Part 4
There is also a little brass circle fixed upon this meridian, divided into 24 Hours, having an index moveable round the axis of the globe, to be turned to any particular Hour. The use of this circle is to shew the difference of time betwixt any two meridians, and is therefore called the _Hour Circle_.
[Sidenote: _Parallels of Declination._]
3. All circles parallel to the equinoctial are, with respect to any point in the Heavens, called _Parallels of Declination_. So that,
[Sidenote: _Declination North_ and _South_.]
4. The _Declination of any Point_ in the Heavens (as of the _Sun_, a _fixed Star_, or the like) is an arch of the meridian passing through that point, and intercepted betwixt it and the equator; and if the said point be to the (Northward/Southward) of the equator, it is called (_North_/_South_) _Declination_.
[Sidenote: _Tropics_ and _Polar Circles_.]
Of the parallels of declination, four are eminently distinguished by particular names, _viz._ The two _Tropics_, and the two _Polar Circles_.
[Sidenote: _Tropic of Cancer; of Capricorn._]
The tropics are on different sides of the equator each 23 degrees and 29 minutes distant from it; that which lies in the Northern Hemisphere, is called the _Tropic of Cancer_, and the Southern one, the _Tropic of Capricorn_.
These circles are the limits of the Sun’s greatest declination, and are called tropics, because whenever the Sun arrives to them, he seems to return back again towards the equator.
[Sidenote: _Arctic Circle._]
[Sidenote: _Arctic Pole._]
[Sidenote: _Antarctic Circle._]
[Sidenote: _Antarctic Pole._]
6. The _Polar Circles_ are each of them at the same distance from the Poles of the world, that the tropics are from the equator, _viz._ 23° 29′. That which lies near the North Pole, is called the _Arctic Circle_, from _Arctos_, a constellation situated in the Heavens near that Place; whence also this Pole is sometimes called the _Arctic Pole_. The other Polar circle, which is situated near the South Pole; is called the _Antarctic Circle_, because its position is contrary to the other; and the South Pole is sometimes called the _Antarctic Pole_.
The tropics and the Polar circles have each their names expressed upon the Globes.
II. _Of the Ecliptic._
[Sidenote: _Ecliptic._]
[Sidenote: _Equinoctial._]
[Sidenote: _Solstitial Points._]
[Sidenote: _Colures._]
[Sidenote: _Equinoctial Colure._]
[Sidenote: _Solstitial Colure._]
7. The _Ecliptic_ is that great circle in whose plane the Earth performs its annual motion round the Sun; or, in which the Sun seems to move round the Earth, once in a year. This circle makes an angle with the equinoctial of 23 degrees 29 minutes, and intersects it in two opposite points, which are called the _Equinoctial Points_; and the two points in the ecliptic that are at the greatest distance from the equinoctial points, are called the _Solstitial Points_. The two meridians passing through those points, are, by way of eminence, called _Colures_; whereof that which passeth thro’ the equinoctial points, is called the _Equinoctial Colure_; and that which is at right angles to it, passing through the Solstitial Points, is called the _Solstitial Colure_.
[Sidenote: The Ecliptic divided into signs.]
The ecliptic is divided into 12 equal parts, called _Signs_, each sign being 30 degrees, beginning from one of the equinoctial points, and numbered from West to East; the names and characters of the twelve signs are as follows, _viz._
_Aries_, _Taurus_, _Gemini_, _Cancer_, _Leo_, _Virgo_, 1. ♈ 2. ♉ 3. ♊ 4. ♋ 5. ♌ 6. ♍ _Libra_, _Scorpio_, _Sagittarius_, _Capricornus_, _Aquaries_, 7. ♎ 8. ♏ 9. ♐ 10. ♑ 11. ♒ _Pisces_. 12. ♓
[Sidenote: _Northern Signs._]
The first six of these are called the _Northern Signs_, and possess that half of the ecliptic which is to the Northward of the equator; beginning with the first point of ♈, and ending with the last point of ♍.
[Sidenote: _Southern Signs._]
The latter six are called the _Southern Signs_, because they possess the Southern half of the ecliptic; beginning at the first point of ♎, and ending with the last point of ♓.
The division of the ecliptic into signs, and the names of the colures, are particularly expressed upon the globes.
