1,293. Which Set of Calculations Did the Sages Use?

Hilchos Kiddush HaChodesh 10:5

If one wants to know when a year’s vernal equinox will occur based on these calculations, one must first determine how many years of the cycle have been completed. One then adds the remainder of a year, which is 10, 21, 121, 48, for each year. Re-group the rega into chalakim, the chalakim into hours, and the hours into days, the same as one when calculating a molad. Subtract nine hours and 642 chalakim from the sum and divide the result by the length of a lunar month; a remainder smaller than the length of a lunar month is added to the time of the Nisan molad for that year. The vernal equinox for the year will occur at the rega determined by this process.

Hilchos Kiddush HaChodesh 10:6

The Rambam expresses the opinion that the Sages used this calculation for the seasons when it came to inserting leap years – in the days when the Sanhedrin did so for reasons of time or for other considerations – because this manner is more precise than the one previously described. It is more like the process of astronomers than the previous method, which was based on the assumption that a solar year is exactly 365 and one-quarter days.