“The key to the whole matter is given by a passage in Chaucer’s ‘Astrolabe,’ pt. ii, ch. 29, where it is clear that Chaucer (who, however merely translates from Messahala) actually confuses the hour-angle with the azimuthal arc (see [Appendix I]); that is, he considered it correct to find the hour of the day by noting the point of the horizon over which the sun appears to stand, and supposing this point to advance, with a uniform, not a variable, motion. The host’s method of proceeding was this. Wanting to know the hour, he observed how far the sun had moved southward along the horizon since it rose, and saw that it had gone more than half-way from the point of sunrise to the exact southern point. Now the 18th of April in Chaucer’s time answers to the 26th of April at present. On April 26, 1874, the sun rose at 4 hr. 43 m., and set at 7 hr. 12 m., giving a day of about 14 hr. 30 m., the fourth part of which is at 8 hr. 20 m., or, with sufficient exactness, at half past eight. This would leave a whole hour and a half to signify Chaucer’s ‘half an houre and more’, showing that further explanation is still necessary. The fact is, however, that the host reckoned, as has been said, in another way, viz. by observing the sun’s position with reference to the horizon. On April 18 the sun was in the 6th degree of Taurus at that date, as we again learn from Chaucer’s treatise. Set this 6th degree of Taurus on the east horizon on a globe, and it is found to be 22 degrees to the north of the east point, or 112 degrees from the south. The half of this at 56 degrees from the south; and the sun would seem to stand above this 56th degree, as may be seen even upon a globe, at about a quarter past nine; but Mr. Brae has made the calculation, and shows that it was at twenty minutes past nine. This makes Chaucer’s ‘half an houre and more’ to stand for half an hour and ten minutes; an extremely neat result. But this we can check again by help of the host’s other observation. He also took note, that the lengths of a shadow and its object were equal, whence the sun’s altitude must have been 45 degrees. Even a globe will shew that the sun’s altitude, when in the 6th degree of Taurus, and at 10 o’clock in the morning, is somewhere about 45 or 46 degrees. But Mr. Brae has calculated it exactly, and his result is, that the sun attained its altitude of 45 degrees at two minutes to ten exactly. This is even a closer approximation than we might expect, and leaves no doubt about the right date being the eighteenth of April.”
Thus it appears that Chaucer’s method of determining the date was incorrect but his calculations in observing the sun’s position were quite accurate. For fuller particulars see Chaucer’s Astrolabe, ed. Skeat (E. E. T. S.) preface, p. 1.
III. It was customary in ancient times and even as late as Chaucer’s century to determine the position of the sun, moon, or planets at any time by reference to the signs of the zodiac. The zodiac is an imaginary belt of the celestial sphere, extending 8° on each side of the ecliptic, within which the orbits of the sun, moon, and planets appear to lie. The zodiac is divided into twelve equal geometric divisions 30° in extent called signs to each of which a fanciful name is given. The signs were once identical with twelve constellations along the zodiac to which these fanciful names were first applied. Since the signs are purely geometric divisions and are counted from the spring equinox in the direction of the sun’s progress through them, and since through the precession of the equinoxes the whole series of signs shifts westward about one degree in seventy-two years, the signs and constellations no longer coincide. Beginning with the sign in which the vernal equinox lies the names of the zodiacal signs are Aries (Ram), Taurus (Bull), Gemini (Twins), Cancer (Crab), Leo (Lion), Virgo (Virgin), Libra (Scales), Scorpio (Scorpion), Sagittarius (Archer), Aquarius (Water-carrier), and Pisces (Fishes).
In this passage, the line “That in the Ram is four degrees up-ronne” indicates the date March 16. This can be seen by reference to Figure 1 in Skeat’s edition of Chaucer’s Astrolabe (E. E. T. S.) The astrolabe was an instrument for making observations of the heavenly bodies and calculating time from these observations. The most important part of the kind of astrolabe described by Chaucer was a rather heavy circular plate of metal from four to seven inches in diameter, which could be suspended from the thumb by a ring attached loosely enough so as to allow the instrument to assume a perpendicular position. One side of this plate was flat and was called the back, and it is this part that Figure 1 represents. The back of the astrolabe planisphere contained a series of concentric rings representing in order beginning with the outermost ring: the four quadrants of a circle each divided into ninety degrees; the signs of the zodiac divided into thirty degrees each; the days of the year, the circle being divided, for this purpose, into 365¼ equal parts; the names of the months, the number of days in each, and the small divisions which represent each day, which coincide exactly with those representing the days of the year; and lastly the saints’ days, with their Sunday-letters. The purpose of the signs of the zodiac is to show the position of the sun in the ecliptic at different times. Therefore, if we find on the figure the fourth degree of Aries and the day of the month corresponding to it, we have the date March 16 as nearly as we can determine it by observing the intricate divisions in the figure.