The signs of the ecliptic took their names from 12 constellations mentioned in the Introduction to be situated in the Heavens near those places. It is to be observed, that the signs are not to be confounded with the constellations of the same name: For the _Sign of Aries_, is not the same with the _Constellation_ Aries; the latter is a system of Stars digested into the figure of a _Ram_, but the sign of _Aries_ is only 30 degrees of the ecliptic, counted from the equinoctial point ♈, (which is reckoned the first point in the ecliptic) to the beginning of _Taurus_: Or, it is sometimes taken for all that space upon the Celestial Globe contained between the two circles passing through the first points of ♈ and ♉. What has been here said of _Aries_, is to be noted of all the rest of the signs.
The constellations above-mentioned were formerly situated within the signs which now bear their names; but by a slow motion of the equinoctial points, being one degree in 72 years, the constellation _Aries_ has now got into the sign ♉, and so of the rest. So that _Pisces_ is now got into the Sign of ♈; this slow motion in the Heavens is called the _Precession of the Equinoctial Points_.
[Sidenote: _Poles of the Ecliptic._]
The _Poles of the Ecliptic_ are both situated in the Solstitial Colure, at 23 degrees, 29 minutes distance from the Pole of the world; and they take their denomination from the Hemisphere wherein they are placed, _viz._ that which lies in the (Northern/Southern) Hemisphere, is called the (North/South) Pole of the ecliptic. The arctic and antarctic circles, are described by the Poles of the ecliptic in the diurnal motion of the Earth round its axis, whence it seems these two circles are called _Polar_.
[Sidenote: _Circles of Longitude._]
8. All great circles passing through the Poles of the ecliptic, and consequently intersecting it at right angles, are called _Circles of Longitude_: So that,
[Sidenote: _Longitude of any Point in the Heavens._]
[Sidenote: _Place of a Star._]
9. The _Longitude_ of any _Point_ in the Heavens (as a _Star_ or _Planet_, _&c._) is an arch of the ecliptic contained between the circle of longitude passing thro’ that point, and the equinoctial point ♈. And that degree of any sign which lies under the circle of longitude, passing thro’ any Star or Planet, is called the _Place_ of that Star or Planet.
_Note_, The _Sun_ never goes out of the ecliptic, and it is not usual to say the Sun’s longitude, but we commonly express it the _Sun’s Place_, which is that sign, degree, minute, _&c._, of the ecliptic, which he at any time passes.
10. All circles conceived to be drawn parallel to the ecliptic, are called _Parallels of Latitude_: So that,
[Sidenote: _Latitude of a Star, &c._]
11. The _Latitude_ of any point in the Heavens, (as a fixed Star, _&c._) is an arch of the circle of longitude, in passing thro’ that point, and intercepted betwixt it, and the ecliptic; or, the latitude is the distance from the ecliptic; and if the said point be to the Northward of the ecliptic, it is called North Latitude; but if it be to the Southward, is called South Latitude.
Upon the _Terrestrial Globe_, none of the circles of longitude are described; and upon the _Celestial_, they are commonly drawn thro’ the beginning of every _Sign_; but they are all supplied upon both Globes, by fastening a thin plate of brass over one of the Poles of the ecliptic, and so as to be moved to any degree thereof at pleasure. The parallels of latitude are also supplied by the graduations upon the said plate, as shall be shewn in a proper place.
We have now done with all those circles that are fixed, and such as are drawn upon the Globes themselves; we next proceed to the moveable circles.
III. _Of the Horizon._
[Sidenote: _Horizon._]
12. The _Horizon_ is that great circle which divides the upper, or visible Hemisphere of the world, from the lower, or invisible: This circle is distinguished into two sorts, the _Sensible_, and the _Rational_.
[Sidenote: _Sensible Horizon._]
The _Sensible_, or _Apparent Horizon_, is that circle which limits or determinates our prospect, whether we are at land or sea, reaching as far as we can see, or it is that circle where the Sky and the Earth, or Water, seem to meet. When we are on _Terra Firma_, this circle commonly seems rugged and irregular, occasioned by the unevenness of the ground terminating our prospect; but at sea there are no such irregularities; the semidiameter of this circle varieth according to the height of the eye of the observer; if a man of six feet high stood upon a large plain, or the surface of the sea, he could not see above three miles round.
This circle determines the rising and setting of the Heavenly bodies, and distinguishes Day and Night.
