Moon – month length

The moon is an erratic traveler, sometimes zipping through its monthly circuit in record time, while, at other times, gliding through it at a more leisurely pace. The Gemara reports this fact in the name of Rabban Gamliel:

“So I have received from the house of my father’s father – sometimes it comes in a long [path] and sometimes it comes in a short [path]” (Rosh Hashanah 25a).

And Rabbi Yochonon there deduces this from a verse: “What is the reason of the house of Rebbe? Because it is written, ‘He made the moon for festivals; the sun knows its coming’ (Tehillim 104:19). The sun knows its coming; the moon does not know its coming.”

Unlike the perfectly regular annual cycle of our planet around the sun, the moon speeds up and slows down with maddening irregularity. In fact, nowadays, the difference between a long month and a short month can be as long as an astounding 13/ hours, so that modern lunar cycles range from midget months of 29 days, 6 hours, and 30 minutes long, to longer months of 29 days, 20 hours.


What makes the moon’s cycle so irregular that predicting its arrival at the start of each new month depends on the chapters of complex calculations cited by the Rambam in Hilchos Kiddush Hachodesh? Before answering this question, let’s explore the criteria that define a new month.

Approximately every 29.5 days, the moon begins its monthly journey at the molad (birth). This is the moment when it is located directly between Earth and the sun (which we will call the Earth-moon- sun axis). Sometimes, when its location in this vital spot blocks off the sun’s light from Earth, we experience a solar eclipse.

Although the moon circles our planet in just over 27 days, since Earth has meanwhile traveled further in its annual journey around the sun, the moon needs about another two days to reach the Earth- moon-sun axis. Thus, the new molad occurs after about 29.5 days.

Now, back to our original question. Why does the moon take varying times to run this monthly race?

The answer is based on the difference between circles and ellipses. If the moon and Earth traveled in perfectly circular
orbits, each molad would always arrive with (almost) perfect regularity, since objects traveling in circular orbits never alter their speed.

However, the moon’s orbit around Earth and Earth’s orbit around the sun are not circular but elliptical (oval), and objects traveling in elliptical orbits constantly vary their speeds. Because of this, both Earth and the moon do not race along at uniform speeds, but are constantly slowing down and speeding up.

This is the secret behind the moon’s irregular molads.

If the moon is moving slowly toward the end of the month and Earth is moving fast, it takes the moon longer to catch up with the new Earth-moon-sun axis, and you end up with a longer month. The opposite happens if the moon is moving fast at the end of a month and Earth is moving slowly. In such a circumstance, the moon catches up with the Earth- moon-sun axis faster and you end up with a shorter month.

To circumvent this irregular month problem, the permanent calendar, introduced by Hillel II, is based on the average month-length mentioned by Rabban Gamliel, who says, “So I received from the house of the father of my father: The renewal of the moon is not less than 29 and a 1/2 days, 2/3 of an hour, and 73 parts (1/18* of a minute)” (Rosh Hashanah 25a).

Based on these criteria, every molad occurs, on average, 29 1/2 days, 2/3 of an hour, and 73 parts after the previous molad. Using a starting point at the time of the Creation, we can calculate the average molad of any month that ever happened in the past or will happen in the future.

However, incredibly accurate as is Rabban Gamliel’s average month-length, it is becoming steadily less precise due to the “stretching of time.”

What is the “stretching of time?” Not some obtuse, Einsteinian concept, but simply the gradual lengthening of the day due to the moon’s constant tugging on our planet’s oceans. Friction caused by the tides’ ebb and flow is slowing down the world’s spin, lengthening the days by about 0.00175 seconds per year. Due to this, our modern days are about 1.75 seconds longer than the days of a thousand years ago.

Since Rabban Gamliel’s average month-length is tailored more for the shorter days of the past than the longer days of modern times, the lengthening
days have gradually pushed Chazal’s molad so far backward that, unlike in the time of Hillel II, when it was accurate to milliseconds, these days it is off by about 0.6 of a second every month. These small differences have added up so that at the present time, on average, the average molad calculation is about two hours late!

Of course, this does not matter in the least, since the Chazon Ish lays down a general rule in connection with a similar issue, that once Chazal give us a certain measurement, it does not have to absolutely coincide with reality. Also, using a chelek (1/18th of a minute) as his smallest unit, Rabban Gamliel could not have expressed the molad average with more accuracy in any case.


We mentioned earlier that the molad calculation began ticking from the time of Maaseh Bereishis. If so, what is the meaning of Hashem’s command in Mitzrayim, “This month shall be for you the first of months” (Shemos 12:2)?

In his fascinating discussion of this verse, the Ramban not only explains this point, but also resolves a triple contradiction.

