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What is an Astrolabe ? To answer this question in simple language was Geoffrey Chaucer's (astronomer-poet) chief object. He wrote his "Treatise on the Astrolabe" in 1387 for little Lewis, his lo-years- old son, to learn at Oxford. Manuscript copies of this work of Chaucer approaching completeness are very few in number and inaccessible to the general reader. He undoubtedly obtained the greater part of his book from Latin version of the Compositio et Operatio Astrolabii of Messahalla, an Arabian astronomer who is thought to have flourished about the end of the eighth century. Chaucer certainly was of opinion that he had achieved simplicity in his explanations, for he gave his Treatise the title of "Bread and Milk for Children." No one understood better than the great poet that words alone would not suffice to make his meaning clear. He directed his son to read the book with a real Astrolabe in hand. To make the description easier to comprehend, he illustrated every chapter with a neatly drawn diagram, specially drawn to explain each successive proposition, as in Euclid. Fifty-seven paragraphs conclude with the instruction, ' For more declaracioun, lo here the figure', and as many as sixty-two explanatory figures may occur in the more complete manuscripts, as in the fine manuscript in the Cambridge University Library known as Dd. 3. 53. In many of its forms the Astrolabe to be one of the most valuable of educational instruments and contains material for an education from the nursery to the University.
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The complete works of Geoffrey Chaucer.
Geoffrey Chaucer.
© 1894 Geoffrey Chaucer.
All rights reserved.
Vladimir Kharchenko
Podpolnaya, 69. Kherson. Ukraine 73000. [email protected]
ISBN: no.
Astrolabe.
Guide to Use of the Astronomical Instrument.
What is an Astrolabe ?
To answer this question in simple language was Geoffrey Chaucer's (astronomer-poet) chief object. He wrote his "Treatise on the Astrolabe" in 1387 for little Lewis, his lo-years- old son, to learn at Oxford. Manuscript copies of this work of Chaucer approaching completeness are very few in number and inaccessible to the general reader. He undoubtedly obtained the greater part of his book from Latin version of the Compositio et Operatio Astrolabii of Messahalla, an Arabian astronomer who is thought to have flourished about the end of the eighth century. Chaucer certainly was of opinion that he had achieved simplicity in his explanations, for he gave his Treatise the title of "Bread and Milk for Children."
No one understood better than the great poet that words alone would not suffice to make his meaning clear. He directed his son to read the book with a real Astrolabe in hand. To make the description easier to comprehend, he illustrated every chapter with a neatly drawn diagram, specially drawn to explain each successive proposition, as in Euclid. Fifty-seven paragraphs conclude with the instruction, ' For more declaracioun, lo here the figure', and as many as sixty-two explanatory figures may occur in the more complete manuscripts, as in the fine manuscript in the Cambridge University Library known as Dd. 3. 53.
In many of its forms the Astrolabe to be one of the most valuable of educational instruments and contains material for an education from the nursery to the University.
PREFACE
The full understanding of the Treatise on the construction and use of an Astrolabe requires a greater mental effort than many more familiar scholastic exercises. As an introduction to the history of early science this Treatise is unique.
It is certainly remarkable that the oldest work written in English upon an elaborate scientific instrument should never have been printed in full. And when we remember that the instrument is one of the most important that the world has known, that the author was greatest astronomer-poet, Geoffrey Chaucer, and that his works are very widely studied, the neglect appears to be more than accidental: it is symptomatic of the attitude of the English mind to matters scientific.
In many of its forms the Astrolabe can still claim to be one of the most valuable of educational instruments.
(1) It is a beautiful piece of workmanship, having a basis of beauty and order in mathematical curves.
(2) It is concrete and meant to be handled.
(3) It can be taken to pieces and put together again by a boy of 10, and so is superior to a watch or clock.
(4) It will provide an introduction to great and worthy fields of investigation— the heavens and the earth, geography, longitude and time, latitude, &c.
(5) It will serve to illustrate much of the great scientific work done by the ancients.
In short, the Astrolabe contains material for an education from the nursery to the University, and Chaucer was very fully alive to its great and manifold utility. Manuscript copies of this work of Chaucer approaching completeness are not only very few in number and inaccessible to the general reader, but no single one has ever been printed in full. A most essential part, the figures of the various parts and uses of the instrument, has always been omitted. It is therefore scarcely surprising that many readers should complain of the author's meaning being obscure, and that they should ask, What is an Astrolabe ?
