Rambam - 3 Chapters a Day

The mean distance traveled by the sun in one day - i.e., in twenty-four hours - is 59 minutes and 8 seconds; in symbols 59' 8".1 Thus, in ten days, it travels 9 degrees, 51 minutes and 23 seconds,2 in symbols 9° 51' 23". In one hundred days, it travels 98 degrees, 33 minutes and 53 seconds, in symbols 98° 33' 53".

The remainder [of the degrees] traveled [by the sun] over the course of one thousand days - after all the multiples of 360 have been subtracted, as explained3 - is 265 degrees, 38 minutes and 50 seconds, in symbols 265° 38' 50". The remainder [of the degrees] traveled [by the sun] over the course of ten thousand days is 136 degrees, 28 minutes and 20 seconds, in symbols 136° 28' 20".

In this manner, one can multiply [the mean distance of a day] and calculate the distance [traveled] by the sun over any number of days. Similarly, if one would like to make pre-calculated figures for the mean distance for two days, for three days, for four days, up to ten days, one may do so. Similarly, if one desires to make pre-calculated figures for the mean distance for twenty days, for thirty days, for forty days, until one hundred days, one may do so. These figures become evident once one knows the mean distance for a single day.

It would be proper for one to know and have prepared the mean distances traveled by the sun in 29 days, and in 354 days, [the latter] being the number of days in a lunar year when the months follow a regular pattern. This is called a regular year.4

When you have these figures prepared, it will be easy to calculate the visibility of the moon. For there are 29 full days from the night when the moon was sighted in one month to the night that it may be sighted in the following month. Similarly, each and every month, there will be a difference of 29 days [between the nights on which the moon may be sighted], no more and no less.5 [This is what concerns us,] for our sole desire in these calculations is to know [when the moon] will be sighted.6

Similarly, [the difference in the sun's position] between the night when the moon will be sighted in a particular month one year and the night when it will be sighted [in that month] the following year will be that of a regular year, or that of a regular year plus one day.7

The mean distance traveled by the sun in one month is 28 degrees, 35 minutes and one second, in symbols 28° 35' 1". The distance it travels over the course of a regular [lunar] year is 348 degrees, 55 minutes and 15 seconds, in symbols 348° 55' 15".

There is one point in the orbit of the sun around the Earth - and similarly, in the orbits of the remainder of the seven stars [around the Earth] - when [the sun or] that star will be furthest removed from the Earth.8 With the exception of the moon, that point in the orbit of the sun and, similarly, in the orbit of the other planets rotates in a uniform pattern, traveling about one degree in seventy years.9 This point is referred to as the apogee.

Accordingly, in ten days, the apogee of the sun travels one and a half seconds - i.e., [a second and] thirty thirds. Thus, in one hundred days, [the apogee] travels fifteen seconds. In one thousand days, it travels two minutes and thirty seconds, and in ten thousand days, 25 minutes. In twenty-nine days, it travels four seconds and a fraction. In a regular year, it travels 53 seconds.

As mentioned, the starting point for all our calculations is the eve of Thursday, the third of Nisan, 4938 years after creation. The position of the sun in terms of its mean distance on this date was 7 degrees, 3 minutes and 32 seconds in the constellation of Aries, in symbols 7° 3' 32". The apogee of the sun at this starting point was 26 degrees, 45 minutes and 8 seconds in the constellation of Gemini, in symbols 26° 45' 8".10

Accordingly, if you desire to know the position of the sun according to its mean distance at any given time, you should calculate the number of days from the starting point mentioned until the particular day you desire, and determine the mean distance it traveled during these days according to the figures given previously, add the entire sum together, accumulating each unit of measure separately. The result is the mean position of the sun on that particular day. For example, if we desired to determine the mean position of the sun at the beginning of the eve of the Sabbath on the fourteenth of the month of Tammuz of the present year, the starting point [for these calculations, we should do the following]: Calculate the number of days from the starting point until the date on which you desire to know the position of the sun. [In this instance,] it is one hundred days. The mean distance the sun travels in one hundred days is 98° 33' 53". We then add that to the starting point, which is 7° 3' 32", and arrive at a total of 105 degrees, 37 minutes and 25 seconds, in symbols 105° 37' 25". Thus, the sun's mean position at the beginning of this night will be 15 degrees and 37 minutes of the sixteenth degree in the constellation of Cancer.

