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The Jewish Calendar


In this week’s parasha we learn of the obligation to sanctify the months, and of the mitzva to ensure that Nissan falls in the spring. How are the months consecrated? What is the difference between the Jewish and civil calendar? And how are they different from the Muslim calendar? How is the Hebrew calendar calculated? What is the molad and what is its halachic significance? When is Kiddush Levana recited, and why does a lunar eclipse change the timing? Does a solar eclipse also make a difference for Kiddush Levana? Many communities announce the molad before Birkas Hachodesh, the prayer recited after the Torah reading on the Shabbos before Rosh Chodesh. Why is it announced? Of this, and more, in the coming article.

The Mitzva of Kiddush Hachodesh

The first mitzva the Jewish people received as a nation was the mitzva of Kiddush Ha’Chodesh – consecrating the Jewish month, as we read: “This month shall be to you the head of the months; to you it shall be the first of the months of the year” (Shemos 12:2). Nowadays, we no longer consecrate the month in Beis Din, but rather calculate the calendar according to myriad parameters, based on calculation of the molad.

What is the molad, and what are the halachos that are relevant to it? And what makes the Jewish calendar so unique?

Marking Time

Dividing time into segments is a universally accepted norm. While all cultures divide time into days and months, how to make that division varies in different countries and religions.

The Torah writes: “Then G-d said: Let there be lights in the dome of the sky, to separate day from night. And they shall be for signs, for appointed seasons, for days, and for years” (Bereshis 1:14). The sun and moon were created in order to determine the days, months, and years. Determining day and night is fairly simple – all it takes is following the sun and moon during daylight and nighttime. Determining the year, though, is much more complicated.

On the one hand earth circles the sun over a period of time in a cycle that affects the changing seasons. On the other hand, the culmination of 12 lunar cycles are the months which make up a year. While the solar cycle affects the changes in weather, the lunar cycle affects the appearance of the moon at night. In the beginning of the month the moon is small, growing gradually until the full moon in the middle of the month. Then it shrinks again until it disappears completely, and a new month begins. In the beginning of the month the moon is visible only at the beginning of the night, while at the end of the month it is only visible at the end of the night.

Unmatching Calculation

Since there are 12 full moons in a year, it was easy to claim there are 12 months in a year. However, a lunar calendar has 354 days whereas the solar one has 364.25 days. Herein lies the conflict between calendars.

The nations of the world only require a calendar to mark the passage of time. Therefore, they can get by with an approximation, and the fact that the lunar and the solar calendar don’t coincide is of no great importance. However, for the Jewish people who also factor in the holidays – the “appointed seasons” mentioned in Bereshis, a more accurate calendar is necessary. If we’d base our year on the solar calendar the appearance of the new moon would be meaningless. And if we’d base it on the lunar calendar, the first month – Nissan, would fall in various random seasons, while the Torah specifically commands “Keep the month of spring, and make the Passover offering to the Lord” (Devarim 16:1). Nissan must fall in the spring.

The difference between the solar and lunar calendars causes a difference of approximately one month every two or three years.


The Julian calendar, was proposed by Julius Caesar in 46 BC as a reform of the Roman calendar. This calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years until 1582, when a minor modification reduced the average length of the year from 365.25 days to 365.2425 days and thus corrected the Julian calendar’s divergence from the true solar year. Worldwide adoption of this revised calendar, which became known as the Gregorian calendar, took place over the subsequent centuries. The Gregorian calendar has two types of years: a normal year of 365 days and a leap year of 366 days.

In this calendar, the appearance of the new moon is of no consequence.

The Muslim calendar, known as the Lunar Hijri calendar or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. This calendar has no connection to the seasons, and the month of Ramadan, for example, occurs in a different season every year. [Last year, for example, Ramadan was in the spring. This year it will also be in the spring, while next year it will be in the middle of the winter. This is despite the fact that the name of the month, ramadan, means “great heat” in Arabic.]

The Hebrew calendar, though, is a combination of the two. It is a lunisolar calendar which is based upon the sighting of the new moon but also  factors in the seasons. The Jewish months will always fall in the same seasons in order to fulfil the Torah’s commandment: “Keep the month of spring.”

The Hebrew lunar year is about 11 days shorter than the solar year and uses the 19-year Metonic cycle to bring it into line with the solar year, with the addition of an intercalary month every two or three years, for a total of seven times per 19 years.

