The Iranian calendar is more precise than the Gregorian system used by most nations.

Nature · Published 3 October 2026 Swipe more like this →

In short: Unlike the Gregorian calendar, which relies on a fixed mathematical formula to approximate the solar year, the Solar Hijri calendar is determined by real-time astronomical observation. By anchoring its New Year to the exact moment of the vernal equinox, it maintains an unparalleled, intrinsic alignment with the Earth's orbit.

The Solar Hijri calendar, also known as the Shamsi or Persian calendar, functions by tracking the Earth's movement around the Sun. Its defining feature is the celebration of Nowruz, or New Year's Day, which occurs precisely at the vernal equinox. If the astronomical equinox falls before noon in Tehran, that day marks the start of the new year; if it occurs after noon, the new year begins the following day. This observational approach ensures the calendar remains perfectly synchronized with the solar cycle, avoiding the long-term drift inherent in systems that rely solely on fixed leap-year rules.

The calendar's epoch is set to 622 CE, commemorating the Hijrah, the migration of Muhammad from Mecca to Medina. While it shares this epoch with the lunar Hijri calendar, the two systems diverge significantly because the Solar Hijri counts solar years. As of 2023, the two calendars are separated by approximately 43 years. The current structure, which assigns 31 days to the first six months and 30 or 29 days to the remainder, was legally established in Iran on 31 March 1925, replacing earlier systems that used sidereal zodiacal months.

Attempts to replace this observational method with complex mathematical algorithms have been met with skepticism. Mathematician Ahmad Birashk proposed a 2820-year cycle in 1993, but studies have shown his projections are less accurate than the traditional 33-year cycle. Furthermore, the calendar has occasionally been subject to political shifts. In 1975, the Pahlavi government briefly changed the epoch to the reign of Cyrus the Great, inflating the year to 2534, before reverting to the original Hijrah-based system in 1978. Today, it remains the official calendar of Iran, serving as a testament to a sophisticated, astronomically-driven approach to timekeeping.

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