By: Mir M.Hosseini
A group of astronomers including famous Omar Khayyam created the new solar calendar known as Jalali calendar, named after Malekshah, a Seljuk king who founded Isfahan observatory. The most precise calculations of solar year were integrated into this calendar that used solar passes through zodiac for month computations. Also known as Persian calendar stayed in use for more than 8 centuries with revisions later taken from the Chinese calendar. On March, 31, 1925, the Iranian parliament issued a charter for the calendar to have more regular month lengths. This law fixed the beginning of the new year as the first day of the spring and revived the ancient Persian names still in use although it specified the beginning of the calendar as the date prophet Mohammad migrated from Mecca to Medina. According to this calendar; the first 6 months (Farvardin, Ordibehesht, Khordad, Teer, Mordad) have 31 days, and the next 5 months (Mehr, Aban, Azar, Day, Bahman) have 30 days, while the last month (Esfand) has 29 or 30 days in leap years. Based on accurate positional astronomy, the Persian calendar uses a very complex leap year structure which makes it the most accurate solar calendar still in use today. Persians grouped years into cycles. Cycles begin with four normal years. Every fourth subsequent year in the cycle is called a leap year. Cycles then make cycles of either 128 years (29, 33, 33, and 33 years) or 132 years ( 29, 33, 33, and 37 years). A greater cycle is composed of 21 consecutive 128 year and a final 132 cycle, making 2820 years. The pattern which began in 1925 will repeat in the year 4745.