According to tradition, the sun was created at the moment of the spring equinox which coincided with the first moment of the fourth day of creation—the day when G‑d set into orbit the sun, moon and all the heavenly bodies. Since, in Jewish tradition, the day begins at the previous nightfall – as we see in the Torah's account of the story of creation, "It was evening, and it was day..." – this actually happened with nightfall on Tuesday evening.
When the sun reaches this starting point again at the exact same time of day and on the same day of the week, we recite the Birkat Hachamah the next morning, shortly after sunrise. This occurs once every 28 years—and this year on Nissan 14, 5769 (April 8, 2009).
(In other words, the spring equinox occurs every year, but on different days of the week and different times of the day. But once in 28 years it occurs on Wednesday [the fourth day of creation] at the moment of the day's onset.)
Please note, from an astronomic point of view, nothing unusual will happen on this date; the sun, moon, planets and stars will not be aligned in any specific pattern.
The starting point is the first moment of the (evening preceding the) first Wednesday in history—the fourth day of Creation. Let us use Tuesday evening at 6:00 p.m. as a rounded off time.
According to the astronomical calculations of the 3rd century Babylonian sage Shmuel – regarding whom the Talmud says, "He was familiar with the pathways of the heaven as with the streets of his hometown Neharda'a" – the length of a solar year is 365.25 days, or 52 weeks plus 1¼ days. It thus follows that precisely one year later, when the sun returned to the original position it occupied at the moment of its creation, it would be 1¼ days (one day and six hours) later in the week: Wednesday at midnight. After two years, it would be 2½ days later in the week: Friday at 6:00 a.m. Only after 28 years, would the sun return to that position on Tuesday at 6:00 p.m.
Birkat Hachamah vs. the Actual Date of the Spring Equinox
Many will undoubtedly take note of the fact that the spring equinox this year will fall on March 20—nineteen days before we will recite the Birkat Hachamah.
To understand the reason for this, let us briefly explain the origins of the Gregorian calendar – the calendar widely used today – which replaced the hitherto used Julian calendar:
The Julian calendar was introduced by Julius Caesar in 46 BCE, and – like that of Shmuel – was based on a 365.25 day year. Its structure was as follows:
a. All future years were to consist of 365 days in a normal year, and 366 days in a leap year.
b. A leap year would occur every fourth year.
c. The spring equinox was set as occurring that year on the 25th of March at 6:00 pm. But since the Hebrew day begins at nightfall, the corresponding Hebrew date for the equinox would actually be the 26th March—an important factor as we will soon see.
So, in each Julian millennium there are 365,250 days (750 normal years [750 x 365 = 273,750] and 250 leap years [250 x 366 = 91,500]).
As modern science has established, however, a solar year is 365.24219 days. As such, 1000 tropical years is 365,242.19 days.
The Julian millennium – and Shmuel's too – is thus 7.81 days longer than the accurate solar millennium (365,250 - 365,242.19).
By the year 1582, the Julian calendar was about 10 days ahead of the tropical year, and an adjustment was again necessary. Pope Gregory XIII introduced two adjustments to the calendar, and the "Gregorian" calendar was established. These two adjustments were as follows:
a. Ten days were removed from the calendar in 1582.
b. To prevent the need for any future corrections, a centenary year not divisible by 400 is not a leap year. The years 1700, 1800, and 1900 were therefore not leap years, whereas the years 1600 and 2000 were.
It follows that the Gregorian calendar is currently 13 days ahead of the Julian calendar: 10 days for the initial correction, plus 1 day each for the three 'skipped' leap years in 1700, 1800, and 1900. And the Gregorian calendar is 18 days ahead of Shmuel's year: the extra 5 days due to Caesar having established the spring equinox as occurring on March 26 instead of March 21.
The Chassidic Angle
So what is the rationale behind saying the Birkat Hachamah on a date predicated on "faulty" calculations?
The Rebbe posited that this is one of those rare conundrums that cannot truly be answered based on the dry facts, rather it requires an explanation culled from the mystical teachings of Torah.
As mentioned above, the Birkat Hachamah is scheduled for the spring equinox, at the moment when the sun was set into orbit.
Though we celebrate the anniversary of all of creation – including the sun – on Rosh Hashanah, approximately six months before the spring equinox, there is actually a debate in the Talmud whether the world was created in Tishrei or Nissan. The consensus reached by the Talmud is that with regards to the counting of years we count from Tishrei, but in all areas of Jewish law that pertain to the calculations of "tekufot," seasons (and equinoxes falls under this category), we calculate from Nissan.
According to mystical teachings, both opinions are correct and not contradictory. All agree that the world was created in Tishrei, but that G‑d conceived the idea of creation in the month of Nissan. The Talmudic "debate" is which one of these two dates is more significant.
The Talmudic ruling quoted above implies that with regards to seasons we ascribe primacy to Nissan—to the conception of the world in G‑d's thought, and the spiritual equinox, as it were: the time when the concept of a spring equinox entered G‑d's mind, rather than the point at which this concept was concretized in physical time. We are actually calculating from a base date that doesn't even exist—the spring equinox that "occurred" in the year before creation!
Accordingly, the timing of this event is set by Shmuel's system, which, according to our Sages, is precisely aligned with the "spiritual" seasons.
(For more elaboration on this topic, see But the Sun is in the Wrong Place!)