To answer this question in simple language was Chaucer's chief object. He wrote his Treatise on the Astrolabe in 1387 for little Lewis, his lo-years- old son, to learn at Oxford, choosing 'naked wordes in Enghssh ', and deeming it better to write ‘unto a child twice a good sentence than that he forget it once'. Modestly he disclaims further originality and acknowledges the earlier labour of the ancients, the ' olde Astrologiens ', Alkabitius and Ptolemy ; but he undoubtedly obtained the greater part of his book from some Latin version of the Compositio et Operatio Astrolabii of Messahalla, an Arabian astronomer who is thought to have flourished about the end of the eighth century. Chaucer certainly was of opinion that he had achieved simplicity in his explanations, for he gave his Treatise the title of
Bread and Milk for Children.
No one understood better than the great poet that words alone would not suffice to make his meaning clear. He directed his son to read the book with a real Astrolabe in hand. To make the description easier to comprehend, he illustrated every chapter with a neatly drawn diagram, specially drawn to explain each successive proposition, as in Euclid. Fifty-seven paragraphs conclude with the instruction, ' For more declaracioun, lo here the figure', and as many as sixty-two explanatory figures may occur in the more complete manuscripts, as in the fine manuscript in the Cambridge University Library known as Dd. 3. 53. Though Chaucer regarded diagrams as essential to his text, none of his publishers appears hitherto to have regarded them in the same light, and even Dr. Skeat has replaced them by sketches of his own devising, so that the modern reader has no means of ascertaining for himself what Chaucer intended. Chaucer's figures are now printed for the first time.
It is generally agreed that the Treatise is unfinished, for whereas the preface promises us a work in five parts only two are extant. It may be possible in the future to reconstruct the other three parts with the aid of contemporary copies of such astronomical codices as Chaucer is likely to have had at his disposal. In the meantime we print the headings of the missing parts, and suggest their contents.
The source to which the bulk of the matter can be traced is the parent book of Messahalla, of which copies are not rare in the great libraries of Europe. It was in two parts : on the Making, and on the Use of the Astrolabe. In the best manuscripts. Part I, the De Compositione Asirolabii, which begins, ‘Scito quod astrolabium sit nomen Graecum ', is illustrated with figures beautifully drawn in red ink. They were not included by Chaucer, probably because they would have made his Treatise too difficult for little Lewis. He would hardly have required them, since he had a ready-made instrument. The De Compositione is now printed in collotype facsimile with an English rendering and an expanded Latin version. Unhappily the reference letters in the text do not always correspond with those on the figures, and further adjustments will have to be made before a definitive text can be produced. Part II, the De Operatione Astrolabii, beginning, ' Nomina instrumentorum sunt hec’, supplied the main basis of Chaucer's work, and of this an English version is also appended. The original figures, with their finely drawn black lines and red lettering, are most attractive.
TREATISE ON THE ASTROLABE
Bread and milk for children.
Little Lewis my son, I have perceived well by certain signs thy ability to learn sciences touching numbers and proportions ; and I also consider thy earnest prayer specially to learn the Treatise of the Astrolabe. Then forasmuch as a philosopher saith, ' he wrappeth him in his friend, who condescendeth to the rightful prayers of his friend,' therefore I have given thee an astrolabe for our horizon, composed for the latitude of Oxford, upon which, by means of this little treatise, I purpose to teach thee a certain number of conclusions appertaining to the same instrument. I say certain conclusions, for three reasons.
The first is this : understand that all the conclusions that have been found, or possibly might be found in so noble an instrument as an astrolabe, are not known perfectly to any mortal man in this region, as I suppose.
Another reason is this : that truly, in any treatise of the astrolabe that I have seen, there are some conclusions that will not in all things perform their promises. And some of them are too hard for thy tender age of ten years to understand.
