Understanding Kiddush Hachodesh and the Jewish Calendar-Part 1

Understanding Kiddush Hachodesh and the Jewish CalendarPart 1

*Important note: The Halachos in this chapter are based mainly on the rulings of the Rambam in his section on Kiddush Hachodesh. Although there exists several other opinions in Geonim and Rishonim regarding certain topics discussed here, they are beyond the scope of this book. For further reading on subjects discussed in this chapter please refer to the Rambam’s Mishneh Torah Kiddush Hachodesh and the Sefer “Understanding the Jewish calendar” by Rabbi Nathan Bushwick 1989, from which parts of the material here has been attained. A most fabulous digest which compiles all the opinions on this subject can be found in the Sefer Torah Shleima [M.M Kasher] Volume 13 “Sod Haibur.”

1. The Biblical command:

The first[1] Mitzvah the Jewish people[2] were commanded in the Torah is to sanctify the new moon, as the verse[3] states “This month is to you the head of the months.”[4] This Biblical positive command obligated the Beis Din Hagadol to calculate as to when the new moon could be visible, and in the event that the new moon could be visible on the 30th day, then they must investigate any witnesses that approach the Beis Din, and upon acceptance of the testimony they have to publicize this matter to the Jewish people. Thus, the positive command includes three stages:

  1. To calculate the moon’s potential date of visibility.
  2. To interrogate the witnesses.
  3. To pronounce the day of Rosh Chodesh and publicize it to the Jewish people.

2. Hashem showed Moshe the new moon:[5]

The Sages state that Moshe Rabbeinu had difficulty understanding at what stage the new moon is to be sanctified. Hashem therefore showed Moshe through prophecy an image of the new moon and stated, “This is what is to be seen and sanctified.” 

3. Understanding the moon’s cycle:

A. The moons orbit and its dark/light periods:

We all witness throughout the month the cycle of the moon going from being invisible [i.e. New Moon] to being a full moon and then once again to being invisible [i.e. New Moon]. What causes this change to occur? In this paragraph we will investigate the moon’s orbit which affects its level of visibility from earth.

Each month, the moon orbits around the earth. That which effects earths’ visibility of the moon is the angle from which the moon is found opposite the sun and the earth. It is the sun that is responsible for the moon’s light, the moon having no light of its own. Depending on the angle in which the moon is being seen from the earth will affect our visibility of the moon. Half of the moon constantly faces earth throughout its monthly orbit. There is never a time of the month that we have more or less than half of the moon facing us. Nonetheless, the half that we see is constantly changing due to the orbit of the moon, and this is what effects our ability to see the moon’s light. The moon itself remains alit in half of its body size throughout the month, as the sunlight constantly hits the half that faces the sun. This half goes through a change during orbit, on a two-week basis. Meaning, every part of the moon goes through a two-week cycle of light and darkness, two weeks of light and two weeks of darkness. As the moon orbits earth and rotates, its angle from the sun and earth changes, causing the people on earth to see either more of its dark side or more of its light side. The following is its cycle: In the beginning of the moons cycle the moon is directly opposite the sun, between the sun and the earth. We will consider this the 0-degree angle from the earth. At this point the moon’s half that faces earth is completely dark, as the sun only shines the half that is directly facing it, leaving us to see only its unlit side. This stage is called New Moon. As the moon begins its orbit cycle it slowly begins to reveal part of its shining half to the earth, until it reveals approximately 1/4 of the moon’s shining half. This stage is called the Waxing Crescent, in which the moon is at a 45-degree angle from the earth. It takes a few days for the moon to reach this position during orbit. After a full week, the moon’s orbit reaches a 90-degree angle from the earth, in which case we have Half Moon. In this position, we see exactly one half of the lit side of the moon. After two weeks into orbit the moon is now at a 180-degree angle from the earth and its entire shining half is visible to earth. This is called Full Moon. In this stage the moon is behind the earth, and the earth is between the moon and the sun. This allows earth to see the full shining half of the moon, which is opposite the sun. After three weeks into orbit the moon is now 270 degrees from the earth in which case, we once again see Half Moon. After four weeks the moon has made a complete 360 degree turn and reaches back to point 0 in which the entire dark half of the moon is facing the earth and we return to have zero visibility of the moon until it restarts its cycle.   

B. How long does it take the moon to make a full orbit around the earth?[6]

It takes the moon exactly 29 days, 12 hours and 793 Chalakim to make a full orbit [360 degree circle] around the earth.[7] That amounts to four full weeks, one day, 12 hours and 793 Chalakim.[8] Each 60 minute hour contains 1080 Chalakim[9]; each minute contains 18 Chalakim [18×60=1080]; and each Cheleik contains 3.333 seconds [60/18=3.3]. Accordingly, 793 Chalakim of an hour is 44 minutes [44×18=792 Chalakim] and 31/3 seconds [1 Chelek for a total of 793 Chalakim]. Thus, between each Molad [new moon] there is always exactly 29 days 12 hours 44 minutes and 3.3 seconds.[10]

C. Calculating the Molad:

Once one knows the Molad of any month he can calculate the Molad of the next month by simply skipping four weeks and adding one day, 12 hours and 793 Chalakim [44 minutes and 3.3 seconds].

D. When is the moon not visible?[11]

The moon becomes invisible from earth during the time of the Molad, and also a day prior and post the Molad. Upon approaching the Molad the moon reaches completion of its orbit and becomes aligned between the sun and the earth, thus blocking earth from seeing any of its lit side which is now lying opposite the sun. The period of time that the moon remains in the dark is approximately two days, give or take some. It is in the dark for approximately one day before the orbit is complete and one day after the orbit is complete.


[1] Rashi Bereishis 1:1; See Likkutei Sichos 26:95; This Mitzvah is the 4th Mitzvah in Sefer Hachinuch and is the 153rd Mitzvah in the Sefer Hamitzvos of the Rambam.

[2] This refers to the Jewish people as a nation, however our forefathers were commanded the Mitzvos of procreating, Mila, and Gid Hanashe prior to the Jewish people’s existence.

[3] Shemos 12:2

[4] In Sefer Hachinuch ibid and Sefer Hamitzvos of the Rambam [Mitzvah 153] this verse is listed as the source of this positive command. However, in Mishneh Torah Kiddush Hachodesh 1:7 the Rambam mentions the verse “Asher Tikreu Osam Mikraeiy Kodesh”.

[5] Rosh Hashanah 20a; Rashi Shemos 12:1-2; Rambam Hilchos Kiddush Hachodesh 1:1

[6] Rambam Kiddush Hachodesh 6:3-5

[7] Rambam ibid 3

[8] Rambam ibid 5

[9] Rambam ibid 2

[10] Rambam ibid 3

[11] Rambam Kiddush Hachodesh 1:3

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