Birkat Hachama - The Solar Calendar

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:

1. Ten days were removed in order to compensate for the incorporation of excess days since the establishment of the Julian calendar.
2. 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 vernal equinox has been shifting slightly forward in the Gregorian year ever since 1582.

• In 1609, the halachic vernal equinox was on April 4
• In 1709, the halachic vernal equinox was on April 5
• In 1809, the halachic vernal equinox was on April 6
• In 1909, the halachic vernal equinox was on April 7
• In 2009, the halachic vernal equinox falls on April 7

The halachic equinox usually jumps a day every century because the algorithm that established the Gregorian calendar dictates that leap years do not occur in years divisible by 100, unless also divisible by 400. Jewish law, however, provides no means for changing the established calendar until such a time as the Sanhedrin might be reconvened, and compensates for it by moving the date of the equinox almost every century. There was no shift between 1909 and 2009, however, because of the exception regarding years divisible by 400; thus, the year 2000 did contain a February 29 and no compensation was necessary. The table at right depicts all the vernal equinoxes from 1981 to 2009, two years in which Birkat Hachama occurs; note that the equinox does not occur at sunset (time=0) on the fourth day (Tuesday) any other years in the entire 28 year cycle.

The halachic year of 365.25 days is equivalent to 52 weeks, 1 day and 6 hours. This means that any given date will, three times out of four, appear a day later in the calendar week in a subsequent year. For example:

• July 3, 1932 was on Sunday
• July 3, 1933 was on Monday
• July 3, 1934 was on Tuesday
• July 3, 1935 was on Wednesday

July 3, 1936, however, was on a Friday and not a Thursday because the 6 hours that accrued over each of the four years effectively adds another calendar day (i.e. 6 hours x 4 years = 24 hours = 1 day).

Halacha maintains that the Sun was created in the position of the vernal equinox immediately after sundown on the fourth day of the week of Creation, which is equivalent to sunset on Tuesday of that week. If that is considered time zero (t=0), and subsequent years' vernal equinoxes occur 1 day and 6 hours later, it would appear as follows:

• Year 1: Day 4 at sunset (Tuesday)
• Year 2: Day 5 at midnight (Thursday)
• Year 3: Day 6 at sunrise (Friday)
• Year 4: Day 7 at noon (Saturday)
• Year 5: Day 2 at sunset (Sunday)
• Year 6: Day 3 at midnight (Tuesday)

Every four years sees a jump of an additional day because the four six-hour periods sum to a full day. This is somewhat similar to a solar leap year occurring every 4 years to account for the four quarter days that accrued at a rate of a quarter day per year.

Although the proper time for the blessing would be at sundown on Tuesday April 7, the Sun is no longer visible at sundown; the blessing is therefore delayed until the following morning.

The 28 year cycle is based on a solar year of 365.25 days, which is only nearly precise. The Hebrew calendar itself uses a solar year of 365.2468 days, but utilizes the less precise approximation of 365.25 for Birkat Hachama so that the blessing might occur with some frequency.

See also Hebrew calendar: Accuracy.