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Euphrol. So the Office of this Meridian, I perceive, is vicarious, viz. to represent all, or any of the rest. Well, since you have let me into the Reason and Significancy of the Nature or Title, let me next know what these Divisions or Degrees are upon it, and to what Use they serve.
Cleon. I will : You observe they begin at the Equinoctial, and proceed each Way to the North and South Pole of the World, to the Number of go. · Euphrof. I do; and pray what do you call them?
Cleon. They are called the Degrees of Latitude on the Terrestrial Globe: By the Word Latitude you are to understand any Distance, North or South of the Equinoctial, towards the Poles; and because such a Distance of any Place on the Globe, is estimated by these Degrees, you will readily see the Reason of their Name.
Euphrof. I do very plainly; but why are those Degrees numbered a contrary Way on the other Half of the Meridian, or that under the Horizon ? for they are reckoned from the Poles towards the Equinoctial there, I see.
Cleon. That's well observed, Euphrofyne; the Divisions are there numbered a contrary Way, the Design of which is, that the Height, or Elevation of the Pole above the Horizon may be readily seen, in any Position of the Sphere; and this Elevation of the Pole is always equal to the Latitude of any Place on the Earth's Surface.
Euphrof. I presume you mean, that so many Degrees as any Place lies North or South from the Equinoctial, just so many Degrees will the North or South Pole appear above the Horizon of that place. But how shall I be able to conceive the Truth of this?
Cleon. Very easily, if you consider, that from the Equinoctial () to the Pole (N) are just 90 Degrees; also from the given Place (Z) to the Horizon (R) are just 90 Degrees; but the Part between the Place and the Pole, (ZN) is included in both those Quadrants; and therefore, if it be fubducted from both, the Remainders will be equal; that is, the Arch (QZ) will be equal to the Arch (NR), or the Latitude (OZ) of the place (Z) is equal to the Height, or Elevation (NR) of the Pole (N),
Euphrof. I believe I apprehend the Meaning; but, pray, Cleonicus, what would you infer from thence ?
Cleon. A very important Use is made of this in divers practical Arts: Thus the Geographer ard Diallist, by taking the Height of the Pole Star with a good Quadrant, know the Latitude of the Place; by this Means also, the Sailor finds his Latitude at Sea ; hence likewise we have a speedy Method of rectifying a Sphere, or Globe, as we Thall farther fee by and by.
Euphrof. What farther Uses are made of this great Circle ?
Cleon. By Means thereof, we also measure the Distances of the Sun, Planets, or Stars from the Equinoctial, North or South ; and the Degrees, or Distance of this Sort, are called the Declination of those Bodies from the Equinoctial. Thus suppose the highest or 'moft Northern Part of the Ecliptic be brought to the Meridian, -you see it cut the same in 231 Degrees ; and so much is the Declination of that Point :—Which you see is the same also on the South Side. The Declination also of the Polar Circle you observe is 66į on each side the Equinoctial.
Euphrof. I do so; and have now gotten a pretty good Notion of Latitude and Declination. But, pray, what is the Use of this thin, long Slip of Brass, which I saw you at first take off from the Meridian?
Cleon. It is called the Quadrant of Altitude ; because it is just a Quarter of a Circle, and divided into go Degrees; and being fixed on the highest, or vertical Point of the Meridian (Z) reaches to the Horizon all around; and being moveable on a Pin at the Meridian, it may be carried to any Part on the Sphere or Globe; and so will shew the Height or Altitude of the Sun, or any Star above the Horizon, at any Time, or in any Position of the Sphere. Also being laid over any Place, it shews how that Place bears from you by the Points of the Compass on the Horizon. Also, by this you will be taught how to Measure, in Degrees, the Distance of any two Places on the Terrestrial Globe, or of any two Stars in the celestial. The graduated Edge of this Quadrant represcnts on the Globe those imaginary Circles, which the Astronomers call Azimuths, or Circles of Altitude,
and is a Kind of general Azimuth, as this, a general Meridian. But, enough of this Circle, till we come to the practical Part on the Globes ; and shall leave it with the poetical Description, which Manilius has given of it, as follows:
-This from the Bear
DIALOGUE IV. of the EQUINOCTIAL, and the DEGREES of
Euphrosyne. THE next Circle then, Cleonicus, which you are to
I instruct me in the Knowledge of, is the Equinoctial : I Thall here, as before, desire to be informed of the Rea. son of the Name.
