Today | News | Books | Recipes Adventure | Science Fiction | Ghost stories | Poetry | Children | History Bookatitude of the place for which the disc is constructed is thus determined by inspection. 22. In the _first_ place where '_orisonte_' occurs, it means the _South_ point of the horizon; in the _second_ place, the _North_ point. By referring to fig. 13, Plate V, it is clear that the arc [Aries]S, representing the distance between the equinoctial and the S. point, is equal to the arc ZP, which measures the distance from the pole to the zenith; since PO[Aries] and ZOS are both right angles. Hence also Chaucer's second statement, that the arcs PN and [Aries]Z are equal. In his numerical example, PN is 51° 50'; and therefore ZP is the complement, or 38° 10'. So also [Aries]Z is 51° 50'; and [Aries]S is 38° 10'. Briefly, [Aries]Z measures the latitude. 23. Here the altitude of a star (A) is to be taken twice; firstly, when it is on the meridian in the most _southern_ point of its course, and secondly, when on the meridian in the most _northern_ point, which would be the case twelve hours later. The mean of these altitudes is the altitude of the pole, or the latitude of the place. In the example given, the star A is only 4° from the pole, which shews that it is the Pole-star, then farther from the Pole than it is now. The star F is, according to Chaucer, any convenient star having a right ascension differing from that of the Pole-star by 180°; though one having the _same_ right ascension would serve as well. If then, at the first observation, the altitude of A be 56, and at the second be 48, the altitude of the pole must be 52. See fig. 13, Plate V. 24. This comes to much the same thing. The _lowest_ or northern altitude of Dubhe ([alpha] Ursæ Majoris) may be supposed to be observed to be 25°, and his _highest_ or southern altitude to be 79°. Add these; the sum is 104; 'abate' or subtract half of that number, and the result is 52°; the latitude. 25. Here, as in § 22, Chaucer says that the latitude can be measured by the arc Z[Aries] or PN; he adds that the depression of the Antarctic pole, viz. the arc SP' (where P' is the S. pole), is another measure of the latitude. He explains that an obvious way of finding the latitude is by finding the altitude of the sun at noon at the time of an equinox. If this altitude be 38° 10', then the latitude is the complement, or 51° 50'. But this observation can only be made on two days in the year. If then this seems to be too long a tarrying, observe his midday altitude, and allow for his declination. Thus, if the sun's altitude be 58° 10' at noon when he is in the first degree of Leo, subtract his declination, viz. 20°, and the result is 38° 10', the complement of the latitude. If, however, the sun's declination be _south_, the amount of it must be added instead of subtracted. Or else we may find [Aries]A', the highest altitude of a star A' above the equinoctial, and also [Aries]A, its nether elongation extending from the same, and take the mean of the two. 26. The 'Sphere Solid' answers nearly to what we now call a globe. By help of a globe it is easy to find the ascensions of signs for _any latitude_, whereas by the astrolabe we can only tell them for those latitudes for which the plates bearing the almicanteras are constructed. The signs which Chaucer calls 'of right (i.e. direct) ascension' are those signs of the zodiac which rise more directly, i.e. at a greater angle to the horizon than the rest. In latitude 52°, Libra rises so directly that the whole sign takes more than 2¾ hours before it is wholly above the horizon, during which time nearly 43° of the equinoctial circle have arisen; or, in Chaucer's words, 'the more part' (i.e. a larger portion) of the equinoctial ascends with it. On the other hand, the sign of Aries ascends so obliquely that the whole of it appears above the horizon in less than an hour, so that a 'less part' (a smaller portion) of the equinoctial ascends with it. The following is a rough table of Direct and Oblique Signs, shewing approximately how long each sign takes to ascend, and how many degrees of the equinoctial ascend with it, in lat. 52°. _Oblique Degrees of the Time of | _Direct Degrees of the Time of Signs._ Equinoctial. ascending. | Signs._ Equinoctial. ascending. Capricornus 26° 1 h. 44 m. | Cancer 39° 2 h. 36 m. Aquarius 16° 1 h. 4 m. | Leo 42° 2 h. 48 m. Pisces 14° 0 h. 56 m. | Virgo 43° 2 h. 52 m. Aries 14° 0 h. 56 m. | Libra 43° 2 h. 52 m. Taurus 16° 1 h. 4 m. | Scorpio 42° 2 h. 48 m. Gemini 26° 1 h. 44 m. | Sagittarius 39° 2 h. 36 m. These numbers are sufficiently accurate for the present purpose. In ll. 8-11, there is a gap in the sense in nearly all the MSS., but the Bodley MS. 619 fortunately supplies what is wanting, to the effect that, at places situated on the equator, the poles are in the horizon. At such places, the days and nights are always equal. Chaucer's next statement is true for _all_ places _within the tropics_, the peculiarity of them being that they have the sun vertical twice in a year. The statement about the 'two summer and winters' is best explained by the following. 'In the tropical climates, ... seasons are caused more by the effect of the winds (which are very regular, and depend mainly on the sun's position) than by changes in the direct action of the sun's light and heat. The seasons are not a summer and winter, so much as recurrences of wet and dry periods, _two in each year_.'--English Cyclopædia; _Seasons, Change of_. Lastly, Chaucer reverts to places on the equator, where the stars all seem to move in vertical circles, and the almicanteras are therefore straight lines. The line marked _Horizon Rectus_ is shewn in fig. 5, where the _Horizon Obliquus_ is also shewn, cutting the equinoctial circle obliquely. 27. The real object in this section is to find how many degrees of the equinoctial circle pass the meridian together with a given zodiacal sign. Without even turning the _rete_, it is clear that the sign Aries, for instance, extends through 28° of the equinoctial; for a line drawn from the centre, in fig. 2, through the end of Aries will (if the figure be correct) pass through the end of the 28th degree below the word _Oriens_. 28. To do this accurately requires a very carefully marked Astrolabe, on as large a scale as is convenient. It is done by observing where the ends of the given sign, estimated along the _outer_ rim of the zodiacal circle in fig. 2, cross the _horizon obliquus_ as the _rete_ is turned about. Thus, the beginning of Aries lies on the _horizon obliquus_, and as the _rete_ revolves to the right, the end of it, on the outer rim, will at last lie exactly on the same curved line. When this is the case, the _rete_ ought to have moved through an angle of about 14°, as explained in § 26. By far the best way is to tabulate the results once for all, as I have there done. It is readily seen, from fig. 2, that the signs from Aries to Virgo are _northern_, and from Libra to Pisces are _southern_ signs. T |