Birkat Hachama - Source of The Practice

Source of The Practice

The primary source for the institution of Birkat Hachama is a Beraita mentioned in the Talmud:

"תנו רבנן הרואה חמה בתקופתה...אומר ברוך עושה מעשה בראשית"

"Our rabbis taught: 'One who sees the sun at its turning point...recites the blessing of 'the maker of works of creation."

The middle of this clause in the beraita (elided here) speaks of other astronomical phenomena, the interpretation of which is ambiguous. The Gemara clarifies when the 'turning point' (תקופה) mentioned in the beraita occurs:

"ואימת הוי אמר אביי כל כ״ח שנין"

"And when does this happen? Abaye answers: every 28 years..."

Next, the gemara explains the basis of the 28 years:

"והדר מחזור ונפלה תקופת ניסן בשבתאי באורתא דתלת נגהי ארבע"

"...when the cycle renews and the 'season of Nisan' (i.e. vernal equinox) falls in Saturn, on the evening of Tuesday going into Wednesday."

This explanation provided by Abaye is based on a ruling of Shmuel also mentioned in the Talmud:

"Shmuel stated: The vernal equinox occurs only at the beginning of one of the four quarters of the day, either at the beginning of the day or at the beginning of the night, or midday or midnight. The summer solstice only occurs at either at the end of 1.5 or at the end of 7.5 hours of the day or the night. The autumnal equinox only occurs at the end of 3 or 9 hours of the day or the night. The winter solstice only occurs at the end of 4.5 or 10.5 hours of the day or the night. The duration of a season of the year is no longer than 91 days and 7.5 hours."

Shmuel's calculations, however, are imprecise because the earth does not travel around the sun with a constant speed. His imprecision, according to Abraham ibn Ezra, was a function of the desire of the time to avoid the necessity of manipulating fractions.