At times, the sun will be located in the mean [position] that can be determined using the above methods of calculation at the beginning of the night, and at times an hour before the setting of the sun, or an hour afterwards.11 This [lack of definition concerning] the sun's [position] will not be of consequence with regard to calculating the visibility [of the moon], for we will compensate for this approximation when calculating the mean position of the moon.12

One should follow the same procedure at all times - for any date one desires, even if it is one thousand years in the future. When [the mean distance traveled by the sun] is calculated and the remainder [after all the multiples of 360 have been subtracted] is added to [the figures of] the starting point, you will arrive at the mean position.

The same principles apply regarding the mean position of the moon, or the mean position of any other planet. Once you know the distance it travels in a single day, and you know the starting point from which to begin [calculations], total up the distance it travels throughout as many years or days as you desire, add that to the starting point, and you will arrive at its position according to its mean distance.

The same concepts apply regarding the apogee of the sun. Add to the starting point the distance it travels over the course of days or years, and you will know the position of the apogee of the sun for the day you desire.

Similarly, if you desire to establish another date as the starting point instead of the date which [we have chosen] to begin in this year, [choosing] a year that will be the beginning of a particular nineteen-year cycle, or that will be the beginning of a new century, you may. Similarly, if you would like to use as a starting point a date in the past, before the date given above, or a date many years in the future, the path [to arrive at such a starting point] is well known.

How is this figure to be calculated? We have already established the mean distance traveled by the sun in a regular year, in twenty- nine days, and in a single day. It is known that a year whose months13 are full is one day longer than a regular year. Similarly, a year whose months are lacking is one day shorter than a regular year. With regard to a leap year,14 if its months are regular, it will be thirty days longer than a regular year. If its months are full, it will be thirty-one days longer than a regular year. If its months are lacking, it will be twenty-nine days longer than a regular year.

Since these principles are already established, it is possible to calculate the mean distance traveled by the sun for as many years or as many days as you desire, and add it to [the mean position of the sun on the date established previously as] the starting point, and you will be able to determine the mean [position of the sun] for any future date. Afterwards, you can use that date as a starting point.

[Conversely,] you may subtract the mean [distance traveled by the sun over the course of a particular period] from [the mean position of the sun on the date established previously as] the starting point, and you will be able to determine the mean [position of the sun] for any past date. Afterwards, you can use that date as a starting point.

The same principles also apply with regard to the mean position of the moon or any of the other planets, if [their mean positions on any particular date] are known to you. It also should be apparent that just as it is possible to determine the mean position of the sun for any future date, so too, it is possible to determine its mean position for any previous date.

[The following method should be used] if you wish to know the true position1 of the sun on any particular day you desire: First, it is necessary to calculate the mean position of the sun through the methods of calculation we have explained. Then calculate the position of the apogee of the sun.2 Afterwards, subtract the apogee of the sun from the mean position of the sun. The remainder is referred to as the course of the sun.3

[The next step is] to calculate the angular distance of the course of the sun.4 If the angular distance of the course is less than 180 degrees, one should subtract5 the angle [determined by the] course6 from the sun's mean position. If the angular distance of the course is more than 180 degrees, one should add7 the angle [determined by the] course to the sun's mean position. The figure remaining after making this addition or subtraction represents [the sun's] true position.

If the course [of the sun] is an even 180 degrees or an even 360 degrees, there will be no angle [determined by the course to add or to subtract]. Instead, the [sun's] mean position is its true position.8

What is the angle [determined by the] course? If the course is ten degrees, the [resulting] angle will be 20 minutes.

If the course is twenty degrees, the [resulting] angle will be 40 minutes.

If the course is thirty degrees, the [resulting] angle will be 58 minutes.

If the course is forty degrees, the [resulting] angle will be 1 degree and 15 minutes.

If the course is fifty degrees, the [resulting] angle will be 1 degree and 29 minutes.

If the course is sixty degrees, the [resulting] angle will be 1 degree and 41 minutes.

If the course is seventy degrees, the [resulting] angle will be 1 degree and 51 minutes.

If the course is eighty degrees, the [resulting] angle will be 1 degree and 57 minutes.