[Sidenote: _Rational Horizon._]
The _Rational_, or true _Horizon_, is a great circle passing thro’ the center of the Earth, parallel to the sensible Horizon, being distant from it by the Earth’s semidiameter, which is about 3980 miles: This distance is nothing in comparison of the immense distance of the Sun and the fixed Stars, therefore astronomers make no distinction between these two circles, but consider the apparent Horizon, or that wherein the Sun appears to rise and set, as passing thro’ the center of the Earth.
[Sidenote: _Cardinal Points of the Horizon._]
This circle is divided by astronomers into four quadrants, and each of the quadrants into 90 degrees, _&c._ The four points quartering this circle are called the _Cardinal Points_, and are termed the _East_, _West_, _North_, and _South_. The _East_ is that point of the Horizon where the Sun rises when he is in the equinoctial, or on that day when he ascends above the Horizon exactly at six o’clock; and the _West_ is that point of the Horizon which is directly opposite to the East, or where the Sun Sets when he is in the Equinoctial. The _South_ is 90 degrees distant from the East and West, and is toward that part of the Heavens wherein the Sun always appears to us in _Great-Britain_ at Noon; and the _North_ is that part of the Heavens which is directly opposite to the South: Or, the North and South points of the Heavens may be found by turning yourself either directly towards the East or the West: If you look towards the (East/West) the (South/North) will be to the right Hand, and the (North/South) to the left.
[Sidenote: _Points of the Compass._]
Besides the aforementioned divisions of the Horizon into degrees. _Mariners_ divide it into 32 equal parts, which they call the _Points of the Compass_; to each of which points they give a particular name, compounded of the four Cardinals, according to what quarter of the Compass is intended.
[Sidenote: _Zenith._]
[Sidenote: _Nadir._]
The center of the Horizon is the place of observation, and the Poles of it are one exactly over our heads, called the _Zenith_; and the other exactly under our feet, called the _Nadir_.
[Sidenote: _Vertical Circles._]
[Sidenote: _Meridian._]
[Sidenote: _Azimuth._]
13. All circles conceived to pass thro’ the Zenith and Nadir, are called _Vertical Circles_, or _Azimuths_. Of these circles, that which passeth thro’ the North and South points of the Horizon, is called the _Meridian_; so that when any object is upon the Meridian, it then bears either due South, or due North from us; and the _Azimuth_ of any object is an arch of the Horizon intercepted between the vertical circle passing through it, and either the North or South part of the Meridian; which part is commonly specified.
The meridian passes thro’ the Poles of the world, as well as through the Zenith and Nadir, and therefore is a secundary both of the equinoctial and the horizon: This circle divides the globe into the _Eastern_ and _Western Hemispheres_, and the Poles of it are the _East_ and _West_ points of the _Horizon_. All the heavenly objects are, during one half of their continuance above the horizon, in the Eastern Hemisphere, and for the other half in the Western; so that whenever the Sun arrives upon the upper part of the meridian, it is then _Noon_, or _Mid-day_, which is the reason why this circle is called the meridian; and when he comes to the lower part, it is then _Midnight_.
[Sidenote: _Prime Vertical._]
The vertical circle passing thro’ the East and West points of the horizon, is called the _Prime Vertical_, or _Circle_ of _East_ and _West_, so that when any object is upon this circle in the Eastern hemisphere, it appears due East; and if it be in the Western hemisphere, it appears due West.
[Sidenote: _Amplitude._]
That degree in the horizon wherein any object rises or sets from the East or West points, is called the _Amplitude_; which for rising is called _Amplitude Ortive_, and _Occasive_ for setting; which must be also denominated whether it be Northerly or Southerly.
It may be observed, that the _Amplitude_ and _Azimuth_ are much the same; the amplitude shewing the bearing of any object when he rises or sets, from the East or West points of the horizon; and the azimuth, the bearing of any object when it is above the horizon, either from the North or South point thereof. As for example, if an object rises or sets within 10 degrees of the East or West, suppose towards the South, we accordingly say, its _Amplitude_ is 10 degrees Southerly; but if an object, that is of any height above the horizon, should be in the vertical circle, passing thro’ the before-mentioned point, we then say, its _Azimuth_ is 80 degrees from the South, or 100 degrees from the North, both which expressions signify the same.
