I was reviewing the discussion about tekufot and it raised something that has bothered me for a while. The definition for tekufat nissan that you gave in the notes on this daf is the time when the sun is directly overhead at the equator. This is the same definition that astronomers claim to use to define the spring equinox. BUT, when I look at a Jewish luach (see http://www.ezrastorah.org/calendar.html) Tekufat nissan is given to be on April 8 while Wikipedia has the spring equinox on March 21. Do you know the source of this difference?
Martin Fogel, carlsbad, USA
Below is what we wrote regarding this topic in our Insights on the Daf for Ta'anis 10a.
Best wishes,
Kollel Iyun Hadaf
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2) DECEMBER 4, 2010
QUESTION: The Gemara rules that outside of Eretz Yisrael, the request for rain in the Shemoneh Esreh begins "sixty days after the Tekufah of Tishrei."
What is special about this date?
RAV YONAH MERTZBACH (ALAH YONAH, p. 22) explains why this date was chosen. In Eretz Yisrael, where rain is needed more, in the event that no rain falls until the first of Kislev, Beis Din institutes days of prayer and fasting. The Chachamim chose to institute the prayer for rain in Bavel on a day which corresponds to this important date in Eretz Yisrael since it is the latest time for the rains to begin in Eretz Yisrael. However, since most people outside of Eretz Yisrael are non-Jews who use a solar calendar, and since the rainy season is actually more closely related to the solar year than to the lunar year, the Chachamim instituted that the request for rain should begin on the first of the "solar month of Kislev." The Gemara in Rosh Hashanah (15a) similarly refers to a "solar Shevat" which is 30 days after the Tekufah of Teves. Likewise, "solar Kislev" begins 60 days after the Tekufah of Tishrei, and that is when those outside of Eretz Yisrael should begin to ask for rain.
Tekufas Tishrei, the vernal equinox, occurs on September 23 (or September 24 in a year which precedes a leap year). Hence, 60 days after Tekufas Tishrei would be November 22 (or November 23 before a leap year), which indeed is the date given by the AVUDRAHAM for when the request for rain begins (as cited by the BEIS YOSEF OC 117). Why, then, in Chutz la'Aretz is the practice today to begin to ask for rain only on December 4 (or 5), 12 days later?
ANSWER: The Gemara in a number of places quotes Shmuel who makes an important statement about the length of the year. Shmuel says that each of the four Tekufos (seasons) of the year lasts 91 days and 7 1/2 hours. Accordingly, a year is 365 days and 6 hours long (91 days and 7 1/2 hours X 4).
In the year 3714 (46 BCE), Julius Caesar arranged the first solar calendar, a calendar based on the earth's position in relation to the sun. His calendar is the basis for the one used by the modern world today. Caesar's astronomers advised him that the solar year is exactly 365 1/4 days (365 days and 6 hours) long, the same as Shmuel's calculation. (Besides the 365 days of the average year, he instituted the addition of a 366th day every four years in order to reflect the extra day that the solar year gained after the passage of four years, due to the four extra 1/4 days of each year.)
This figure, however, is imprecise. The exact length, rounded to the nearest second, is 365 days, 5 hours, 48 minutes, and 46 seconds. Thus, the actual year is 11 minutes shorter than Shmuel calculated. In practical terms, every 128 years the equinoxes and solstices will arrive one day (11 minutes X 128) late on Shmuel's calendar.
As time passed, it became evident that the calendar was not compensating properly for the true length of the year. In order to prevent the summer months from occurring during winter and the winter months during summer, the astronomers modified the Julian calendar and adopted a new version known as the Gregorian calendar. In the year 5342 (1582 CE), they cut out the ten days that were added to the year over the passage of time due to the miscalculation of the Julian astronomers. (The day after October 4 that year was not October 5, but October 15.) They also agreed to deduct three leap years every 400 years. Three out of four centesimal years (for example, the years 1700, 1800, and 1900) will not be leap years, even though they are divisible by four (those years have no February 29). Only a centesimal year in which the number of centuries is divisible by 4 (such as the years 1600 and 2000) will be a leap year. According to their calculations, the average year is 365 days, 5 hours, 49 minutes and 12 seconds long. This is actually off by approximately 26 seconds, but it is sufficiently accurate for most practical purposes. The calendar will be ahead by one day only after 3,323 years.
