a defined lunar moment accurate to within minutes. This particular eclipse is documented in the most authentic hadith collections (Bukhari and Muslim), having occurred on the day of the death of Ibrahim, son of the Prophet Muhammad (pbuh).
NASA's computations confirm it was the only eclipse visible from Medina during that entire era, and that its position matches what the historical sources describe (late Shawwal, year 10 AH). This convergence of astronomical certainty and historical documentation makes it an exceptional calibration point.
The official Hijri calendar is purely lunar: its months drift through the seasons completing a full cycle every ~33 years. The Hebrew and Chinese calendars, by contrast, are lunisolar: they follow the Moon but insert a leap month periodically to keep months anchored to their seasons. Our calendar does the same.
The synchronisation rests on a remarkable astronomical fact discovered by the Greek astronomer Meton:
Every 19 years, 7 leap months ("Nasi'") are added to keep the calendar aligned with the seasons. The logic uses no fixed table — it counts real astronomical conjunctions: if a year spans 13 conjunctions, it is a leap year.
When we ran the calculation independently — without forcing any outcome — it produced multiple convergences with the historical record:
The site computes the next conjunction (new moon) in your local time, and issues an impossibility verdict — never a visibility claim — based on two scientific thresholds:
Passing both thresholds does not mean the crescent will be seen — visibility depends on the horizon, atmosphere, and eyesight. We judge impossibility only, because it is the more certain claim.