on a Tuesday once every 28 years. Despite the rigorous calculations that follow, there is no synchronization of this prayer and the actual astronomical point in time when the sun crosses the celestial equator; the symbolism is no different from a situation in which the molad for Tishrei would fall out by day on a Sunday and Rosh Hashanah falls out on Monday. The vernal equinox As explained in the Talmud, there is a tradition that the Sun was created in its vernal equinox position at the beginning of the springtime Jewish lunar month of Nissan. The sages of the Talmud settled disputes over the halachic definition of the vernal equinox by establishing it on March 25 of the Julian calendar. Because both the Julian calendar and Jewish tradition define a solar year as exactly 365.25 days, the halachic vernal equinox historically fell out on March 25 every year. This halachic equinox now falls about 17 days after the true equinox, with the error increasing by about 3/4 of a day per century. In summary, Birkat Hachama is recited when the halachic vernal equinox (the position at which the Sun was created) occurs at sundown on a Tuesday (the time at which the sun was created). The solar calendar This method of marking the cycle of machzor gadol (that is, using the 25th of March) was invalidated in 1582 when the Julian calendar was replaced by the Gregorian calendar by decree of Pope Gregory XIII. The calendar was adjusted to allow for Easter to be celebrated in the appropriate time according to an agreement reached at the First Council of Nicaea in 325 CE. To recalibrate the calendar, two adjustments were made: Ten days were removed in order to compensate for the incorporation of excess days since the establishment of the Julian calendar. A method to avoid further incorporation of excess days was put into place, whereby February 29 would be skipped according to a particular algorithm. The last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the first day of the Gregorian calendar, Friday, 15 October. Thus, while the halachic vernal equinox occurred on March 25 until 1582, in 1583 it occurred on April 4 of the new Gregorian Calendar. However, the Jewish calendar was in no need of such a modification, and remained uninfluenced by it. As a result, the halachic