Birkat Hachama - How The Date Is Calculated

How The Date Is Calculated

This table represents a week with all the 24 hours marked and labeled by the "planet", actually a celestial light in direct translation, which corresponds to them according to the ancient geocentric model of "planet" order. It is based on Rashi's commentary to Eruvin 56a. We create this table by noting that in the Bible the stars were created on Wednesday at twilight so we place Saturn (the first "star") at hour 1 (or 6:00 p.m.). Then we follow with all the 6 other "planets" (Jupiter, Mars, Sun, Venus, Mercury, and Moon) and repeat the order until all the hours of the week have a "star" assigned to them.

We assume the world was created in the Hebrew month of Nissan, not Tishrei. So hour 1 (sunset) on Wednesday on this chart represents the beginning of Tekufat Nissan (Spring) in the first year of creation, the Hebrew year 1. For the purpose of Birkat Hachama, the calculation of Samuel of Nehardea for the length of a tropical year is used not the more accurate calculation of Adda bar Ahavah. Therefore a year is assumed to be 52 weeks and 1 day and 6 hours (365.25 days). This means that a year after creation Tekufat Nissan will occur 1 day and 6 hours later on this chart, or on Thursday at the 6th hour, Jupiter. Each year this will shift another day and 6 hours. It will take 28 years for Tekufat Nissan to return to being on Wednesday at twilight. It is worth noting that the true spring equinox occurs on March 21 yet Birkat Hachama is said 18 days later. This inaccuracy accrued over thousands of years as a result of using the inexact calculation of Samuel for the length of the year.