Calculating the Jewish Calendar

As previously stated, the Hebrew calendar is a lunisolar calendar, meaning that months are based on lunar months, but years are based on solar years. The calendar year features twelve lunar months of twenty-nine or thirty days, with an intercalary lunar month added periodically to synchronize the twelve lunar cycles with the longer solar year. The beginning of each Jewish lunar month is based on the appearance of the new moon. Originally the new lunar crescent had to be observed and certified by witnesses in the great Beis Din in Yerushalayim in order for the month to be a 29-day month. If no witnesses appeared on the 30th day, the month was consecrated on the following day, and the previous month would be 30 days long.

The Rambam (Kiddush Hachodesh 6:1) writes that while the actual consecration of the month depended upon the sighting of the new moon, the dayanim also made all the calculations themselves. Only if the witnesses’ report coincided with the calculations would the report be accepted. Other additional factors were calculated into consecration of the month (which are beyond the scope of this article) which could interfere in the process. It is important though to note that while, at times, the first of the month was not on the real new moon, the calendar would eventually correct itself.

Towards Rosh Chodesh Adar, the Sanhedrin, the Grand Jewish Court, would gather to determine if the year would be a regular year or a leap year.

In 359 CE, Hillel Nesiah in the Eretz Yisroel Beis Din, calculated the calendar with a fixed intercalation cycle. The sanctification of the Jewish month in Beis Din effectively ended with that. Chazal explain this was a result of the growing persecution apparently with the spread of Christianity throughout the Roman Empire. Hillel’s system is used till today to calculate the Hebrew calendar. These calculations result in a precise calendar which has not and will not need any modifications until the end of time.

[This Hillel was a descendant of Rabbi Yehudah the Prince, who was a descendant of the Tana called Hillel who had lived approximately 350 years before.]

Chazal’s Time

In order to explain what exactly the molad is, we must add another short introduction about time calculation in Chazal.

Our sages divide the day (a cycle of light and darkness) into 24 hours, and every hour into 1080 parts – not the same hours and minutes as we have today. As a result, every minute is comprised of 18 parts, and each part amounts to 3.3 seconds.

The Moon

The moon is a mass of luminescent stone that reflects the sun’s rays, and therefore, only shines when it faces the sun. Molad means “birth”, generically referring to the time at which the new moon is “born”. Chazal call the moment in which the moon is completely hidden and reflects no light from the sun “the moment of the molad” or in English: the lunar conjunction. (A conjunction, as a phenomenon of perspective is an event that involves two astronomical bodies seen by an observer on the Earth.) As we travel away from this moment, the moon becomes more and more visible, at first as a tiny crescent that appears right after sunset, growing every night in visibility and length of time it can be sighted. The full moon is called in Chazal “the contrast [nigud] moment”. At this moment the moon begins shrinking again, both in size and length of time it is visible. At the end of the month the moon is only visible towards the end of the night, right before daybreak.

Molad Amiti (Actual) vs. Molad Emtza’i (Mean)

The molad amiti (actual molad), which has no relevance to the Hebrew calendar, is the time at which the actual astronomical lunar conjunction occurs. This moment is referred to by the Rambam as ibbur. The mean molad is called molad emtza’i and labeled by the Rambam molad.

The Gemara (Rosh Hashana 25a) writes: “Rabban Gamliel said to the Sages: ‘This is the tradition that I received from the house of my father’s father: Sometimes the moon comes by a long path and sometimes it comes by a short one. Rabbi Yocḥanan asked: ‘What is the source for Rabban Gamliel’s statement? The Gemara answers that the source is the verse ‘Who appointed the moon for seasons; the sun knows its going down’ (Tehilim 104:19).”  Only the sun knows its going down, i.e., its seasons and the times that it shines are the same every day and year. The moon, in contrast, does not know its going down-its course is not identical every month. Only the molad [its birth] determines the times for the holidays.

Chazal work with the molad memutza (mean) (Rambam, Hilchos Kiddush Hachodesh, chapter 6). This molad, observed over hundreds of years, is at a regular interval. While the real conjunction occurs at irregular times, the molad memutza appears at regular intervals and there is approximately a 12-hour difference between the two. Therefore, since we only calculate the molad memutza, while Rosh Chodesh always falls around the time of the new moon, the new moon can be sighted before or after Rosh Chodesh.