I will show thee this treatise, divided into five parts, under full easy rules and in plain English words ; for Latin thou knowest as yet but little, my little son. But nevertheless, these true conclusions are sufficient for thee in English, as they are in Greek for noble Greek scholars, and in Arabic for Arabians, and in Hebrew for Jews, and in Latin for the Latin folk, for they have written them first out of other different languages, in their own tongue, that is to say, in Latin. And God knows, that in all these languages, and in many more, these conclusions have been sufficiently learned and taught, though by di- verse rules, just as diverse paths lead diverse folk the right way to Rome. Now will I meekly pray every discreet person that readeth or heareth this little treatise, to have my rude inditing and my superfluity of words excused, for two causes. The first, that curious inditing and hard sentences are at once too difficult for a child to learn. And the second, that indeed it seems better to me to write a good sentence twice unto a child, rather than that he forget it once. And Lewis, if I show thee in my easy English as true conclusions touching this matter, and not only as true but as many and as subtle conclusions as are shown in Latin in any common treatise of the astrolabe, grant me the more thanks ; and pray God save the King, who is lord of this language, and all that are true to him and obey him, each in his degree, the more and the less. But consider well, that I do not claim to have found this work by my own labour or ingenuity, I am but an unlearned compiler of the labour of old astrologians. I have translated it into English only for thy instruction ; and with this sword shall I slay envy.
The first part of this treatise will rehearse the figures and the parts of thy astrolabe, so that thou mayest have the greater knowledge of thy own instrument.
The second part will teach thee to work the exact practice of the aforesaid conclusions, as far and as exactly as may be showed in so small and portable an instrument. For every astrologian well knows that the smallest fractions are not shown in so small an instrument, as they are in subtle tables calculated on purpose.
The third part will contain diverse tables of longitudes and latitudes of fixed stars for the astrolabe, and tables of declinations of the sun, and tables of longitudes of cities and of towns ; both for the regulation of a clock and to find the meridian altitude, and many another notable conclusion, in accordance with the calendars of the reverend scholars, friar John Somer and friar Nicholas Lenne.( Somer's Calendar was calculated for 140 years from 1367. Lynn's for 76 years from 1387.)
The fourth part will be a theory to explain the moving of the celestial bodies with the causes. In particular it will show a table of the exact moving of the moon from hour to hour, every day and in every sign, after thy almanac ; after which table there follows an explanation, sufficient to teach both the manner of the working of that same conclusion, and to know in our horizon the degree of the zodiac with which the moon rises in any latitude, and the arising of any planet in accordance with its latitude from the ecliptic line.
The fifth part will be an introduction according to the rules of our doctors, in which thou mayest learn a great part of the general rules of theory in astrology. In this fifth part thou wilt find tables of equations of ' houses ' for the latitude of Oxford ; and tables of dignities of planets, and other useful things, if God and his mother, the maid, will grant more than I promise.
The Rete of Chaucer's Astrolabe. MS. Rawlinson D. 913.
ASTROLABE DEVICE
The astrolabe, the most wide−spread and most popular instrument in the history of astronomy.
“The astrolabe is a portable instrument which adjusts itself into an exactly vertical position through a type of Cardanic suspension. One of its main parts is a stationary disc on which the horizon is projected with its parallel circles and vertical circles (muqantara and azimuthal circles) from a point, mostly from one of the celestial poles. The horizontal line divides the disc into two parts, into an upper part with the projections of the muqantara and azimuthal circles, which corresponds to the half of the celestial sphere above the earth, and into a lower part that corresponds to the half of the celestial sphere beneath the earth. On this lower part several arcs are drawn from the centre of the disc up to the rim; these are designated as hour lines. It has to be kept in mind that the counting of the hours begins with sunrise, according to ancient customs. The other main part of the instrument is a movable disc which, however, is not solid but an open−work piece. On it are seen the projection of the ecliptic (of the zodiac) which, corresponding with the number of the signs of the zodiac, is divided into 12 parts; these are further subdivided into 30 degrees. There are also the projections of a number of the largest and most well−known fixed stars.”
