Rambam - 3 Chapters a Day

[There is a difference of opinion among] the Sages of Israel concerning the length of a solar year. Some Sages1 maintain that it is 365 days and 1/4 of a day - i.e., six hours. Others maintain that it is slightly less than that figure.2 There is also a difference of opinion among the wise men of Greece and Persia concerning this matter.3

According to the opinion that [a solar year] is [exactly] 365 and 1/4 days, there will be a remainder of one hour and 485 units after every nineteen-year cycle, as we mentioned.4

Between the start of each of the successive seasons of the year, there will be ninety-one days and seven and one-half hours. When you know the date and the hour of the beginning of one season, you can calculate [the beginning of] the following season by [adding the above amount]. Similarly, you can calculate the beginning of the following season, and continue forever.

The equinox of Nisan (spring) [takes place] at the hour and the unit when the sun enters the beginning of the constellation of Aries. The solstice of Tammuz (summer) [takes place] when the sun is located in the beginning of the constellation of Cancer. The equinox of Tishrei (autumn) [takes place] at the hour and the unit when the sun enters the beginning of the constellation of Libra. The solstice of Tevet (winter) [takes place] when the sun is located in the beginning of the constellation of Capricorn.

According to this calculation, in the first year of creation the vernal (spring) equinox took place seven days, nine hours, and 642 units before the conjunction of the month of Nisan, in numbers, 7 -9 - 642.5

The method of calculating [the beginning of] the seasons can be explained as follows. First, it is necessary to calculate the number of [nineteen-year] cycles that have passed until the [nineteen-year] cycle in question. Afterwards, add one hour and 485 units for every [nineteen-year] cycle. Afterwards, group all the units into hours, and all the hours into days. [Once a] total [has been reached], subtract seven days, nine hours, and 642 units [from it].6 Add the remainder to [the time of] the conjunction of Nisan in the first year of the [nineteen-year] cycle in question, and you will be able to know the hour and the date of the the vernal equinox of the first year of this cycle.7 From this date, you can calculate [the beginnings of] all the subsequent seasons [by] adding ninety-one days and seven and one- half hours for every season.

If you desire to know [the time and the date of] the vernal equinox of a particular year within a given [nineteen-year] cycle, [the following procedure should be used:] Add one hour and 485 [units] for every [nineteen-year] cycle. For each complete year that has passed within the [nineteen-year] cycle [under discussion], add ten days, twenty-one hours, and 204 units,8, and then group the entire sum [into days and hours].9

Afterwards, subtract seven days, nine hours, and 642 units [from this sum]10 and divide the remainder into lunar months of 29 days, 12 hours, and 793 units.11 [The number of days, hours, and units that] remain [after all the complete] lunar months [have been calculated] should be added to [the day and the time of] the conjunction of Nisan in that year. [In this manner,] you will be able to determine the date and the time of the vernal equinox of the year desired.

According to this calculation, the vernal equinox will always take place either at nightfall, at midnight, at daybreak, or at noon.12 The summer solstice will always take place at either 7:30 PM, 1:30 AM, 7:30 AM, or 1:30 PM.13 The autumnal equinox will always take place either at nine or at three o'clock, either in the day or the night.14 The winter solstice will always take place either at 10:30 PM, 4:30 AM, 10:30 AM, or 4:30 PM.15

If you desire to know the day of the week and the hour of the equinox, [the following procedure should be used:] Count the number of complete years that have passed from the year of creation until the desired year, and divide them into groups of twenty- eight.16 Add one day and six hours17 for each year remaining. Total the sum [of the hours and the days], and then add three days. Afterwards, divide the days into groups of seven. The remainder of the days and the hours should be added to the time of nightfall on the first day of the week.18 The result will be [the day and the time] on which the vernal equinox will occur.

Why is it necessary to add three days? Because the first equinox of the year of creation took place at the beginning of the fourth day.19

What is implied? If a person desires to know the day and the time of the vernal equinox of the year 4930 after creation,20 [the following procedure should be used:] [That number] should be divided by 28, leaving a remainder of one year, thus producing the figure of one day and six hours. By adding three days to this figure, it can be determined that the vernal equinox will take place on the night of the fifth day at midnight.

By adding seven and one-half hours to this figure, it can be determined that the summer solstice will take place on Thursday, an hour and one half after daybreak. By adding seven and one-half hours to this figure, it can be determined that the autumnal equinox will take place on Friday, at nine hours after daybreak. By adding seven and one-half hours to this figure, it can be determined that the winter solstice will take place on the night of the sixth day, four and one half hours after nightfall.

Similarly, by adding seven and one-half hours to this figure, it can be determined that the vernal equinox of the following year will take place on Friday, at daybreak. In this manner, it is possible to calculate [the time of the beginning of all] the seasons forever.

[The following procedure should be used] if one desires to know the date of the month on which the vernal equinox will fall this year:21 First, determine the day of the week on which [the equinox] will fall. Then determine the day [of the week] on which Rosh Chodesh of Nisan will fall, and how many complete years have passed within the nineteen-year cycle. Add eleven days for every year,22 and then add seven days to this sum in the present time.23 Divide the sum by thirty,24 and begin counting the remainder of days from Rosh Chodesh Nisan.