If the course is ninety degrees, the [resulting] angle will be 1 degree and 59 minutes.

If the course is one hundred degrees, the [resulting] angle will be 1 degree and 58 minutes.9

If the course is one hundred ten degrees, the [resulting] angle will be 1 degree and 53 minutes.

If the course is one hundred twenty degrees, the [resulting] angle will be 1 degree and 45 minutes.

If the course is one hundred thirty degrees, the [resulting] angle will be 1 degree and 33 minutes.

If the course is one hundred forty degrees, the [resulting] angle will be 1 degree and 19 minutes.

If the course is one hundred fifty degrees, the [resulting] angle will be 1 degree and 1 minute.

If the course is one hundred sixty degrees, the [resulting] angle will be 42 minutes.

If the course is one hundred seventy degrees, the [resulting] angle will be 21 minutes.

If the course is an even one hundred eighty degrees, it has no measure. Instead, its mean position is its true position, as we explained.

[The following procedure should be used] if the course [of the sun] is greater than one hundred eighty degrees: One should subtract the course from three hundred sixty degrees and [calculate the resulting] angle accordingly.10

What is implied? If the course is 200 degrees, that figure should be subtracted from 360 degrees, leaving a remainder of 160 degrees. Since you already know that the [resulting] angle of a course of 160 degrees is 42 minutes, that same figure will be the [resulting] angle of a course of 200 degrees.

Similarly, if the course was three hundred degrees, one should subtract that figure from three hundred sixty, leaving a remainder of sixty. Since you already know that the [resulting] angle of a course of 60 degrees is 1 degree and 41 minutes, that same figure will be the [resulting] angle of a course of 300 degrees. Similar procedures should be followed in calculating other figures.

[How is the angle determined by the course calculated] when the course is [an intermediate figure - e.g.,] 65 degrees? You already know that the [resulting] angle of 60 degrees is 1 degree and 41 minutes. And you know that the [resulting] angle of 70 degrees is 1 degree and 51 minutes. Thus, there are ten minutes between these [two] measures. Thus, [an increase of] a degree [of the course] will bring an increase of a minute [in the resulting angle]. Thus, the [resulting] angle of a course of 65 degrees will be 1 degree and 46 minutes.11

Similarly, if the course was 67 degrees, the [resulting] angle would be 1 degree and 48 minutes. A similar procedure should be followed regarding any course that has both units and tens, both for calculations regarding the sun and for calculations regarding the moon.

[To apply these principles]: Should we desire to know the true position of the sun at the beginning of Friday night, the fourteenth of Tammuz for this present year: First, we should calculate the mean position of the sun for this time, which is, as explained,12 105° 37' 25". We should then calculate the apogee of the sun at this time, which is 86° 45' 23". When the apogee is subtracted from the mean position, the remainder, the course [of the sun], will be 18 degrees, 52 minutes and 2 seconds, in symbols 18° 52' 2 ".

With regard to the course [of the sun], the minutes are of no consequence. If they are less than thirty, they should be disregarded entirely. If they are more than thirty, they should be considered an additional degree and added to the sum of the degrees. Accordingly, it should be considered as if there are 19 degrees in this course. The [resulting] angle of such a course can be calculated to be 38 minutes in the manner that we explained.

Since the course is less than 180 degrees, the [resulting] angle [of the course], 38 minutes, should be subtracted from the mean position of the sun, leaving a remainder of 104 degrees, 59 minutes and 25 seconds, in figures 104° 59' 25". Thus, the true position of the sun at the beginning of this night will be fifteen degrees less 35 seconds in the constellation of Cancer.

One need not pay attention to the seconds at all, neither with regard to the position of the sun, nor with regard to the position of the moon, nor in any other calculations regarding the sighting [of the moon]. Instead, if the number of seconds is approximately13 thirty [or more], they should be considered a minute, and added to the sum of the minutes.

Since you are able to calculate the location of the sun on any desired date, you will be able to calculate the true date of the equinox or solstice for any equinox or solstice you desire,14 whether for the equinoxes or solstices that will take place in the future, after the date we established as a starting point, or for the equinoxes or solstices that have taken place in previous years.

There are two mean rates of progress [that are significant] with regard to the moon, for the moon revolves in a small orbit that does not encompass the earth. Its mean progress within this orbit is referred to as the mean within its path.

