The invention and fabrication of the precise mechanical clocks for reckoning the time correctly, dates back to about three hundred years ago. As such, the mean or actual length of tropical years measured in the ancient history might have been also based on the time measured by sundial which measures solar apparent (real) time based on the length of apparent (real) solar day. The length of real solar day itself changes slightly throughout the year.
At the present time, precision and accuracy in the measurement of time has reached to surprising limits which has revealed the extremely small changes in the period of rotational and ephemeris time.
The Iranian calendar in vogue is a solar one based on the actual length of each tropical year from equinox to equinox.
The leap years are set based on the time when equinox occurs in comparison to the time of the real solar noon for longitude 52.5 degrees east. As such, one has to pay attention to the exact changes in the time of the real solar noon in the distant past and future.
There are two schools of thought about the determination of leap years which happens each four or occasionally five years.
Based on the changing effect of the length of the actual tropical year, many scholars believe that there is no exact rule for the settlement of leap years. On the other hand, some others believe that it is possible to adopt a certain rule for the determination of leap years, namely in a 2820-year cycles. As such, it is believed that the equinox happens exactly at the same time after 2820 years and this cycling rule is going on.
This article deals with a discussion about the proposed 2820-year cycle and the scientific and historical implications involved therewith. As such, the necessity for an extensive reasearch study to be carried out about the proposed cycle in highlighted.
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