“The movable disc, called spider or net [‘ankabut or sabaka], can be rotated around an axis at its centre upon the stationary disc. By rotating the spider, the daily rotation of the heavenly bodies at a given local horizon can be simulated. If the spider is set up in a particular position, it is possible to read off the altitude above the horizon and the azimuth directly on the disc which is under the spider, for each of the stars and signs of the zodiac represented on the spider, for the Sun and, in a certain sense, the planets included, and can read off the hours, which have elapsed since sunrise or sunset, from the intersection with the hour lines of the point of the zodiac sign where the Sun is situated just then, or of the point in the zodiac diametrically opposite to the Sun …”
“The astrolabe makes it possible to directly determine the stars in the following main positions. It is only necessary to see which heavenly body lies, with a [80] particular position of the spider, on the eastern or western part of the horizon, on the upper or lower part of the meridian line, which is the vertical diameter of the disc. In order to be able to situate the spider at a position corresponding with the given position of the celestial sphere, it is necessary to know one of the above−mentioned astronomical data, be it e.g. the altitude of a star or of the Sun above the horizon, be it the hour that has elapsed since sunrise. By rotating the spider, the star is placed on the muqantara in accordance with its altitude, or at night by giving the hour, and that is to say the hour of the night, the position of the Sun in the zodiac, and with the time of the day, the point situated diametrically opposite the Sun’s position upon the respective hour line. The spider then shows the desired position. Besides these few problems mentioned, quite a number of other astronomical and astrological problems can be solved mechanically with the astrolabe, almost without calculation” (Josef Frank, Zur Geschichte des Astrolabs, pp. 4–5 (reprint,op. cit., pp. 4–5).
The enormous development in professional, technical and artistic respects which this chief instrument of Arabic−Islamic astronomy underwent through the centuries has been dealt with more comprehensively by modern research.
The signs of the advanced development which the astrolabe underwent in the Arabic−Islamic period include its numerous variants. The types known until the turn of the 4th/10th to the 5th/11th century are described by Abu r−Raihan al−Biruni in his book Isti‘ab al−wuguh al−mumkina in which he leans heavily on a book by his teacher Ab‚ Sa‘id Ahmad b. Muhammad as−Sigzi (2nd half 4th/10th c.).
From the studies on the various types of astrolabes done so far, it is evident that there is a connection between their origin and the concept of the mixed astrolabes (mizag al−asturlab). This has to do with the combination of the features of the northern and the southern astrolabe in a single one. As early as in the first half of the 3rd/9th century, the Arabs were—in the words of J. Frank —”not content with the form adopted from their predecessors where the part of the celestial sphere to the north of the tropic of Capricorn is projected upon a plane parallel to the celestial equator or upon itself from the south pole. They also attempted stereographic projection from the north pole of that part of the celestial sphere which lay south of the tropic of Cancer and called an astrolabe thus produced the southern astrolabe, as distinct from the northern astrolabe. When exactly the southern one originated cannot be ascertained any more, but in any case before Fargânî, who also provides the theory for this astrolabe.”
PART I
Here beginneth the Description of the Astrolabe
I. Thy astrolabe hath a ring to put on the thumb of thy right hand when taking the height of things.
And note that from henceforward, I will call the height of any thing that is taken by thy 'rule', the altitude, without more words.
2. This ring runs in a kind of eyelet, fastened to the ' mother ' of thy astrolabe, in so roomy a space that it does not prevent the instrument from hanging plumb.
3. The mother of thy astrolabe is a very thick plate, hollowed out with a large cavity, which receives within it the thin plates, marked for different climates, and thy rete, shaped like a net or the web of a spider. For more explanation, lo here the figure :
Figure showing the Rete lying in the Mother.
Like the other text-figures it has been taken from MS. Cambridge Dd.3. 53, and is obviously from the same source as the Rete on p. 4, with the head of the Dogstar, Alhabor, reversed.
4. This mother is divided on the backhalf with a line, which descends from the ring down to the lowest border. This line, from the aforesaid ring to the centre of the large cavity in the middle, is called the south line, or the line meridional. And the remainder of this line down to the border is called the north line, or the line of midnight. And for more explanation, lo here the figure :
5. At right angles to the meridional line, there crosses it another line of the same length from east to west. This, from a little cross + in the border to the centre of the large cavity, is called the east line, or line oriental; and the remainder of the line from the aforesaid + to the border, is called the west line, or line occidental. Now hast thou here the 4 quarters of thy astrolabe, divided according to the 4 principal quarters of the compass, or quarters of the firmament. And for more explanation, lo here thy figure :
6. The east side of thy astrolabe is called the right side, and the west side is called the left side. Forget not this, little Lewis. Put the ring of thy astrolabe upon the thumb of thy right hand, and then its right side will be towards thy left side, and its left side will be towards thy right side ; take this as a general rule, as well on the back as on the hollow side. Upon the end of the east line, as I first said, is marked a little +, which is always regarded as the beginning of the first degree in which the sun rises. And for more explanation, lo here the figure:
7. From this little + up to the end of the meridional line, under the ring, thou wilt find the border divided into go degrees ; and every quarter of thy astrolabe is divided in the same proportion. Over these degrees are numbers, and the degrees are divided into fives as shown by long lines between. The space between the long lines containeth a mile-way. (The time it takes to walk a mile.) And every degree of the border contains 4 minutes, that is to say, minutes of an hour. And for more explanation, lo here the figure :
[The Figure is similar to the figure drawn above].
