1. As stated in the laws mentioned previously,¹E.g., Chapter 1, Halachah 6; Chapter 2, Halachah 4; Chapter 6, Halachah 1. the court made precise calculations and knew whether or not the [new] moon would be visible. Accordingly, we are assured that anyone with a proper spirit and heart, who desires words of wisdom and probes to grasp the mysteries, will wish to know the methods of calculation used to determine whether or not the [new] moon would be visible on a particular night.²This concept, the calculation of the place and position of the new moon, and the determination of when it will be visible, is the subject of this and the following eight chapters. In the present chapter, the Rambam outlines the general principles and ground rules governing his calculations.

2. There are many differences of opinion among the sages of the nations of the previous eras who studied astronomy and mathematics, with regard to these methods of calculation.³See Chapter 17, Halachah 24, where the Rambam states that in this text he refers to the works of Greek scientists, because the books written by the Sages of Israel on the subject were not available to him. In the following halachah, however, he mentions having accepted traditions from the Rabbis. Great wise men have blundered regarding these matters. Concepts were hidden from them and doubts arose [in their minds].

There are those who have made many calculations, but have not been able to find the correct approach to determine when the moon becomes visible. Rather, they plunged into the mighty waters, to return with merely a potsherd in their hands.⁴The poetic wording is borrowed, out of context, from Bava Kama 91a.

3. Over the course of history, through much research and investigation, several sages have discovered the proper methods of calculation. We also possess traditions regarding these principles that we have received from the sages, and proofs that were not written in texts that are of common knowledge. For these reasons, I have considered it proper to explain a method of calculation that will be available for anyone whose heart spurs him to approach the task and perform it.⁵The latter phrase is borrowed, also out of context, from Exodus 36:2.

4. A person should not regard these calculations lightly, because they are not required in the present age, for these methods are indeed abstract and deep matters. They constitute the mystery of the calendar, which was known [only] to great sages, who would not convey these matters to [most] other people, but only to ordained and perceptive [sages].⁶I.e., to receive this knowledge, one had to have received semichah as described in Hilchot Sanhedrin, Chapter 4. Nevertheless, not all the Sages who received semichah were privileged to this knowledge.

The calendar that is employed in the era when there is no court to determine [the months according to the testimony of] witnesses, and which we use at present [is], by contrast, [a simple matter that] can be appreciated even by school children in three or four days.

5. A wise man of the gentile nations or a sage of Israel who studied Greek wisdom may meditate on the methods of calculation I have used to determine the appearance of the moon and may detect a slight approximation [and imprecision] with regard to certain matters. He should not presume that we have overlooked this point and were not aware that there was an approximation regarding that matter.

Instead, he should assume that whenever we were not exact, it was because our mathematical calculations proved that [this inaccuracy] did not affect the knowledge of the time when the moon would become visible, and thus it was not significant. Therefore, we were not precise regarding this matter.

6. Similarly, should a person see that [our use of] one of the methods leads to a minor inadequacy that is inappropriate for this method of computation, [he should realize] that this was intentional. For this method produced an advantage from another perspective that will produce a correct result - [albeit] through approximate calculations - without requiring lengthy computations. Thus, a person who is not practiced in such matters will not be flustered by complex computations that are of no avail with regard to the visibility of the moon.

7. The [following] fundamental principles must be known by a person as a prelude to all astronomical computations, whether for the purpose of determining the visibility [of the moon] or for other purposes:

The heavenly sphere⁷This term is used, because from man's perspective, the earth appears flat and the heavens appear as a sphere that revolves around him, only half of which is visible at any given time. is divided into 360⁸The number 360 was chosen because it can be divided by all the cardinal integers except for seven (Perush). degrees [and twelve constellations].⁹One corresponding roughly to each of the months of the year. Each constellation includes thirty degrees, beginning with the constellation of Aries the ram.¹⁰For the year begins in spring, and during the spring the sun is located in the constellation of Aries. Every degree contains sixty minutes, every minute sixty seconds, and every second sixty thirds. You may continue and divide into further fractions to the extent that you desire.