First, he explains the verse, “This month shall be for you the first of months,” saying that just as it is a mitzvah to constantly remember Shabbos by calling the days Rishon beShabbos and Sheini beShabbos, etc., so it is a mitzvah to start the counting of the months from Nissan, in order to remember the redemption from Mitzrayim.

But how can one say that the count begins in Nissan? Everyone knows that the year begins with Rosh Hashanah in Tishrei, as it says, “And the festival of ingathering [Sukkos] at the changing (tekufas) of the year” (Shemos 34:22). The Ramban answers that although we call Nissan the first month and Tishrei the seventh month, this does not mean that they are the first or seventh months of the year, but that they are the first or seventh month since our redemption.

The Ramban now raises another difficulty. How can we nowadays call the months by the names Nissan, Iyar, etc.? Isn’t this a violation of the Torah’s command to name them Rishon and Sheini after yetzias Mitzrayim?

To answer this question, he cites the Yerushalmi that states, “The names of the months came with them from Bavel(Rosh Hashanah 1:2). In other words, the Jews introduced a new system of naming months after returning from the second golus. Why did they do this? In order to fulfill the verse, “It will no longer be said, as Hashem lives Who raised Bnei Yisroel from the land of Mitzrayim, but, as Hashem lives Who raised Bnei Yisroel and Who took Bnei Yisroel from the land of the North [Bavel]” (Yirmiyohu 16:14-15).

“We reverted to calling the months the names they were called in Bavel to be a reminder that we were there and that Hashem, may He be blessed, raised us from there,” the Ramban concludes, “because these names, Nissan, Iyar, and the rest, are Persian names, and are only mentioned in the seforim of the Prophets, who were in Bavel (Zechariah 1:7, Ezra 6:15, Nechemiah 1:1) and in Megillas Esther (3:7). … And until today, the nations in the lands of Persia and Medea, too, call them Nissan and Tishrei, etc., like us. And, so we make remembrance, with [these names of the] months, of the
second redemption, as we had made until now of the first [redemption].”

Actually, the people in Bavel pronounced them a little differently. For example, it was Simanu instead of Sivan, Du’uzu instead of Tammuz, and Arakhsamma instead of Marcheshvon. Also, they counted Shevat before Teves. But our version is close enough to eternally remind us of our redemption from the land of Nevuchadnetzar and Haman.


In light of the Ramban’s statement that the names of the Hebrew months and their numerical symbols serve such important functions, how can we use non-Jewish names for months, such as January and February, which remind us neither of yetzias Mitzrayim nor of our escape from Bavel?

One intriguing answer is that, in the Torah view, non-Jewish months are not months at all, as their timing is purely arbitrary and has nothing to do with the molad. In fact, the earliest Roman calendar, created in about 3008/753 BCE, by the mythic first king of Rome, Romulus, did not even have twelve months but only ten! This early ten-month system helps explain a surprising incongruity connected with these months’ names.

Most of the first four months are named after false gods, raising a shailah as to how we are permitted to use them. Martis was the god of war, Aprilis probably refers to hog-raising, Maius was an Italian god, and Junius was yet another god.

By the time he reached the fifth month, Romulus’ imagination seems to have run dry; he labeled the remaining six months as Fifth, Sixth, Seventh, Eighth, Ninth, and Tenth, or, in Latin, Quintilis, Sextilis, September, October, November, and December.

Since this ten-month year had a miniscule length of only 304 days, far less than the solar year of 365 days, in about 3061/700 BCE, the Romans tacked two more months onto the year, Januarius and Februarius, stretching it to 355 days.

This explains yet another incongruity. This is the reason why in a solar leap year we add a day to the month of February. Being that February was the last month of the year we tack on the extra day to the year end!

Centuries later, Julius Caesar made the names of the last six months a joke when he shifted the beginning of the year from March to January, so that the names September, October, etc., no longer make any sense. Also, Quintilis was renamed Julius after Julius Caesar, and Sextilis was later renamed Augustus after Augustus Caesar, transforming the months’ names into their present-day form.

All this makes it obvious that the secular months have nothing to do with our lunar year. They are not months according to the Torah-meaning of the word, and thus one can argue that using them does not constitute a substitute for the names of the Hebrew months.

Of course, it is no big mitzvah to use them either. No one would deny that it preferable to mention the remembrance of our redemption from Mitzrayim and Bavel than to perpetuate the memory of obsolete Roman gods and dead emperors.

(Source: Duncan, David Ewing, The Calendar, Fourth Estate: London 1999. Source of molad information: Bromberg, Dr. Irv, Moon and the Molad of the Hebrew Calendar, University of Toronto, Canada.)

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