Leap Years in the Persian Calendar
A unique set of leap year rules makes the Solar Hijri calendar one of the world’s most accurate calendar systems.
Iranian flag over Tehran, Iran.
©bigstockphoto.com/Borna
Seasons Determine Leap Years
Like the Western Gregorian calendar, the Persian Solar Hijri calendar is a solar calendar with common years lasting 365 days and leap years with 366 days.
Its leap year rules, however, are specifically designed to keep the calendar in sync with the seasons, with each calendar year reflecting the length of time between two vernal equinoxes.
This makes the Solar Hijri calendar, which is officially used in Iran and Afghanistan, one of the world’s most accurate calendar systems.
Read more about the Solar Hijri calendar
Leap Year Rules of the Solar Hijri Calendar
The rules governing the Solar Hijri calendar were initially conceived almost a thousand years ago—in 1079 CE—by the legendary Persian astronomer and mathematician Omar Khayyam.
The calendar uses an intercalation system based on a 33-year cycle, consisting of 25 common years and 8 leap years.
A cycle starts with four common years, followed by a leap year; all of the remaining leap years occur after three common years. In other words, years 5, 9, 13, 17, 21, 25, 29, and 33 of each cycle are leap years.
Based on this arrangement alone, the calendar deviates from solar time by about three minutes per 33-year cycle. To make up for that drift, the cycle is sometimes shortened to 29 years. However, this happens only very rarely.