8. Therefore, were you to calculate that a particular star's position in the heavenly sphere is seventy degrees, thirty minutes and forty seconds, you would know that this star is located in the constellation of Gemini the twins, in the middle of the eleventh degree. For the constellation of Aries includes thirty degrees, and the constellation of Taurus the bull includes thirty degrees. Thus, there remain ten and one half degrees of the constellation of Gemini, plus forty seconds of the next degree.

9. Similarly, were you to calculate that a particular star's position in the heavenly sphere is 320 degrees, you would know that this star is located in the constellation of Aquarius the water bearer, in its twentieth degree. The same applies to all other calculations.

The order of the constellations is the following: Aries the ram, Taurus the bull, Gemini the twins, Cancer the crab, Leo the lion, Virgo the virgin, Libra the balance, Scorpio the scorpion, Sagittarius the archer, Capricorn the goat, Aquarius the water-bearer, Pisces the fishes.¹¹See Hilchot Yesodei HaTorah 3:7, which states that these constellations appeared in these forms at the time of the flood, and then they were given these names. At present, the stars have changed position somewhat, and some creativity is required to perceive how the images suggested by these names are appropriate for these constellations.

10. In all calculations, when you collect fractions or add numbers, each integer should be added to its kind, the seconds to the seconds, the minutes to the minutes, and the degrees to the degrees. When calculating seconds, they should be grouped in sets of sixty [or less]. Whenever sixty seconds are reached, they should be considered a minute and added to the sum of the minutes.

When calculating minutes, they should be grouped in sets of sixty [or less]. Whenever sixty minutes are reached, they should be considered a degree and added to the sum of the degrees.

When calculating degrees, they should be grouped in sets of 360. If a sum above 360 is reached, the remainder after 360 has been subtracted is the figure that is of consequence.

11. In all computations, whenever you desire to subtract one number from another, should the second number be greater than the first number, even if it is merely one minute greater, it is necessary to add 360 degrees to the first number so that it is possible to subtract the [greater] number from it.

12. What is implied? When it is necessary to subtract two hundred degrees, fifty minutes and forty seconds - in symbols 200° 50' 40" - from one hundred degrees, twenty minutes and thirty seconds - in symbols 100° 20' 30" - [one should follow this procedure]:

[First,] one adds 360 to 100, producing a sum of 460. Afterwards, one begins to subtract the seconds. Since it is impossible to subtract 40 from 30, it is necessary to convert one of the 20 minutes into 60 seconds. When added to 30, this produces a sum of 90. [From the 90] subtract 40, producing a total of 50 seconds.

Afterwards, one must subtract 50 minutes from the 19 [remaining], for one of the minutes has already been converted into seconds. Since 50 cannot be subtracted from 19, one must convert a degree into 60 minutes. When this figure is added to 19, it produces a sum of 79. When 50 is subtracted [from 79], a total of 29 minutes remain.

Afterwards, one must subtract the 200 degrees from the 459 decrees, for one of the degrees has already been converted into minutes. Thus 259 degrees remain. In symbols [the remainder is] 259° 29' 50". All other subtractions should be performed following a similar method.

13. The sun, the moon, and the remainder of the seven stars,¹²The Rambam appears to be referring to his statements in Hilchot Yesodei HaTorah 3:1, which relate that there are nine spheres in which the stars revolve: The moon revolves in the first, then Mercury, Venus, the Sun, Mars, Jupiter, and then Saturn. In the eighth sphere revolve all the stars that are visible, and the ninth sphere includes and encircles all existence. each proceeds at a uniform speed in its orbit. They are never inclined to heaviness, nor to lightness. Rather, the speed at which they proceed today is the same speed at which they proceeded yesterday. And tomorrow, and indeed on every other day, they will proceed at this speed.