At that time, the Jewish authorities agreed that it was not necessary to institute a parallel change to the Jewish lunisolar calendar with regard to calculating when the Tekufos occur (which, in Halachah, affects the date on which we begin to recite the request for rain, and the date on which we recite Birkas ha'Chamah). We still rely on Shmuel's calculation of the solar year. However, as a result of the change made by the Gregorian calendar, sixty days after the Tekufah was changed from the night of November 21 (which becomes November 22 at midnight) to December 1. In 1700, when another leap year was skipped by the Gregorian calendar, the day for saying "v'Sen Tal u'Matar" became December 2, and in 1800 it moved up to December 3 (this is the date recorded as the time to say "v'Sen Tal u'Matar" by the Chayei Adam and his 19th century contemporaries). In 1900, the date was moved up to December 4. This is why "v'Sen Tal u'Matar" is said in Chutz la'Aretz beginning from December 4 (or 5, in the year preceding a leap year) today.
This date, December 4 (or 5), did not change upon the arrival of the year 2000, since that year is a centesimal year divisible by 400, which is a leap year according to the Gregorian calendar. Only in the year 2100 will we have to change the date for starting to say "v'Sen Tal u'Matar" in Chutz la'Aretz, and Mashi'ach certainly will have arrived well before that date.
Shalom,
Thanks for the reply. This makes it all very clear.
I was thinking about this, probably too much, and the following idea occurred to me.
In most sefarim I looked at on the tekufot, they define tekufat nissan, for instance, as the time at which the sun first enters the zodiac sign - tleh. This is not the astronomical definition for spring equinox. From what I have been able to find out, around 1 CE or so, these 2 definitions were roughly equivalent. But the zodiac also "moves" and now the sun enters tleh, not at the spring equinox, but about 20 days later as near as I can find out. The tekufat nissan that is based on Shmuel's halacha is now about 13 days after spring equinox as defined by the astronomers. This is about 7 days before the sun enters tleh. So if it is important to correlate with the zodiac sign, the tekufah is quite close, indeed closer to it than to the astronomical equinox.
I now wonder which definition for equinox, halacha uses.
What you refer to is known as the precession of the equinoxes. Because of a "wobble" of the earth's axis, the equinox continues to shift "backwards" through the Zodiac constellations. Today, the months are off by one entire Mazal from the time of the Churban. (The Ibn Ezra discusses this phenomenon in Amos 5:8.)
The Rambam (Hilchos Kidush Hachodesh 9:3) indeed defines the Tekufos of Nisan and Tishrei as when the sun "enters Mazal Taleh" - as you cited from many books. Astronomers define them as the time of year when the sun reaches the point in its path (i.e. in the ecliptic) which crosses the celestial equator, causing the day and night to be of equal length - as you noted.
However, it is clear from the words of the Rambam that he is indeed using the astronomic definition for the equinoxes. He is simply redefining Mazal Taleh as the section of the ecliptic which the sun enters at the astronomical equinox. That is, Mazal Taleh of today, as per the Rambam's usage, has nothing to do with the stars that make up the constellation Aries! Rather, it is somewhere back in what we call Pisces (Dagim). Using this definition, both ways of describing the Tekufah are actually one and the same.
(I thank Rav Alexander Schutz of Kiryat Sefer, author of the helpful pamphlet "Di Shmaya" in English and Hebrew which is available through the Kollel, for helping me with this point as well as with many other points in Talmudic astronomy.)
Mordecai Kornfeld
Kollel Iyun Hadaf