Cleon. To understand this well, you must remember, as I told you, that this great Circle surrounds the Sphere, exactly in the Middle, or at equal Distance from the Poles of the World ; and, secondly, you must observe, that the Ecliptic, which represents the Sun's Way, cuts this Circle in two opposite Points, and therefore of Course, the Sun is twice in the Year upon this Circle ; and since one Half of this Circle is always above the
Horizon, and the other Half below it, it comes to pass, that, when the Sun is in those two Points, the Days and Nights will then be equal, which is implied by the Name Equinoctial. For the same Reason also, those two Points are called the Equinoxes, or Equinoxial Points.
Euphrof. Very good : Pray, has this Circle any other Name ?
Cleon. Yes, upon this Sphere, and the celestial Globe, it is called the Equino&tial; but on the terrestrial Globe, it is called the Equator ; because it equates, or divides the Globe of the Earth into two equal Parts, or Hemispheres, as I said : It is also called the Equator in all Maps, and Sea Charts.
Euphrof. I am glad I know that; for I would not (though a Woman) [peak improperly in naming this, or any other Part of the Sphere, or Globes. But what is the proper Use of this Circle ?
Cleon. It is, you see, divided into 360 Degrees, beginning and ending in the Equinox Point, and numbered from West to East at every tenth Degree. These Degrees are called the Longitude, on the terrestrial Globe; and on the celestial Globe and Sphere, they are called Degrees of Right Afcenfion.
Euphrof. I have heard much Talk of the Longitude, and have as little Knowledge of it, after all, as though I had never heard it mentioned : But as we are now come to the Circle of Longitude, I presume you can give me a better Idea of that Matter.
Cleon. The Longitude of a Place is the Distance thereof, reckoned in Degrees of the Equator, from that Point of the Equator where the Degrees begin ; and the Meridian, which passes through this Beginning of Longitude, is called by the Geographers, the first Meridian, Now, suppose the first Meridian be that of the City of London, as it is upon the best Globes, then the Arch of the Equator, contained between this and the Meridian of any other Place, is said to be the Longitude of that Place from London ; which you will learn with Ease to find, when we come to the Problems of the Globe.
Euphrof. Then I understand, that as the Latitude of a Place is its Distance, North or South of the Equator, reckoned on the general Meridian; so the Longitude of
any Place is its Distance, East or West, from the first Meridian, reckoned on the Equator.
Cleon. You apprehend it very well, Euphrolyne ; it is so; and the same Thing which on the terrestrial Globe and Maps is called Longitude, is, on the celestia) Globe and Sphere, called Right Afcenfion; for with respect to the Sun, Planets, and Stars, their Distance Eastward is also counted on the Equinoctial, from a first, or fixed Meridian, and it is that which passes through the Equinox (Q) where the Graduation begins ; so that the Meridian, passing through the Sun or any Star, thews on the Equinoctial the Degrees of its Right Afcenfon, or Distance from the Equinoctial Point : But in the Ecliptic, their Distance is called Longitude, and is reckoned from a very large, bright Star in the Constellation, Aries, which Star I shall Thew you hereafter, both on the Globe, and in the Heavens.
Euphrof. But why is their Longitude reckoned from that Star, and not from the Beginning of the Ecliptic, as their Right Ascenfion is ?
Cleon. Because this Star was formerly near the Meridian of that Point, yea, almoft in the Equinox itself, at the Time of Hipparchus, about 2000 Years ago; since when, it has moved forward (with the flow Motion of the Stars I formerly mentioned to you) about 291 Degrees; or rather the Equinoxes have moved so much backward ; and this Motion of the Equinoxes backward, makes the Sun enter those Points fooner every Year by nearly 20 Minutes, and this makes what you will find in Books of Astronomy is called the Precession of the Equinoxes.
Euphrof. Well, I think I understand now what you call the Longitude, and Right Afcenfion, of the Planets and Stars : Pray, to what other Purpose does this Circle serve ?
Cleon. I have already told you, that the Distance of the Sun, Planet, or Star, from the Equinoctial, North ør South, is called its Declination : Besides, the Degrees of the Equinoctial are convertible into Time, and thew in what Time any Part of a Revolution of the Sphere, or Globe, is performed.