Although the orbits in which they all travel encircle the earth,¹³The Rambam is following the theory of an earth-centered universe. The term "center" must, however, be understood loosely, because the earth does not lie at the exact center of all these spheres. the earth is not at the center of [their orbits].

14. Therefore, if one measured the progress [of any of these stars] against the sphere that encompasses the world in which the earth is the center - i.e., the sphere of the constellations - its [rate of] progress [would appear to] change.¹⁴The early astronomers realized that at some given times, the sun appears to travel faster or slower than at others - i.e., the pace at which it appears to proceed in the heavens varies between approximately 1 1/2 degrees per day and 58 1/2 minutes per day. Similarly, they saw that at different times of the year, the sun appears larger or smaller. By postulating that the earth was not the center of the sun's orbit, they were able to resolve these anomalies. Its rate of progress in the sphere of the constellations on one day could appear less or more than its progress on the previous day or on the following day.¹⁵Were the earth to lie at the center of all the planets' orbits, the speed at which the planets progress would not only be uniform, it would appear uniform. Since the earth is not in the center, although the planets are proceeding at a uniform pace, this does not always appear to be the case.

15. The uniform speed at which a planet, the sun, or the moon progresses is referred to as its mean motion.¹⁶See the notes on Chapter 6, Halachah 1. The progress that [this celestial body appears to make] in the sphere of the constellations that is sometimes greater and sometimes less [than its actual rate of progress] is referred to as its true motion. This determines the true position of the sun¹⁷I.e., the angular location in the heavenly sphere at which the sun can be found. The stars cannot be seen during the daytime. Hence, we cannot actually see the constellations in which the sun is located. Throughout this text, the term "the position of the sun" generally refers to the angular position of the celestial sphere that is just below the horizon when the sun sets. or the true position of the moon.¹⁸The place of the moon in the heavenly sphere can be seen at night. It is possible for us to determine its angular position in comparison to the constellations of the Zodiac.

16. We have already stated that the calculations that we explain in these laws are intended solely to determine the visibility of the [new] moon. Therefore, we have established the starting point from which we will always begin these calculations: the eve of Thursday,¹⁹I.e., Wednesday night. the third of Nisan, of the present year, the seventeenth year of the 260th [nineteen-year] cycle - i.e., the year 4938 since creation²⁰This year corresponds to 1178 C.E. There are several other dates cited within the Mishneh Torah with regard to the composition of that text. - which is the year 1489 with regard to contracts,²¹In Talmudic times, legal contracts were dated from the year when Alexander the Great ascended to the throne. and 1109 years after the destruction of the Second Temple. This is the year that will be referred to as the starting point in these calculations.

17. Since the sighting of the moon is significant only in Eretz Yisrael as explained,²²For the New Moon can be sanctified only in Eretz Yisrael, as stated in Chapter 1, Halachah 8. all our calculations are centered on the city of Jerusalem and locations within six or seven days' journey [from it. [In these places,] the moon is frequently sighted, and the people come and give testimony in the court.²³The positive value of testimony from locales of more than a day's journey is mentioned in Chapter 3, Halachot 15-18.

This location is situated approximately 32 degrees north of the equator,²⁴More precisely, Jerusalem is 31° 47' north of the equator. [and the surrounding areas extend] from 29° to 35° [north]. Similarly, in longitude, it is situated approximately 24 degrees west of the center of the populated area,²⁵The populated area refers to the land mass of Europe and Asia, for at the time the Rambam wrote his text, America had not been discovered. The center of the populated area refers to a line approximately 90° east of "western edge of civilization". Thus, Jerusalem, which is 66° east of "western edge of civilization," is 24° west of this line.

The significance of the latitude and longitude of Jerusalem with regard to these calculations is mentioned in Chapter 17. [and the surrounding areas extend] from 21° to 27° [west].