[Sidenote: _Almacanthers._]
[Sidenote: _Altitudes._]
[Sidenote: _Meridian Altitude._]
[Sidenote: _Zenith Distance._]
14. All circles drawn parallel to the horizon, in the upper hemisphere, are called _Almacanthers_, or _Parallels of Altitude_: So that the _Altitude_ of any point in the Heavens is an arch of the vertical circle passing thro’ that point, and intercepted betwixt it and the horizon; and if the object be upon the meridian, it is commonly called the _Meridian Altitude_. The complement of the altitude, or what it wants of 90 degrees, is called the _Zenith Distance_.
The horizon (by which we mean the rational) is represented by the upper surface of the Wooden frame, wherein the globes are placed; upon this horizon are described several concentric circles, the innermost of which is divided into degrees, which ought to be numbered both ways from the East and the West, until they end at 90 degrees in the North and South points. The use of these divisions is to shew the amplitudes of the Sun and Stars, at their rising and setting: Also in some convenient place upon this horizon, there is commonly noted the points of the Compass. Without the before-mentioned circle there is drawn the ecliptic with its divisions, into signs, and degrees, and a circle of months and days: The use of these two circles is to serve as a kalendar to shew the Sun’s place at any time of the year, and by that means to find his place in the _Ecliptic_, drawn upon the globe itself.
The _Vertical Circles_, and the _Parallels of Altitude_, are supplied by a thin plate of brass, having a nut and screw at one end to fasten it to the brass meridian in the Zenith point; which being done, the lower end of it may be put between the globe it self, and the inner edge of the horizon, and so turned round about to any point required.
[Sidenote: _Quadrant of Altitude._]
The fiducial edge thereof representing the _Vertical Circles_, and the _Degrees_ upon it, describing the Parallels of Altitude. This thin plate is called the _Quadrant of Altitude_.
The center of the horizon being the place of observation, it is evident that this circle, and all the others belonging to it, are continually changed, which way soever we move; wherefore we may suppose the horizon, with its secundaries and parallels, to invest the globe like a _rete_ or net; and to be moveable every way round it. This is very naturally illustrated by the globes; if we move directly North, or directly South, the change made in the horizon, is represented by moving the brass meridian (keeping the globe from turning about its axis) in the notches made in the wooden horizon, just so much as we travelled. If our course should be due East, or due West, the alterations made thereby are represented by turning the globe accordingly about its axis, the brass meridian being kept fixed; and if we steer betwixt the meridian and the East or West points, then we are to turn the brass meridian, and also the globe about its axis accordingly; the sum of which is, let the spectator be at what point soever of the Earth’s surface, he’ll there gravitate, or tend exactly towards its center, and imagine himself to be on the highest part thereof, (the unevenness of the ground not being here considered) wherefore if we turn the globe in such a manner as to bring the several progressive steps of a traveller successively to the Zenith, we shall then have the successive alterations made in the horizon, in every part of his journey. This explication being well considered, will be of help to young beginners, to conceive how the Earth is every where habitable; and how passengers can travel quite round it; for since every thing tends toward the center of the Earth, we are to conceive that point as being the lowest, and not to carry our idea of downwards any farther. Those that are diametrically opposite to us being as much upon the upper part of the Earth as we are, there being no such thing in nature as one place being higher than another, but as it is at a greater distance from the center of the Earth, let it be in what country soever.
We have now done with all the circles of the sphere, and it may be observed, that the _Equinoctial_, the _Ecliptic_, and the _Horizon_, with their Secundaries and parallels, are all alike; and altering their position, may be made to serve for one another. Thus, if the _Poles of the World_ be brought into the _Zenith_ and _Nadir_, the _Equinoctial_ will coincide with the _Horizon_, the _Meridians_ will be the same with the _Vertical Circles_, and the parallels of _Declination_ will be the parallels of _Altitude_. After the same manner, if shifting the position, we bring the _Ecliptic_ to coincide with the _Horizon_, the circles of _Longitude_ will be the _Vertical Circles_, and the parallels of _Latitude_ and _Altitude_ will coincide.
The horizon and the equator may be either parallel, perpendicular, or oblique to each other.
[Sidenote: _Parallel Sphere._]
15. A _Parallel Sphere_ is that position where the equator coincides with the horizon, and consequently the poles of the world are in the Zenith and Nadir: The inhabitants of this sphere (if there be any) are those who live under the poles of the world.
