Soc.Culture.Jewish Newsgroups Frequently Asked Questions and Answers URL: www.scjfaq.org/faq/05-09.html

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Question 5.9:
How does Judaism measure the day?

In the Talmud (Eiruvin 56a), Shmuel (3rd cent CE) asserts the Julian year to be a sufficient approximation for the true solar year for legal purposes: 365 days, 6 hours. In Sefer haIbbur, Rav Ada (a younger contemporary) asserts a closer approximation of 365 days, 997 chalaqim, 46 rega'im.

In general, tradition follows R' Ada, except in the Blessing on the Sun, which is done once every 28 years. Every 28 years, the sun returns to where it was at the moment of its creation on Wednesday. This is only true if you presume Shmuel's approximation, which would have each year be 52 weeks, 1.25 days. The calculations of R' Ada's approximation would lead to the blessing being said too rarely. In any case, the whole thing is symbolic, as there is reason to believe Shmuel himself didn't take the "week of creation" literally.

The Jewish calendar, which uses the Metonic 19 year cycle of 12 and 13 month years, is adjusted to get a total of 19 of R' Ada's approximation of solar years. For the month, the approximation used is to the nearest heleq, not rega: 29 days, 12 hours 793 halaqim. However, it is exact to that precision. [Which is quite an accomplishment, as the month length varies (the path of the moon around the earth is chaotic, what Newton called a "three body problem). It would take roughly 2,400 years of averaging to get a standard deviation that small. Jewish tradition attributes great age to this number, dating it all the way back to G-d telling Moses in Sinai.]

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