8. Under the circle of these degrees are written the names of the 12 Signs, as Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces ; and the numbers of the degrees of the Signs are written in Arabic numerals above, and with long divisions, from 5 to 5, divided from the time that the Sign entereth unto the last end. But understand well, that these degrees of Signs are each of them considered to be of 60 minutes, and every minute of 60 seconds, and so forth into small fractions infinite, as saith Alkabucius, (Abdilazi Alchabitius, Introdiictorium ad scientiam judicialem astronomiae, printed 1473.) and therefore, know well, that a degree of the border containeth 4 minutes, and a degree of a Sign containeth 60 minutes ; remember this. And for more explanation, lo here thy figure :
9. Next follows the Circle of the Days, in number 365 that are numbered as are the degrees, and divided also by long lines from 5 to 5; and the numbers under that circle are written in Arabic numerals. And for more explanation, lo here thy figure:
10. Next the circle of the days follows the Circle of the Months) that is to say, January, February, March, April, May, June, July, August, September, October, November, December. These months were named amongst the Arabians, some for their prophets, and some by statutes of lords, some by other lords of Rome. Also, as it pleased Julius Caesar and Caesar Augustus, some months were composed of different numbers of days, as July and August. Then hath January 31 days, February 28, March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, December 31. Nevertheless, although Julius Caesar took 2 days out of February and put them in his month of July (This is not right. Julius Caesar added 2 days to January, August, and December, and i day to April, June, September, and November.) and Augustus Caesar called the month of August after his own name and ordained it of 31 days ; yet trust well, that the sun never dwelleth on that account more or less in one sign than in another.
11. Then follow the names of the Holydays in the calendar, and next them the letters of the a. b. c. on which they fall. And for more explanation, lo here thy figure :
[The Festivals marked in the figure are those of St. Paul (Jan. 25), Purification (Feb. 2), Annunciation (March 25), Invention of the Cross (May 3), St. John Baptist (June 24), St. James (July 25), St. Lawrence (Aug. 10)?, Nativity B.V.M. (Sept. 8), St. Luke (Oct. 18), All Souls (Nov. 2), Conception B.V.M. (Dec. 8). But the scribe has put them in the wrong months.]
12. Next to the aforesaid circle of the a. b. c, and under the crossline is marked a scale, like 2 measur-ing-rules or else like ladders, that serveth by its 12 points and its divisions for full many a subtle conclusion. Of this aforesaid scale, the part from the cross-line to the right angle, is called umbra versa, and the nether part is called umbra recta, or else umbra extensa. (The names are transposed in the original MS. and in the figure.) And for more explanation, lo here the figure :
13. Then hast thou a broad rule, that hath on either end a square plate pierced with certain holes, to receive the streams of the sun by day, and also by means of thy eye, to know the altitude of stars by night. And for more explanation, lo here thy figure :
14. Then is there a larg pin like an axle-tree, that goeth through the hole, and holdeth the tables of the climates and the rete in the cavity of the mother. Through this pin there goeth a little wedge, called the horse, which compresses all the parts in a heap ; the pin which resembles an axle-tree, is imagined to be the pole arctic (north pole) in thy astrolabe. And for the more explanation, lo here the figure :
15. The hollow side of thy astrolabe is also divided with a long cross into 4 quarters from east to west, from south to north, from right side to left side, as is the back-side. And for the more explanation, lo here thy figure :
16. The border of the hollow-side is divided from the point of the east line to the point of the south line under the ring, into 90 degrees; and every quarter is divided by that same proportion. So too is the back-side divided,, and that amounteth to 360 degrees. And understand well, that degrees of this border correspond with, and are concentric to, the degrees of the equinoctial, that is divided into the same number as is every other circle in the high heaven. This same border is also divided with 23 capital letters and a small cross -f above the south line, so as to show the 24 equal hours of the clock ; and, as I have said, 5 of these degrees make a mile-way, and 3 mile-ways make an hour. And every degree of this border contains 4 minutes of time, and every minute contains 60 seconds. Now have I told thee twice, and for more explanation, lo here the figure :
17. The plate under thy rete is marked with 3 principal circles; of which the least is called the Circle of Cancer, because that the head of Cancer, or the beginning of the Sign of Cancer in the rete, turneth evermore concentric upon this same circle.
