« السابقةمتابعة »
Euphrof. This, I suppose, is done by means of the Hour-Circle, and Hand, which is fixed about the North Pole.
Cleon. You judge right ; for that Circle being fixed about the Axis, and the Hand, or Index, upon the Axis, and the Axis itself being ‘moveable in the Meridian at each Pole, it must follow, that, if the Sphere be moved, or revolved, the Axis will also be revolved, and fo carry the Index round upon the Hours engraved on the Circle.
Euphrof. I observe, that on the Circle the 12 Hours are twice engraved; pray, why is that?
Cleon. The first 12 Hours Thew the Time from Noon till Midnight; the last from Midnight to Noon the next Day; for you must know the Astr:nomers begin the Day at Noon ; therefore the two Hours of XII. stand exactly upon the graduated Edge of the general Meridian.
Euphrof. Well, now, Cleonicus, tell me how you find the Time in which any Motion is perform’d.
Cleon. I will; and first, you observe, I bring the Beginning of the Degrees, or Equinoctial Point, to the Meridian. Secondly, I then (hoiding the Sphere in that Position) turn the Index about to the Hour of XII. upon the Edge of the Meridian next to us. Thirdly, then you observe, I turn the Sphere once round, till the faid Point comes again to the Meridian, and the Index passes once round the Hour-Circle, which shews, that one Revolution of the Sphere is performed in twenty-four Hours Time.'
Euphrof. That is extremely casy to understand; and how then?
Cleon. If one Revolution, or 360 Degrees, be made in 24 Hours, then half a Revolution, or 180 Degrees, is made in 12 Hours; a Quarter, or 90 Degrees, in fix; and in that Proportion 15 Degrees of the Equi-' noctial palles the Meridian in one Hour; and 15 Minutes, or one-fourth of a Degree, in one Minute of Time ; which Notions of Motion and Time it will be necessary for you to have a clear Idea of, in smuch as they are the Grounds of many Problems on the Globes, which you willthink very curious and divertins,
said Point comund the Hour-Circle, wed in twenty-four
Euphrof. I am obligéd to you thus to prepare me with such previous Knowledge for a right understanding of the Globes, towards which my Inclinations are very ftrong.
Cleon. We'll make the Introduction to them as short as can be ; and to that End we shall dwell no longer on the Equinoctial here ; which Circle, Manilius thus describes.
The Equinoctial in the Midst divides
Creech's Man. Lib. I.
DIALOGUE V. Of the Ecliptic, and the Longitude and Latitude
of the Heavenly Bodies,
Cleonicus. TITE are now come to the great Circle, called the
V Ecliptic; because (as I formerly told you) all the Eclipses happen in, or very near it; for this great Circle represents the Sun's apparent annual Path, or Tract, through the 'Heavens; and therefore, whenever the Moon obscures the Sun, or is eclipsed by him, it must be when she is in, or near this Line.
Euphrof. This I understand ; as also I remember you told me, this Circle was divided into 12 Parts, or Signs, whore Names and Characters you explained, and are such as I see here engraved : But, pray, what was the Reason of such a Division of the Ecliptic?
Cleon. For the greater Ease and Readiness in judging of, and expresling the Places of the Sun, Moon, and Planets, at any Time; for we can form a more distinct
Idea of this in the Parts of a Sign, than in Parts of the whole Circle; the minute Parts of so grand a Whole, as the Circumference and Expanse of the Heavens, obliged Astronomers to make various Divisions of the celestial Circles, and to range the irregular Distribution of Stars in the Surface of the Skies, into certain and determinate Classes of Constellations, the better to reduce the Science to Order and Method.
Euphrof. To be sure Regularity and Method helps our Conception of Things very much. I can better apprehend in what Part of the Heavens the Sun, &c. is, when it is said, he is in such a Degree of such a Sign, than I should otherwise been able to have done, But what other Uses do they make of the Ecliptic?
Cleon. By the Ecliptic they estimate the Longitude, Latitude, and Declination of the heavenly Bodies.
Euphrof. How is the. Longitude of these Bodies reckoned?
Cleon. It is estimated in Signs and Degrees of the Ecliptic, beginning from the first Minute of Aries, viz. in the Equinoctial Point (Q). Thus, suppose the Sun be now in the fifth Degree of Leo, si, we say, bis Longitude is four Signs, four Degrees, and Part of another; for he has already passed the four Signs, Aries, Taurus, Gemini, and Cancer, and four Degrees of Leo, and is in the fifth.
Euphrof. Very good; but the Sun is always in the Ecliptic; pray, how do you compute the Longitude of Planets and Stars, which are not in, but at a Distance from the Ecliptic on either Side ?
Cleon. In this case, their Places are reduced to the Ecliptic, by means of the Quadrant of Altitude fixed to the Brass Meridian upon the Pole of the Ecliptic, which then represents' a Circle of Longitude : To do this will be a Problem you will be taught on the celestial Globe.
Euphros. Well, and how do you estimate the Latitude of the heavenly Bodies ? . Cleon. Their Latitude is reckoned in an Arch of the Quadrant of Altitude (fixed as aforesaid) contained between the Planet, or Star, and the Ecliptic; that is, it is their Distance from the Ecliptic on either Side,
from the editie Eclipile contained The one and it fervesc Figur
measured upon a Circle of Latitude, which the Quadrant then represents; for Circles of Latitude are, with respect to the Ecliptic, the same as Meridians with respect to the Equinoctial, viz. they pass thro’ its Poles, and cut it at right Angles.
Euphrof. Pray, what is the broad Circle you called the Zodiac, of which you gave me some Account, when we were considering the Theory of the Earth?
Cleon. That Circle, or rather Zone, is put upon Spheres and Globes; it represents the Space or Limits to which the Planets stray on each side the Ecliptic, which is about five Degrees; so that the Breadth of this Belt is 10 Degrees, as I then told you ; and therefore upon this broad Circle, which you obserre in this Sphere, are engraved the Figures of all the Animals of the 12 Signs ; it serves to few other Purposes but for Ornament, and to inhance the Price of the Machine. The Zodiac and Ecliptic are thus described by Manilius;
Two more oblique, and which in adverse Lines
And varying Luna follows in her Course.
Creech, Lib. I. Euphrol. Do not the Astronomers distinguish thelo Signs into different Sorts, or Orders ?
Cleon. Yes, and the Astrologers too. The Astronomers divide them into the Summer and Winter Signs, The Summer Signs are those fix which make the upper Half of the Ecliptic, or which lie above the Equinoctial, towards the North, viz. Aries, Taurus, Gemini, Cancer, Leo, and Virgo; for while the Sun is in them, it makes our Summer. The other fix, which make the lower Half of the Ecliptic, are called Winter Signs for the fame Reason.
Euphros. Pray, how do the Astrologers divide the Signs ?
Cleon. They, according to their usual Absurdity and Cant, make them of various Kinds, and ascribe to them wonderful Influences and Virtues. Thus, Manilius tells us some are Male, others Female Signs ; some Human, others Brute :
Some Signs bear human Shape, and some express'd
In single Figures, bear the Form of Beast.
Those Signs are single; now observe the Pairs ;
A Change, and vast Variety of Fates.
Such is the Goat, he twists a scaly Train,
The Centaur such, half Horse, and half a Man.
Yet swift, my Muse, like Lark on tow'ring Wings,
Some which the Day respect, and some the Night.
Some Signs, 'tis obvious, do the Sea command,
They lead the Herds, and still they love the Land.
The Crab is fruitful, and a numerous Brood