The small orbit [within which the moon revolves] itself rotates in a larger orbit that encompasses the earth.1 The mean progress of the small orbit within the large orbit that encompasses the earth is referred to as the moon's mean. The rate of progress for the moon's mean in one day is 13 degrees, 10 minutes and 35 seconds, in symbols 13° 10' 35".2

Thus, its progress in ten days will be 131 degrees, 45 minutes and 50 seconds, in symbols 131° 45' 50". The remainder [of the sum]3of its progress in one hundred days will be 237 degrees, 38 minutes and 23 seconds, in symbols 237° 38' 23".4

The remainder [of the sum] of its progress in one thousand days is 216 degrees, 23 minutes and 50 seconds, in symbols 216° 23' 50". The remainder [of the sum] of its progress in ten thousand days is 3 degrees, 58 minutes and 20 seconds, in symbols 3° 58' 20".

The remainder [of the sum] of its progress in twenty-nine days is 22 degrees, 6 minutes and 56 seconds, in symbols 22° 6' 56".5The remainder [of the sum] of its progress in a regular year is 344 degrees, 26 minutes and 43 seconds, in symbols 344° 26' 43". Following these guidelines, you can multiply these figures for any number of days or years you desire.

The distance travelled by the mean within its path in a single day is 13 degrees, 3 minutes and 54 seconds, in symbols 13° 3' 54".6 Thus, its progress in ten days will be 130 degrees, 39 minutes and no seconds, in symbols 130° 39'. The remainder [of the sum] of its progress in one hundred days will be 226 degrees, 29 minutes and 53 seconds, in symbols 226° 29' 53".7

The remainder [of the sum] of its progress in one thousand days is 104 degrees, 58 minutes and 50 seconds, in symbols 104° 58' 50". The remainder [of the sum] of its progress in ten thousand days is 329 degrees, 48 minutes and 20 seconds, in symbols 329° 48' 20".

The remainder [of the sum] of its progress in twenty-nine days is 18 degrees, 53 minutes and 4 seconds, in symbols 18° 53' 4".

The remainder [of the sum] of its progress in a regular year is 305 degrees, no minutes and 13 seconds, in symbols 305° 13".8

The position of the moon's mean on Wednesday night, [the third of Nisan, 4938,] the starting point for these calculations, was 1 degree, 14 minutes and 43 seconds, in figures 1° 14' 43", in the constellation of Taurus. The mean within its path at this date was 84 degrees, 28 minutes and 42 seconds, in symbols 84° 28' 42".

Since you know the mean rate of progress for the moon's mean, and you know its position on the date of the starting point, you [will be able to calculate] the position of the moon's mean on any date that you desire, as you did with regard to the mean position of the sun.

After calculating [the position of] the moon's mean on the beginning of the night that you desire, [the next step in calculating where the moon can be sighted] is to focus on the sun and see the constellation in which it will be located [at that time].9

If the sun is located between midway in the constellation of Pisces and midway in the constellation of Aries, the moon's mean should be left without emendation.10 If the sun is located between midway in the constellation of Aries and the beginning of the constellation of Gemini, 15 minutes should be added to the moon's mean.11 If the sun is located between the beginning of the constellation of Gemini and the beginning of the constellation of Leo, 30 minutes should be added to the moon's mean.12 If the sun is located between the beginning of the constellation of Leo and midway in the constellation of Virgo, 15 minutes should be added to the moon's mean.13

If the sun is located between midway in the constellation of Virgo and midway in the constellation of Libra, the moon's mean should be left without emendation.14 If the sun is located between midway in the constellation of Libra and the beginning of the constellation of Sagittarius, 15 minutes should be subtracted from the moon's mean.15 If the sun is located between the beginning of the constellation of Sagittarius and the beginning of the constellation of Aquarius, 30 minutes should be subtracted from the moon's mean.16 If the sun is located between the beginning of the constellation of Aquarius and midway in the constellation of Pisces, 15 minutes should be subtracted from the moon's mean.17

The figure that remains after these additions or subtractions have been made, or when the mean was left without emendation, is the mean of the moon approximately 20 minutes after the setting of the sun18 for the time when this mean was calculated. This is referred to as the mean of the moon at the time of the sighting.