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Kiddush HaChodesh: The Jewish Calendar

In this week’s parsha we read about the very first mitzvah given to Klal Yisroel: the mitzvah of Kiddush HaChodesh, sanctifying the month, which is the basis for our calendar. Fulfilling this mitzvah is necessary for the observance of many of our most important mitzvos, for without it there would be no Yomim Tovim. Let us take this opportunity to examine this most fascinating mitzvah. It is fascinating because, aside from the halachic aspects involved, there is also an interesting historic angle that needs to be discussed. Additionally, there are many not so well-known tidbits about the calendar that are quite interesting.


Hashem told Moshe (Shemos 12:2), “This month will be for you the beginning of months.” Rashi comments that Hashem showed Moshe the new moon and told him, “When the moon appears like this, you shall sanctify and declare Rosh Chodesh.”

According to many Rishonim, this mitzvah actually encompasses two mitzvos: 1) Sanctifying the month and 2) declaring leap years (Sefer HaMitzvos l’Rambam, Asei #153; Sefer HaChinuch #4).

The Gemara (Rosh Hashanah 22a) maintains that only Beis Din is authorized to sanctify the moon. This is derived from the word in the possuk, “lachem” – “for you.” In other words, the authority to accept testimony and to deal with the issues of the new moon was given to men of Moshe and Aharon’s stature – the leaders of the people. According to the Rambam (Hilchos Kiddush HaChodesh 5:1), these issues were handled by the Sanhedrin or any beis din whom they authorized.

Although lechatchilah, the Nasi who was also the head of the Sanhedrin, together with the Sanhedrin would sanctify the month, neither was necessary. Rather, any authorized beis din of three people who had received semicha could do it (Mishnah Rosh HaShanah 2:7, 4:4, Rav MiBartenura ibid.; Rambam 4:12, 5:1).


Before explaining what is “sanctifying the month” and how was it done, we must briefly discuss the lunar cycle.

As everyone knows, the moon orbits the earth on a monthly basis. It takes approximately twenty-nine and a half days for the moon to complete its orbit. When the moon is behind the earth vis-à-vis the sun, we see the entire face of the moon illuminated by the sun, and this is called a full moon. This occurs during the middle of the Jewish month. At the end of the first and third weeks of the month, when the moon is directly alongside the earth in relation to the sun, only half of the moon’s face is illuminated. We call this a quarter moon. When the moon comes in between the earth and the sun at the very end of the month and the moon’s dark side is exposed to us, we do not see the moon at all. The moment when the moon passes directly between the earth and sun is referred to in halacha as the molad, the “birth” of the new moon.

There is a period of approximately two days, one day before and one day after the molad when the moon cannot be seen at all. The evening when it is first visible after the molad signifies the beginning of the month. If that evening is the eve of the thirtieth of the month, the following day is the first day of the new month. If the moon does not appear that night, the next day, the thirtieth, is the last day of the previous month, while the following day will be the first of the new month (Rambam, Hilchos Kiddush HaChodesh 1:3).

When the month is twenty-nine days long, it is called “chaser” – lacking. If however, it is a thirty-day month, it is referred to as being “malei” – full (ibid. 4).

In anticipation of Rosh Chodesh, the judges of Beis Din would calculate whether or not the moon could be visible on the eve of the thirtieth. If according to their calculations the moon could be visible, they would wait the entire thirtieth day to see whether anyone will arrive to testify that he had witnessed the new moon the previous evening. If witnesses came, their testimony was thoroughly examined. If the testimony was determined to be truthful, the head of the beis din would declare, “Mekudash,” meaning, the month was “sanctified.” The other people present would then respond, “Mekudash, mekudash,” and that day was proclaimed to be Rosh Chodesh. If witnesses did not come that day, or their testimony was rejected, the following day (the thirty-first) became the first day of the new month (ibid. 1:6, 2:8).

However, if based on their calculations, the judges knew that it was impossible to see the moon on the eve of the thirtieth, beis din did not receive any testimony regarding the new moon, because they knew that anyone claiming to have seen the moon either had mistaken a cloud for the moon or was lying (ibid. 1:6). In this situation it was unnecessary for beis din to sanctify the month, for as in the words of the Mishnah, “Heaven already sanctified it” and the thirty-first day automatically became Rosh Chodesh (Rosh HaShanah 2:7; Rambam 2:8).


Originally, any two Jewish men could come to beis din and testify that they saw the new moon and if it was determined that they were telling the truth, their testimony was accepted. This was because of the rule that until proven otherwise, everyone has a chezkas kashrus, a presumption of validity (Rambam 2:2).