In this head of Cancer is the greatest declination northward of the sun. And therefore is it called the solstice of summer ; which dechnation, according to Ptolemy is 23 degrees and 50 minutes, as well in Cancer as in Capricorn. This sign of Cancer is called the tropic of summer, from tropos, that is to say a turning, for then beginneth the sun to pass away from us; and for the more explanation, lo here the figure :
Alternative Figure of the Mother of Chaucer's Astrolabe. MS. Rawlinson D. 913.
The middle circle in wideness, of these 3, is called the Equinoctial Circle, upon which turns evermore the heads of Aries and Libra. And understand well, that evermore this equinoctial circle turns exactly from very east to very west ; as I have shown thee in the solid sphere. This same circle is called also the weigher, equator, of the day, for when the sun is in the heads of Aries and Libra, then are the days and the nights equal in length in all the world. And therefore are these two signs called the equinoxes. And all that moveth within the heads of these Aries and Libra, their moving is called northward, and all that moveth without these heads, their moving is called southward as from the equinoctial. Take heed of these latitudes north and south, and forget it not. By the equinoctial circle the 24 hours of the clock are considered ; for [evermore] the arising of 15 degrees of the equinoctial maketh an equal hour of the clock. This equinoctial is called the girdle of the first moving, or else of the angulus primi motus velprimi mobilis. And note, that first moving is called ' moving ' of the first movable of the 8th sphere, which motion is from east to west, and after again into east, also it is called 'girdle' of the first moving, for it divideth the first movable, that is to say, the sphere, in 2 equal parts, evenly distant from the poles of this world.
The widest of these 3 principal circles is called the Circle of Capricorn, because that the head of Capricorn turneth evermore concentric upon the same circle, in the head of Capricorn is the greatest declination southward of the sun, and therefore is it called the solstice of winter. This sign of Capricorn is also called the tropic of winter for then beginneth the sun to come again towards us. And for the more explanation, lo here thy figure :
18. Upon this aforesaid plate are drawn certain circles [of altitude] that are called Almicanteras, some of which seem perfect circles, and some seem imperfect. The centre that standeth amidst the narrowest circle is called the zenith ; and the lowest circle, or the first circle, is called the horizon, that is to say, the circle that divides the two hemispheres, i. e. the part of the heaven above the earth, and the part beneath. These almicanteras are compounded by 2 and 2 [or are two degrees apart], but some other astrolabes have the almicanteras divided by one degree, others by two, and others by 3 degrees according to the size of the astrolabe. The aforesaid zenith is imagined to be the point exactly over the crown of thy head, and also the zenith is the exact pole of the horizon in every region. And for more explanation, lo here thy figure :
19. From this zenith, as it seemeth, there comes a kind of crooked lies like the claws of a spider, or else like the work of a woman's caul, crossing the almicanteras at right angles. These lines or divisions are called azimuths. They divide the horizon of thy astrolabe into 24 divisions. And serve to indicate the directions of the firmament, and to other conclusions, such as the position of the cenith (The cenith {not zenith) was the ' point of the horizon denoting the sun's place in azimuth ' (Skeat). In the figure the 18 azimuth Jines have been carelessly sketched : they should be symmetrical and 24 in number.) of the sun and of every star. And for more explanation, lo here thy figure :
20. Next the azimuths, under the Circle of Cancer, there are 12 oblique divisions, much like to the shape of the azimuths ; they show the spaces of the hours of planets. And for more explanation, lo here thy figure :
21. The rete of thy astrolabe with thy zodiac, shaped like a net or a spider's web, according to the old description, thou mayest turn up and down as thyself liketh. It contains a certain number of fixed stars, with their longitudes and latitudes properly ascertained, if the maker have not erred. The names of the stars are written in the margin of the rete where they are situate ; and the small point of each star is called the centre. Understand also that all stars situated within the zodiac of thy astrolabe are called stars of the north, for they rise north of the east line. And all the rest of the fixed stars, out of the zodiac, are called stars of the south ; but I say not that they all rise to the south of the east line ; witness one, Aldebaran and Algomeisa. Understand generally this rule, that those stars that are called stars of the north rise sooner than the degree of their longitude; and all the stars of the south rise later than the degree of their longitude ; that is to say, the fixed stars in thy astrolabe. The measure of this longitude of stars is taken in the ecliptic line of heaven, on (Chaucer wrote 'under') which line, when the sun and moon are in an exact line, or else closely bordering upon it, then an eclipse of the sun or of the moon is possible, as I shall declare, and also the cause why. But truly the ecliptic line of the zodiac is the outer- most border of thy zodiac, where the degrees are marked.
