Question 5.9: How does Judaism measure the day?

                                  Answer:
   
   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.]