[Sidenote: _Right Sphere._]
16. A _Right_ or _Direct Sphere_ is that position where the equator is perpendicular to the horizon, the inhabitants whereof are those who live under the equinoctial.
[Sidenote: _Oblique Sphere._]
17. An _Oblique Sphere_ is when the equinoctial and the horizon make oblique angles with each other, which every where happens but under the equator and the poles.
[Sidenote: _Diurnal_ and _Nocturnal Arch_.]
The arch of any parallel or declination, which stands above the horizon is called the _Diurnal Arch_; and the remaining part of it, which is below the horizon, is called the _Nocturnal Arch_.
That point of the equinoctial which comes to the (Eastern/Western) part of the horizon with any point of the Heavens, is called the (Ascension/Descension) of that point, counted from the beginning of ♈; and if it be in a right sphere, the ascension or descension is called right; but if it be an oblique sphere it is called an oblique ascension or descension. So that,
[Sidenote: _Right Ascension._]
18. The _Right Ascension_ of the _Sun_, _Moon_, or any _Star, &c._ is an arch of the equator contained betwixt the beginning of ♈, and that point of the equinoctial which rises with them in a _Right Sphere_, or which comes to the meridian with them in an oblique sphere.
[Sidenote: _Oblique Ascension._]
19. _Oblique Ascension_, or _Descension_, is an arch of the equinoctial intercepted between the beginning of ♈, and that _Point_ of the _Equator_ which rises or sets with any point in the Heavens in an oblique sphere.
[Sidenote: _Ascensional Difference._]
20. _Ascensional Difference_, is the difference betwixt the right and oblique ascension or descension, and shews how long the Sun rises or sets before or after the hour of six.
IV. _Of the Division of Time._
The parts that time is distinguished into, are _Days_, _Hours_, _Weeks_, _Months_, and _Years_.
A Day is either natural or artificial.
[Sidenote: _Natural_ and _Artificial Day_.]
A _Natural Day_ is the space of time elapsed while the Sun goes from any meridian or horary circle, ’till he arrives to the same again; or, it is the time contained from noon, or any particular hour, to the next noon, or the same hour again: An _Artificial Day_ is the time betwixt the Sun’s rising and setting; to which is opposed the _Night_, that is, the time the Sun is hid under the horizon.
[Sidenote: _Hours_, &c.]
[Sidenote: _Equinoxes._]
[Sidenote: _Vernal_ and _Autumnal Equinox_.]
[Sidenote: _Solstices._]
[Sidenote: _Summer_ and _Winter Solstices_.]
The _Natural Day_ is divided into 24 _Hours_, each hour into 60 _Minutes_, each minute into 60 _Seconds, &c._ The _Artificial Days_ are always unequal to all the inhabitants that are not under the equator, except when the Sun is in the equinoctial points ♈ and ♋, which happens (according to our way of reckoning) about the 21st of _March_, and the 23d of _September_; at those times the Sun rises at six and sets at six to all the inhabitants of the Earth. These days are called the _Equinoxes_, or _Equinoctial Days_; the first of which, or when the Sun is in the first point of _Aries_, is called the _Vernal Equinox_, and the latter is called the _Autumnal Equinox_. In all places where the Sun descends below the horizon, excepting under the equator, the days continually lengthen or shorten, and that faster or slower, according as the Sun is nearer to, or further from the equinoctial, until he arrives to either of the _Solstitial Points_ ♋ or ♑. At those times the Sun seems to stand still for a few days, and then begins to return with a slow motion towards the equinoctial, still hastening his pace as he comes nearer to it: The Sun enters the tropics of ♋ and ♑, about the 21st of _June_, and the 22d of _December_, which days are sometimes called the _Solstices_; the first of which we call the _Summer Solstice_, and the latter the _Winter Solstice_.
[Sidenote: The different beginning of the day.]
All nations do not begin their day, and reckon their hours alike. In _Great-Britain_. _France_, and _Spain_, and in most places in _Europe_, the days is reckoned to begin at midnight, from whence is counted twelve hours ’till noon, then twelve hours more ’till next midnight, which makes a compleat day; yet the _Astronomers_ (in these countries) commonly begin their day at noon, and so reckon 24 hours ’till next noon, and not twice twelve, according to the vulgar computation.
[Sidenote: _Babylonish Hours._]
[Sidenote: _Italian Hours._]