However, at a later stage, a group of Jews called the Bei’sosim appeared and they attempted to mislead the chachamim. They and another group of renegades called Sadducees were students of Tzadok and Bei’sos, who chose to ignore the Oral Torah and stuck to a literal understanding of the Written Torah. With the goal of corrupting the Jewish calendar, they would hire witnesses to testify falsely in beis din that they saw the new moon (Gemara Rosh HaShanah 22b; Rambam ibid.). (As for the reasons why they did this, see Rashi, Tosafos, Pnei Yehoshua, Turei Even, Sefas Emes and Aruch LaNer ad loc.)

When this occurred, Chazal instituted that only people who were known to be kosher witnesses could be admitted in court to testify regarding the new moon. This presented a problem. What if people that the main beis din did not know were the ones who saw the new moon; how could they testify? In this situation, the local beis din of their city, who knew that they were kosher, would send a pair of witnesses who were recognized by the Sanhedrin to testify that the witnessing pair were valid (Rambam 2:2-3).


In order to encourage people to come to Yerushalayim to testify, large meals were served to all those who came. Additionally, even if beis din was able to determine that it was Rosh Chodesh based on the testimony of an earlier pair of witnesses, nevertheless, beis din would interview all of the people who came, albeit to a lesser degree, in order that no one should feel he came for nothing (ibid. 2:7).


The passuk says (Vayikra 23:4), “These are the appointed festivals of Hashem, the holy convocations, which you shall designate in their appropriate time (b’mo’adam).” Chazal understood that whenever the word “moaid” appears in the Torah in connection with a mitzvah, one is allowed to desecrate Shabbos to perform it. Since the mitzvah here is to see to it that the festivals occur on the correct day and the only way for this to happen is to designate the proper day as Rosh Chodesh, we are permitted to transgress Shabbos in order to testify in beis din that the new moon was sighted (ibid 3:2).

This halacha also changed during two time periods. When the Beis HaMikdash was still standing, one was allowed to desecrate Shabbos in order to testify about any Rosh Chodesh during the year. This was because it was necessary to determine the correct day to bring the special Musaf offering normally brought on Rosh Chodesh. Since the bringing of this offering superseded Shabbos, one was also allowed to travel on Shabbos as well in order to testify (ibid.).

However, after the churban, when korbanos were no longer brought, one was only allowed to travel on Shabbos to testify for the months of Nissan and Tishrei. Since the festivals occur during these months, and everyone needed to know when to observe the holiday, the chilul Shabbos was permitted (ibid.).


Another system that was changed over the course of time was how beis din informed the Jews of Bavel when Rosh Chodesh was. Bavel was the major population center outside of Eretz Yisrael at the time and no modern methods of telecommunications were available. Originally, after beis din declared Rosh Chodesh, someone would ascend Har HaZeisim and light a large torch and wave it back and forth and up and down. He did this until he saw someone on the next mountaintop do the same. This would continue from mountaintop to mountaintop all the way until Bavel (Mishnah Rosh Hashanah 2:2-4).

Again, this worked fine until the Kusim intentionally mislead the Jews. The Kusim, also referred to as the Shomronim or Samarians, were a group of non-Jews who were settled in the Shomron by the king of Ashur (Assyria) after he exiled the Aseres HaShevatim (see Melachim II 17). At one point this nation converted, but the Sages were undecided whether their geirus was sincere. After it was discovered that they continued to worship idols, they were ostracized from the Jewish community. It is possible that this caused the Kusim to hate the Jews and make any attempt to disrupt their lives (History of the Jewish People – From Yavneh to Pumpedisa, Mesorah Publications 1986, pg. 166). Normally, torches were only lit when the previous month was only twenty-nine days, but when it was malei, thirty days long, and no torches were lit. Thus, if the torches were not used, everyone understood that they must start the new month on the day following the thirtieth day. However, the Kusim would light torches even when the month was supposed to be malei, thus misleading people to think that the previous month was only twenty-nine days long (Rav MiBartenura, Rosh HaShanah 2:2).

At this point it became necessary to use a more secure method to inform Diaspora Jewry. The Sages therefore started hiring messengers to do the job (Rambam 3:8). Unlike the witnesses who were permitted to desecrate Shabbos in order to come testify that they saw the new moon, the messengers could not (Rosh HaShanah 21b; Rambam 3:8).

The messengers would travel to Bavel only during specific months. During Nisan they went to inform them, so that they would know when Pesach would be. Once they knew the correct day for Pesach, Shavuos was not an issue, as it is always fifty days from the first day of chol hamoaid Pesach. They traveled during Av to inform them of the date of Tisha b’Av. Although there are no holidays in Elul, the messengers left for Bavel during that month so that the Jews there would have a better idea of when Rosh HaShanah would be. Once they knew the correct day of Rosh Chodesh Elul, they would count twenty-nine days and celebrate Rosh HaShanah on the thirtieth. Even though there was a chance that Elul would be thirty days long, since most years Elul was only twenty-nine, they followed the majority. However, if they would not know the precise day of Rosh Chodesh Elul, they would have to celebrate Rosh Hashanah for two days. The messengers also went to inform them when Tishrei, Kislev and Adar began because of the holidays that occurred during those months (Rambam 3:9).