The zodiac of thy astrolabe is shaped like a circle that contains a large breadth, in proportion to the size of thy astrolabe, to signify that the zodiac in heaven is imagined to be a surface containing a latitude of 12 degrees, whereas all the rest of the circles in the heaven are imagined true lines without any latitude. Amidst this celestial zodiac is imagined a line, called the ecliptic line, on which line is evermore the way of the sun. Thus there are 6 degrees of the zodiac on one side of the line, and 6 degrees on the other. In the rete of an astrolabe the zodiac band represents the 6 degrees of the zodiac on the northern side of the ecliptic line. The zodiac is divided into 12 principal divisions, dividing the 12 signs. And, for the accuracy of thy astrolabe, every small division in a sign is divided by two degrees and two; I mean degrees containing 60 minutes. And this aforesaid heavenly zodiac is called the circle of the signs, or the circle of the beasts, for ' zodia ' in the Greek language means ‘beasts ' in the Latin tongue. And in the zodiac are the 12 signs that have names of beasts ; either because when the sun enters into any of the signs, he taketh the property of such beasts ; or else because the stars that are fixed there are disposed in signs of beasts, or shaped like beasts ; or else, when the planets are under these signs, they act upon us by their influence, operations and effects like to the operations of beasts. And understand also, that when a hot planet comes into a hot sign, then its heat increaseth ; and if a planet be cold, then its coldness diminisheth, because of the hot sign. And by this conclusion thou mayest take example in all the signs, be they moist or dry, mov- able or fixed ; reckoning the quality of the planet as I first said.
And each of these 12 signs hath respect to a certain part of the body of a man and hath it in subjection; as Aries hath thy head, and Taurus thy neck and thy throat, Gemini thy armholes and thy arms, and so forth ; as shall be shown more plainly in the 5th part of this treatise. This zodiac, which is part of the 8th sphere, cuts across the Equinoctial ; and crosses it again in equal parts, of which one half declineth southward, and the other northward, as the Treatise of the Sphere plainly declareth. And more explanation, lo here thy figure :
22. Then thou hast a labels that is shaped like a rule, save that it is straight and hath no plates with holes at the ends ; but by the point of the aforesaid label, thou wilt calculate thy equations in the border of thy astrolabe, as by thy almury. And for more declaration, lo here thy figure :
23. Thy almury is called the denticle of Capricorn or else the calculator. It is situate fixed in the head of Capricorn, and it serveth for many a necessary conclusion in equations of things, as shall be shown. And for the more declaration, lo here thy figure :
The sections that have been derived from the Treatise on the Astrolabe of Messahalla.
PART II
Here bygynneth the Conclusions of the Astrolabe
I. To find the degree in which the sun is day by day, after her (The sun was of the feminine gender in Anglo-Saxon.) course about.
Ascertain the day of thy month, and lay thy rule upon that day, then the true point of thy rule will sit in the border, on the degree of thy sun.
Example as thus : In the year of our Lord 1391, on the 12th day of March (Chaucer's dates are about 8 days behind ours, e. g. his March 12 is our March 21.) at midday, I wished to know the degree of the sun. I sought in the back-half of my astrolabe, and found the circle of the days, which I know by the names of the months written under the circle. Then I laid my rule over the said day, and found the point of my rule in the border upon the first degree of Aries, a little within the degree ; thus I know this conclusion.
On another day, I wanted to know the degree of my sun at midday on the 13th day of December ; I found the day of the month as I have said, then I laid my rule upon the said 13th day, and found the point of my rule in the border upon the first degree of Capricorn, a little within the degree. And then I had the full experience of this conclusion ; and for the more explanation, lo here thy figure :
1. To fynde the degree in which the sonne is day by day, after hir cours abowte.
[Hic incipiunt Conclusi nes Asirolabii ; et prima est ad inveniendum gradus solis in quibus singulis diebus secundum cursum sol est exislens]
