A Criticism on the 2820-Year Period in the Hejira Solar Calendar

Periods are the most important parts of the Iranian calendars. During the Achaemenien dynasty the lunisolar 19-year system was in official use. The lunisolar 25-year cycle of intercalation was in use by the Seleucid.At first the Arsacid used the same way of the Seleucid, but then used the solar calendar with the era of the first king of the dynasty. During the Sassanid era the Arsacid calendar became popular in two forms of common civil (or khirājī) and Vahijkik (one month intercalation in every 120 years).The period of leap years in every 120 years during the Sassanid era have been studied by Iranian researchers and orientalists. The doubled intercalary which was two months simultaneously in 461 A.D. during Firūz monarchy (the son of Yesdgerd II, 484-459 A.D.), has been the turning point of intercalary not only in the Iranian calendar but also in other Islamic possessions for about 600 years.The new way of intercalary which was applied in Jalālī calendar is an honor for the Iranian mathematicians and astronomers which found it about 1000 years ago.Unfortunately, there would never be a periodical usage for the leap years, even in the present formal Hejira solar calendar.The 2820-year intercalation period is one of the new innovations which presented by some modern researchers, but its way lacks accuracy and efficiency of the 128-year historical intercalation period.