The Rambam (5:2) writes that according to Halacha l’Moshe mi’Sinai, as long as there is a Sanhedrin, the new months must be inaugurated based on testimony of witnesses, but when there is no Sanhedrin, we are to fix Rosh Chodesh based on calculations. The system of sanctifying the moon based on eyewitness accounts of seeing the new moon was in force for over 1,600 years, from the time of Yetzi’as Mitzrayim until the last generation of Amoraim. However, with the increase of persecution by the Roman government, and because of the concern that the Sanhedrin would no longer be able to meet to proclaim Rosh Chodesh, it became necessary to base the calendar solely on calculations.

Therefore, the Nasi Hillel II, a fourteenth generation descendant of Hillel HaZakein, in the year 4119 (359 CE) convened the Sanhedrin in order to deal with the issue (History of the Jewish People, pg. 190). In the words of the Ramban (Comments to Rambam Sefer HaMitzvos, Asei #153): “He calculated the sanctification of months and declared those leap years that needed to be declared until Eliyahu returns, when once again we shall go back to sanctifying the months by a first sighting of the new moon.”


We have mentioned numerous times that the time of Rosh Chodesh is “calculated.” How was this done?

Although we mentioned earlier that the lunar month is twenty-nine and a half days long, this is an approximate figure. The precise amount is twenty-nine days, twelve hours and 793 chalakim (Rambam 6:6). Some will be familiar with this word, “chalakim” as it is used on Shabbos Mevarchim during Birchas HaChodesh or Rosh Chodesh Bentching when the gabay or chazzan announces, “the molad will be…” What is a “cheilek”?

A “cheilek” means “a portion,” and in this case a portion of an hour. Each hour contains exactly 1080 chalakim. The reason why this number is used is because it was a simple way of calculating without fractions. Also it is divisible by many numbers, such as 2, 3, 4, 5, 6, 8, 9, 10 and others (Rambam 6:2 and Pirush ad loc.). Therefore after converting chalakim into minutes and seconds, a lunar month is twenty-nine days, twelve hours, forty-four minutes and three and one third seconds. This is the time between one molad or new moon and the next.

Although knowing the length of the month is important for the purposes of calculating when the molad takes place, even more crucial is to have a starting point from when to begin the calculations. Chazal had a tradition as to when the molad of Tishrei during the year of creation took place and all future calculations are based on that molad (Tur Orach Chaim 427).


Incidentally, it is worthwhile to point out that although most people think that the time of the molad announced in shul is the actual time of the molad, in reality, it cannot be taken at face value for two reasons. The first reason is because it is not based on our method of telling time. For example, on this previous Shabbos Mevarchim (Shevat 5767), the gabay announced: The molad will be on Thursday night, ten hours, thirty-three minutes and ten chalakim. Many people think that this refers to 10:33 PM on Thursday night. This is incorrect, as the molad could be about twenty minutes earlier, depending on several factors. However, the explanation of these calculations is beyond the scope of this discussion.

The second reason is because even if the time announced was actually in sync with our clocks, it is based on Jerusalem Time, i.e., the time in Jerusalemat the time of the molad.

The fact that the time announced needs to be converted to local time is important when determining a different halachah – the earliest and latest times for reciting Kiddush Levanah. This bracha can only be recited during a specific time during the month, whose calculations are based on the molad. If one merely tried to make the calculations based on the molad announced in shul, his reckoning would be off. Therefore, in order to ascertain the earliest and latest times for Kiddush Levanah it is essential to have a luach that makes the conversion to local time. To the best of my knowledge, most luchos simply include theJerusalem time without any conversions or clarifications.


Although it has no practical halachic ramifications for us, in order to gain a greater appreciation for the chochmas Chazal, it is worthwhile to point out that there is another calculation for the molad. The length of the lunar month quoted above is in actuality the average length of the month. In truth, the time between one molad and the next can vary by several hours, due to gravitational forces and the distance between the earth and the moon. This is because the moon’s orbit is not a perfect circle, but rather elliptical, and the distance between it and earth typically varies from 230,000 to 251,000 miles. When the moon is closer to us, the earth has greater gravitational pull and the moon moves faster in its orbit. This variance in speed causes the actual molad or the molad ha’amiti to occur earlier or later than the average molad announced in shul (Kuntres Di Shamaya on Rosh HaShanah 25a). The fact that Chazal, who lived over two thousand years ago and did not have any modern equipment with which to measure and calculate, and nevertheless were aware of this information is absolutely mind boggling.