The next passage “Noon hyer was he, whan she redy was” means evidently, ‘he was no higher than this (i. e. four degrees) above the horizon when she was ready’; that is, it was a little past six. The method used in determining the time of day by observation of the sun’s position is explained in the Astrolabe ii, 2 and 3. First the sun’s altitude is found by means of the revolving rule at the back of the astrolabe. The rule, a piece of metal fitted with sights, is moved up and down until the rays of the sun shine directly through the sights. Then, by means of the degrees marked on the back of the astrolabe, the angle of elevation of the rule is determined, giving the altitude of the sun. The rest of the process involves the use of the front of the astrolabe. This side of the circular plate, shown in Fig. 2, had a thick rim with a wide depression in the middle. On the rim were three concentric circles, the first showing the letters A to Z, representing the twenty-four hours of the day, and the two innermost circles giving the degrees of the four quadrants. The depressed central part of the front was marked with three circles, the ‘Tropicus Cancri’, the ‘AEquinoctialis,’ and the ‘Tropicus Capricorni’; and with the cross-lines from North to South, and from East to West. There were besides several thin plates or discs of metal of such a size as exactly to drop into the depression spoken of. The principal one of these was the ‘Rete’ and is shown in Fig. 2. “It consisted of a circular ring marked with the zodiacal signs, subdivided into degrees, with narrow branching limbs both within and without this ring, having smaller branches or tongues terminating in points, each of which denoted the exact position of some well-known star. * * * The ‘Rete’ being thus, as it were, a skeleton plate, allows the ‘Tropicus Cancri,’ etc., marked upon the body of the instrument, to be partially seen below it. * * * But it was more usual to interpose between the ‘Rete’ and the body of the instrument (called the ‘Mother’) another thin plate or disc, such as that in Fig. 5, so that portions of this latter plate could be seen beneath the skeleton-form of the ‘Rete’ (i. 17). These plates were called by Chaucer ‘tables’, and sometimes an instrument was provided with several of them, differently marked, for use in places having different latitudes. The one in Fig. 5 is suitable for the latitude of Oxford (nearly). The upper part, above the Horizon Obliquus, is marked with circles of altitude (i. 18), crossed by incomplete arcs of azimuth tending to a common centre, the zenith (i. 19).” [Skeat, Introduction to the Astrolabe, pp. lxxiv-lxxv.]
Now suppose we have taken the sun’s altitude by §2 (Pt. ii of the Astrolabe) and found it to be 25½°. “As the altitude was taken by the back of the Astrolabe, turn it over, and then let the Rete revolve westward until the 1st point of Aries is just within the altitude-circle marked 25, allowing for the ½ degree by guess. This will bring the denticle near the letter C, and the first point of Aries near X, which means 9 a.m.” [Skeat’s note on the Astrolabe ii. 3, pp. 189-190].
IV. Chaucer would know the altitude of the sun simply by inspection of an astrolabe, without calculation. Skeat has explained this passage in his Preface to Chaucer’s Astrolabe (E. E. T. S.), p. lxiii, as follows:
“Besides saying that the sun was 29° high, Chaucer says that his shadow was to his height in the proportion of 11 to 6. Changing this proportion, we can make it that of 12 to 66⁄11; that is, the point of the Umbra Versa (which is reckoned by twelfth parts) is 66⁄11 or 6½ nearly. (Umbra Recta and Umbra Versa were scales on the back of the astrolabe used for computing the altitudes of heavenly bodies from the height and shadows of objects. The umbra recta was used where the angle of elevation of an object was greater than 45°; the umbra versa, where it was less.) This can be verified by Fig. 1; for a straight edge, laid across from the 29th degree above the word ‘Occidens,’ and passing through the center, will cut the scale of Umbra Versa between the 6th and 7th points. The sun’s altitude is thus established as 29° above the western horizon, beyond all doubt.”
V. Herberwe means ‘position.’ Chaucer says here, then, that the sun according to his declination causing his position to be low or high in the heavens, brings about the seasons for all living things. In the Astrolabe, i. 17, there is a very interesting passage explaining in detail, declination, the solstices and equinoxes, and change of seasons. Chaucer is describing the front of the astrolabe. He says: “The plate under thy rite is descryved with 3 principal cercles; of whiche the leste is cleped the cercle of Cancer, by-cause that the heved of Cancer turneth evermor consentrik up-on the same cercle. (This corresponds to the Tropic of Cancer on the celestial sphere, which marks the greatest northern declination of the sun.) In this heved of Cancer is the grettest declinacioun northward of the sonne. And ther-for is he cleped the Solsticioun of Somer; whiche declinacioun, aftur Ptholome, is 23 degrees and 50 minutes, as wel in Cancer as in Capricorne. (The greatest declination of the sun measures the obliquity of the ecliptic, which is slightly variable. In Chaucer’s time it was about 23° 31′, and in the time of Ptolemy about 23° 40′. Ptolemy assigns it too high a value.) This signe of Cancre is cleped the Tropik of Somer, of tropos, that is to seyn ‘agaynward’; for thanne by-ginneth the sonne to passe fro us-ward. (See Fig. 2 in Skeat’s Preface to the Astrolabe, vol. iii, or E. E. T. S. vol. 16.)
The middel cercle in wydnesse, of thise 3, is cleped the Cercle Equinoxial (the celestial equator of the celestial sphere); up-on whiche turneth evermo the hedes of Aries and Libra. (These are the two signs in which the ecliptic crosses the equinoctial.) And understond wel, that evermo this Cercle Equinoxial turneth iustly fro verrey est to verrey west; as I have shewed thee in the spere solide. (As the earth rotates daily from west to east, the celestial sphere appears to us to revolve about the earth once every twenty-four hours from east to west. Chaucer, of course, means here that the equinoctial actually revolves with the primum mobile instead of only appearing to revolve.) This same cercle is cleped also the Weyere, equator, of the day; for whan the sonne is in the hevedes of Aries and Libra, than ben the dayes and the nightes ilyke of lengthe in al the world. And ther-fore ben thise two signes called Equinoxies.