Today is the longest day of the year - the summer solstice. Technically, the summer solstice is an astronomical event that occurs when the sun reaches its most northern point in the sky.
The sages of ancient days were well aware of astronomy and the cycle of the seasons. The Talmud even records their calculations of the seasons:
"Samuel stated: ...The summer solstice only occurs either at the end of one and a half, or at the end of seven and a half hours of the day or the night...and the winter solstice only occurs at the end of four and a half, or ten and a half hours of the day or the night. The duration of a season of the year is no longer than ninety-one days and seven and a half hours; and the beginning of one season is removed from that of the other by no more than one half of a planetary hour" (Eruvin 56a).
The NASA website supports the sage's calculations (which were established even without sophisticated scientific equipment or a large budget). Perhaps it could be argued that the solstices are not so difficult to calculate, and any observant person can do so by simply counting. But on the days surrounding the solstice, the alteration in the length of days occurs in minimal increments, making accuracy much more difficult. In fact, people often do not even notice that the days are getting shorter until a month or so later, a fact noticed elsewhere in the Talmud where it states: "It has been taught, Rabbi Eliezer the Elder says: From the 15th of Av onwards the strength of the sun grows less" (Taanit 31a). (The 15th of Av this year corresponds to August 3, 2012).
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