As we mentioned, the lunar month is approximately twenty-nine and a half days long. In order to compensate for this extra half of a day, the months generally alternate between twenty-nine and thirty days long. We will discuss the exceptions to this rule shortly. Therefore, every thirtieth day from Rosh Chodesh will also be Rosh Chodesh; in a month that is chaser, twenty-nine days, the thirtieth will be the first of the new month, and in a month that is malei, thirty days, the thirtieth is Rosh Chodesh as well as the following day. In this situation, we have two days Rosh Chodesh because part of the thirtieth day should belong to the following month (Rambam 8:4).


Since in general the months must alternate between twenty-nine and thirty days, Chazal chose that the more important months would have thirty days. Hence, Tishrei, Shevat, Nissan, Sivan and Av are always thirty days since each one has a holiday. Teves, Adar, Iyar, Tammaz and Elul are always twenty-nine. Although Adar contains the holiday of Purim, it cannot have thirty days. This is because the Gemara says that people know that thirty days after Purim is Pesach and if Adar would be malei, people would end up eating chometz on Pesach. The added month during a leap year, Adar Rishon, is always malei (Shevilei d’Raki’ah [written by the Tiferes Yisroel] 1:8).


Although Kislev has Chanukah and therefore should be malei as well, Cheshvan and Kislev are the two months that can change. When Cheshvan is chaser and Kislev malei, the year is said to be “k’sidrah” – “according to its order,” because then all the months throughout the year will alternate between twenty-nine and thirty days. If both months are twenty-nine days long, the year is called “chaseirah” – lacking. And if both have thirty days, we refer to the year as “sheleimah” – full.

Why do these months fluctuate?

There are two reasons for this: 1) If the lunar month would be exactly twenty-nine and a half days, there would be no need for any months to change their length. Every year there would be six months of twenty-nine and six of thirty. However, since there are an extra forty-four minutes and three and a third seconds every month, which over the course of a year adds up to more than eight hours and forty-eight minutes, compensation must be made for this extra time. These extra minutes add up to a full day in less than three years. Therefore, days are added to Cheshvan and/or Kislev accordingly.

2) In order to understand the second reason why these two months have varying lengths, an introduction is required. Optimally, Rosh HaShanah occurs on the same day as molad Tishrei. However, this is not always so. There are four reasons to delay Rosh Hashanah for one or two days. Since three of the four are beyond the scope of this article, we will only discuss the most famous of these reasons: “Lo AD”U Rosh.” This rule determines that the first day of Rosh HaShanah cannot fall out on Sunday, Wednesday, or Friday. (The abbreviation “adu” is spelled aleph-dalad-vav, which represent days of the week.)

What is the reason for this rule? If the first day of Rosh HaShanah would occur on Sunday, this would mean that Hoshanah Rabbah would occur on Shabbos, making it impossible to perform the special minhag of taking the hoshanos. If Rosh Hashanah were to occur on Wednesday or Friday, Yom Kippur would then occur either on Friday or on Sunday. Since during the time of Chazal there were no methods of refrigeration, it would cause great difficulty to have two consecutive days where all types of food preparation are prohibited.

Therefore, if molad Tishrei occurs on one of those days, Rosh HaShanah is delayed until the next day. Also in some years, two of the reasons for delaying Rosh HaShanah might apply, thereby delaying it for two days. In order to decide how many days should be in Cheshvan and in Kislev, it first must be determined from the moldos when Rosh HaShanah occurs this year and next year and whether any of the four reasons for delay apply. Once this is known, Cheshvan and Kislev can be adjusted accordingly.


As everyone knows, the solar year is approximately 365.25 days long. This is about eleven and a quarter days longer than the lunar year of 354 days (29.5 x 12). If an extra month were not added to the Jewish calendar once every three years or so, the chagim would soon start to wander through the seasons. Pesach would eventually move into the winter, then the fall, etc. This is what happens with the Moslem calendar which does not add an extra month.

In order to compensate for this, seven years out of nineteen are leap years and an extra Adar is added. Thus, the Jewish years are grouped into nineteen year cycles with the third, sixth, eighth, eleventh, fourteenth, seventeenth and nineteenth years being leap years.


The Abarbanel in his commentary to the possuk (Shemos 12:1, s.v., v’halimud ha’shlishi), “This month will be for you the beginning of months,” relates that when the great Greek astronomer Ptolemy heard that Jewish sages had this calculation of seven leap years in a nineteen year cycle and were thereby able to keep the lunar months in sync with the solar year, he was amazed. He said that this could only be prophecy!

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