The Molad Interval

The shortest month is 29 days and 6.5 hours, and the longest is 29 days and 20 hours. The molad interval, as chazal taught us, is exactly 29 days 12 hours 44 minutes and 3+1/3 seconds (or, equivalently, 29 days 12 hours and 44+1/18 minutes) after the previous molad moment. The Rambam writes that before calculating the month, the chachomim in the Great Beis Din needed to calculate the molad memutza (i.e., ensure that the month will fit into the overall calculation of time) and also calculate the actual molad (ibbur, according to the Rambam) to determine if the witnesses are speaking the truth and they indeed saw the new moon.

Hillel Nesiah’s calculations only relate to the molad memutza, and today, the actual lunar conjunction has no bearing on the Jewish calendar.

Kiddush Levana or Birkat Halevana

The Shulchan Aruch (OC 426:4) rules that Kiddush Levana may be recited after 7 days from the molad have passed, not before. The Beis Yosef (ibid) cites the Sha’arei Ora in attributing this halacha to kabbalistic sources. Eidot Hamizrach and Chassidim follow this custom.

The Mishna Brura (footnote 20) writes that most Achronim dispute this halacha and agree that Kiddush Levana may be recited earlier – as soon as three days have passed from the molad the moon already reflects enough light, and the blessing can be recited. While it is customarily accepted to wait until Motzaei Shabbos to recite the blessing, if there is concern that waiting until Motzaei Shabbos will result in forfeiting the mitzva, it can be reciting when three days have elapsed. This is the prevalent Ashkenazi custom.

There are three opinions how to determine the final time for reciting Kiddush Levana:

1) The Shulchan Aruch (OC 426:3) rules that it is when 15 full days have passed from the molad (15X24=360 hours).

2) The Rama contends that it can be recited until half of the time between this month’s and the next month’s molad, i.e., 354 hours 22.5 minutes.

3) The Biur Halacha notes an opinion allowing for reciting it on the entire 16th day.

Practically, the Biur Halacha writes not to recite it on the 16th day, and only post factum, if the correct time has already passed, can one recite the blessing without Hashem’s Name. However, on the 15th day, if the Rama’s cutoff point has passed but the Shulchan Aruch’s time has not, one can be lenient and recite the blessing in full.

The reason for refraining from reciting the blessing after the middle of the month is because the blessing must be recited over something renewing and growing, not a waxing, decreasing moon, signifying lack, not blessing. Those who are lenient to recite it even a short time after the middle of the month maintain that the blessing can still be recited before the decrease is visible.

In light of this halachic debate, it would seem appropriate to calculate the time for Kiddush Levana from the real molad, but the Beis Yosef quotes the Maharil (19) that all calculations in Judaism traditionally follow the molad memutza, regardless of the actual lunar conjunction.

The Maharil, though, adds, that if there was a lunar eclipse that month, the blessing should not be recited afterwards. While a solar eclipse can occur only at the actual lunar conjunction, a lunar eclipse can only occur on a full moon. Therefore, if the full moon was already sighted, Kiddush Levana cannot be recited. But as long as it is unclear, we are permitted to rely upon the molad memutza and recite Kiddush Levana.

Announcing the Molad

The Rishomin (Machzor Vitri p. 173; Kolbo, chapter 37; Abudraham, Seder Rosh Chodesh; and others) mention the custom of reciting Birkas Hachodesh on the Shabbos before Rosh Chodesh. The Mishna Brura mentions it (417:1), with the reason being so people that people will know when Rosh Chodesh will be. The Mishna Brura adds that it is customarily recited standing up just as consecrating the month is Beis Din was done in earlier times.

[Rabbi Akiva Eiger questions this custom, based on the fact that the Beis Din consecrated the month seated. He answers that it was the witnesses who stood when they testified. Therefore, when reciting Birkas Hachodesh, we also recite it standing up.]

Rabbi Efraim Zalman Margalios (Sha’ar Efraim 10:37) adds that some are careful to know the time of the molad before reciting Birkas Hachodesh. Therefore, it became the custom to announce the molad in shul before Birkas Hachodesh. He adds, though, that not knowing the time is not a problem, and the main thing is to know the day of Rosh Chodesh. The Aruch Hashulchan (OC 417:8) adds that since we do things like it was done during Kiddush Hachodesh, the molad, too, should be announced, just as the Beis Din had to know the molad memutza at Kiddush Hachodesh.

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