Today | News | Books | Recipes Adventure | Science Fiction | Ghost stories | Poetry | Children | History Bookwas described a number of concentric rings, marked with various divisions, which may be readily understood from the figure. Beginning at the outermost ring, the first two represent the ninety degrees into which each quadrant of a circle can be divided (i. 7). The next two represent the signs of the zodiac, each subdivided into thirty degrees (i. 8). The next two represent the days of the year, and are rather difficult to mark, as the circle has, for this purpose, to be divided into 365¼ equal parts (i. 9). The next three circles shew 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 (i. 10). The two innermost rings shew the saints' days, with their Sunday-letters. Thus, above the 21st of December is written 'Thome,' i.e. St. Thomas's day, its Sunday-letter being E; the rest can easily be traced by the tables in a Prayer-book (i. 11). These may be thus briefly recapitulated:-- 1 and 2. Circles of degrees of the quadrant and circle. 3 and 4. Circles of the zodiacal signs, with their degrees. 5 and 6. Circles of the days of the year, with their numbers. 7, 8 and 9. Circles of the months, with their days and numbers of the days. 10 and 11. Circles of saints' days, with their Sunday-letters. Within all these, are the Scales of Umbra Recta and Umbra Versa, in each of which the scale is divided into twelve equal parts, for the convenience of taking and computing altitudes (i. 12). This primitive and loose method of computation has long been superseded by the methods of trigonometry. Besides these circles, there is a perpendicular line, marking the South and North points, and a horizontal line from East to West. The other side of the plate, called the _front_, and shewn in Fig. 2, had a thick rim with a wide depression in the middle (i. 3). The rim was marked with three rings or circles, of which the outermost was the Circle of Letters (A to Z) representing the twenty-four hours of the day, and the two innermost the degrees of the quadrants (i. 16). The depressed central portion of the plate was marked only with three circles, the 'Tropicus Cancri,' the 'Æquinoctialis,' and the 'Tropicus Capricorni' (i. 17); and with the cross-lines from North to South, and from East to West (i. 15). But several thin plates or discs of metal were provided, which were of such a size as exactly to drop into the depression spoken of. The principal one of these, called the 'Rete,' is shewn 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 names of these stars, as 'Alhabor,' 'Rigel,' &c., are (some of them) written on the branches (i. 21). The 'Rete' being thus, as it were, a skeleton plate, allows the 'Tropicus Cancri,' &c., marked upon the body of the instrument, to be partially seen below it. Another form of the 'Rete' is shewn in Fig. 9, and other _positions_ of the Rete in Fig. 11 and Fig. 12. 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 are 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). The lower part of the same plate is marked with arcs denoting the twelve planetary hours (i. 20). At the _back_ of the astrolabe revolved the 'rule,' made of metal, and fitted with sights, represented in Fig. 3 (i. 13). At the _front_ of it revolved the 'label,' represented in Fig. 6 (i. 22). All the parts were held together by the central pin (Fig. 4) which passed through the holes in the 'moder,' plates, 'Rete,' rule, and label[59], and was secured by a little wedge (i. 14), which was sometimes fancifully carved to resemble a horse (Fig. 7). Another 'table' or disc is shewn in Fig. 14, and was used for ascertaining the twelve astrological houses. § 29. USES OF THE ASTROLABE PLANISPHERE. I here briefly enumerate such principal uses of the instrument as are mentioned by Chaucer. The _back_ (Fig. 1) shews at once the degree of the zodiac answering to every day in the year (ii. 1). The altitude of the sun can be taken by the 'Rule,' elevated at the proper angle (ii. 2). If the Rete be properly adjusted to this altitude, we can thus tell the hour of the day (ii. 3). The duration of twilight can be calculated by observing when the sun is 18° below the horizon (ii. 6). Observe the times of sunrise and sundown, and the interval is the 'artificial day' (ii. 7). This day, with the duration of morning and evening twilights added to it, is called the 'vulgar day' (ii. 9). The plate in Fig. 5 shews the planetary hours (ii. 12). The placing of the sun's degree on the South-line gives the sun's meridian altitude (ii. 13), and conversely (ii. 14). The back of the instrument can shew what days in the year are of equal length (ii. 15). The degree of the zodiac which souths with _any_ star can be ascertained by observing two altitudes of the star; but the observations must be made when the star is _very near_ the meridian (ii. 17). If the star be marked on the Rete, the said degree is easily found by use of the Rete (ii. 18). We can also find with what degree of the zodiac the same star rises (ii. 19). The use of the Rete also shews the declination of every degree in the zodiac (ii. 20). We can always tell for what latitude a disc such as that in Fig. 5 is constructed, by properly examining it (ii. 21). The latitude of any place can be found by two observations of the altitude of the Pole-star (ii. 23); or of any circumpolar star (ii. 24); or by observing the sun's meridional altitude (ii. 25). The Rete also tells us the 'ascensions of signs,' or how many degrees of the equinoctial circle pass the meridian with a given sign (ii. 27); as also the 'oblique ascensions' of the same (ii. 28). The astrolabe can also be used to discover (but only in an imperfect and approximate manner) the four cardinal points of the compass (ii. 29). We can also compare the altitude of a planet with that of the sun (ii. 30). We can find in what part of the horizon the sun rises (ii. 31); and in what direction to look for a conjunction of the sun and moon (ii. 32); also near what point of the compass the sun is at any given hour (ii. 33). The moon's observed altitude will shew her longitude (ii. 34). We |