If the date coincides with the day of the week on which the equinox falls, this is sufficient. If not, add one, two, or three days to this number until you reach the day [of the week] on which the equinox falls.25 If the year in question is a leap year, begin counting from Rosh Chodesh of the second Adar.26 When a day is determined through this calculation, the equinox will take place on that date.

What is implied? Should we desire to know the date of the vernal equinox of the year 4930, which is the ninth year of the two- hundred-sixtieth [nineteen-year] cycle, [the following procedure should be used:] We have already determined that Rosh Chodesh Nisan will take place on Thursday, and that the equinox will take place on Thursday.27

Since this is the ninth year of the [nineteen-year] cycle, there are eight complete years [to take into consideration]. When eleven days are added for every year, we reach a sum of 88. When seven is added, the total will be 95. When this number is divided by 30, there will be a remainder of five.

When we add five days to Rosh Chodesh Nisan, which is Thursday, we reach Monday. Since we know that the equinox will not fall on Monday, but rather on Thursday, we continue adding days until Thursday, the day of the equinox. Thus, we can determine that this year the vernal equinox will take place on the eighth of Nisan. A similar process can be followed [to determine the date of the equinox] every year.

Although we said that one should continue to add days until one reaches the day of the week on which the equinox takes place, one should never have to add more than one, two, or three days28 - or in a most unusual case - four days.29 If you find it necessary to add any more days than this, know that you have made an error in your calculations, and you should recalculate carefully.

According to the opinion among the Sages of Israel that a solar year is less than [365 and] one-quarter [days], there is a view that [the length of the solar year] is 365 days, 5 hours, 997 units, and 48 moments. A moment is a seventy-sixth portion of a unit.

According to this reckoning, the difference between a solar year and a lunar year will be 10 days, 21 hours, 121 units, and 48 moments (in numbers, 10 - 21 - 121 - 48). [According to this calculation,] there will be no remainder at all after a [nineteen-year] cycle. Instead, after every [nineteen-year] cycle, a perfect correspondence will be established between the solar years and the combination of ordinary and full lunar years.

According to this calculation, there are ninety-one days, seven hours, 519 units, and thirty-one moments (in numbers, 91 - 7 - 519 - 31). When you know the date and the time of the beginning of any particular season, you can calculate [the date and the time of the beginning of] the subsequent season according to the seasons of the year, in a way resembling the calculations [that follow the opinion that a solar year is 365 and] 1/4 days.

According to this calculation, the vernal equinox of the first year of creation was nine hours and 642 units1 (in numbers, 9 - 642) before the conjunction of the month of Nisan. Similarly, in every first year of a [nineteen-year] cycle, the vernal equinox is nine hours and 642 units before the conjunction of the month of Nisan.

When you know which is the first year of a [nineteen- year] cycle, [you will be able to calculate the beginning of every subsequent season] by adding 91 days, 7 hours, 519 units, and 31 moments for each and every season until the end of the [nineteen- year] cycle.

If you desire to know when the vernal equinox [of a given year] will fall according to this calculation, first determine how many complete years have passed within this [nineteen-year] cycle. For each year, add the remainder of a year 10 [days], 21 [hours], 121 [units], and 48 moments.

[Afterwards,] group all the moments as units, all the units as hours, and all the hours as days, as done when calculating the conjunction. Subtract nine hours and 642 units from the entire sum,2 and divide the remainder by the length of a lunar month.3 The remainder that is less than the length of a lunar month should be added to the time of the conjunction of Nisan of the year in question. The vernal equinox of that year will take place on the moment arrived at according to these calculations.

It appears to me that [the Sages] relied on this calculation [of the length] of the seasons regarding the institution of a leap year, in the era when the High Court held sessions and would institute a leap year because of the time [when the equinox was scheduled to occur] or for other reasons. For this calculation is more accurate than the former one. It shares a greater resemblance to the data explained by the astronomers than the first opinion, which considered a solar year to be 365 and 1/4 days.

Both these calculations that we have explained are approximations, based on the mean rate of progress of the sun, and not on its actual position [in the celestial sphere]. When one considers the actual position of the sun at these times, the vernal equinox will take place approximately two days before the time determined by either of these calculations.4 [This applies both] according to the opinion that [a solar year is] exactly [365 and] 1/4 days, and according to the opinion that [a solar year is] less than [365 and] 1/4 days.

As stated in the laws mentioned previously,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.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.3 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.4

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.5

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].6

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.

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.

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.

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 sphere7 is divided into 3608 degrees [and twelve constellations].9 Each constellation includes thirty degrees, beginning with the constellation of Aries the ram.10 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.

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.

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.11

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.

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.

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 degrees, 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.

The sun, the moon, and the remainder of the seven stars,12 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,13 the earth is not at the center of [their orbits].

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.14 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.15

The uniform speed at which a planet, the sun, or the moon progresses is referred to as its mean motion.16 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 sun17 or the true position of the moon.18

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,19 the third of Nisan, of the present year, the seventeenth year of the 260th [nineteen-year] cycle - i.e., the year 4938 since creation20 - which is the year 1489 with regard to contracts,21 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.

Since the sighting of the moon is significant only in Eretz Yisrael as explained,22 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.23

This location is situated approximately 32 degrees north of the equator,24 [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,25 [and the surrounding areas extend] from 21° to 27° [west].