TheDESCRIPTIONandUSEof theCelestialandTerrestrial Globes.
GlobeorSphere.
AGlobeorSphereis a round solid body, having every part of its surface equally distant from a point within it, called itsCenter; and it may be conceived to be formed by the revolution of a semicircle round its diameter.
Great Circle.
Hemispheres.
Any circle passing through the center of the sphere, thereby dividing into two equal parts or segments, is called aGreat Circle; and the segments of the sphere so divided, are calledHemispheres.
Every great circle has its Poles and Axis.
Poles.
ThePolesof a great circle are two points on the surface of thesphere, diametrically opposite to one another, and every where equally distant from the said circle.
Axis.
TheAxisof a circle is a right line passing through the center of the sphere, and through the Poles of the said circle, and is therefore perpendicular to the Plane: Therefore
Secundaries.
All circles passing through the Poles of any great circle, intersect it in two places diametrically opposite, and also at right angles; and with respect to the said great circle, they may be called itsSecundaries.
Parallelorlesser Circles.
All circles dividing the sphere into two unequal parts, are calledlesserorparallel Circles, and are usually denominated by that great circle to which they are parallel.
Terrestrial Globe.
The Earth being globular, its outward parts, as the severalCountries,Seas,&c.are best, and most naturally represented upon the surfaces of a Globe; and when such a body has the outward parts of the Earth and Sea delineated upon its surface, and placed in their natural order and situation, it is called aTerrestrial Globe.
Celestial Globe.
The Celestial Bodies appear to us as if they were all placed in the same concave sphere, therefore astronomers place the Stars according to their respective situations and magnitudes, and also the images of the constellations, upon the external surface of a Globe; for it answers the same purposes as if they were placed within a concave sphere, if we suppose the Globe to be transparent, and the eye placed in the center. A Globe having the Stars placed upon its surface, as above described, is called aCelestial Globe. These Globes are both placed in frames, with other appurtenances, as shall be described in a proper place.
The principal use of the Globes.
The principal uses of the Globes (besides their serving asMaps, to distinguish the outward parts of the Earth, and the situations of the fixed Stars) is to explain and resolve the phænomena arising from the diurnal motion of the Earth round its Axis.
There will be the same prospect of the fixed Stars whether the spectator be placed on the Earth, or in the Sun.
It has been shewed in the Introduction, that the distance of the Earth from the Sun, is no more than a point, when compared with the immensedistance of the fixed Stars; therefore let the Earth be in what point soever of her orbit, there will be the same prospect of the Heavens, as a spectator would observe did he reside in the Sun: And if several circles be imagined to pass thro’ the center of the Earth, and others, parallel to them, be conceived to pass thro’ the center of the Sun, these circles in the Heavens will seem to coincide, and to pass exactly thro’ the same Stars. Wherefore as to the appearances of the fixed Stars, it is indifferent whether the Earth or the Sun be made the center of the Universe. But because it is from the Earth that we always observe the celestial bodies, and their apparent motions seem to us to be really made in the Heavens, it is more natural in explaining the phænomena arising from these motions, to place the Earth in the center. And again, because the semidiameter of the Earth, when compared to her distance from the Sun, is of no sensible magnitude, any point, upon the Earth’s surface, let her be in what part soever of the orbit, may be considered as being the center of the Universe. Upon these principles, the different phænomena arising from the diurnal motion of the Earth,and the different situation of a spectator upon its surface, are very naturally illustrated and explained by the Globes.
As to the alterations of seasons,&c.arising from the annual motion of the Earth round the Sun, it is indifferent which we suppose to move, the Earth or the Sun, for in both cases the effect will be the same. Wherefore because it is the Sun that appears to us to move, we say the Sun is in such a part of the ecliptic, without attributing any motion to the Earth, any more than if she had actually been at rest. For the same reason we say the Sun rises, or the Sun sets; by which we mean that he begins to appear or disappear, without considering in the least how these effects are produced. These things are here mentioned, to obviate the objections that might be made by beginners, after they have been told that the Sun stands still.
An Explanation of the Circles of the Sphere, and of some Astronomical Terms arising therefrom.
TheCircles of the Sphere.
Inorder to determine the relative situations of places upon the Earth, as well as the positions of the fixed Stars, and other Celestial phænomena, the Globe of the Earth is supposed to be environed by several imaginary circles, and these are called theCircles of the Sphere. These imaginary circles are either fixed, and always obtain the same position in the Heavens, or moveable, according to the position of the observer.
Those circles that are fixed, owe their origin to the two-fold motion of the Earth, and are theEquator, and theEcliptic, with theirSecundariesandParallels.These fixed circles are usually delineated upon the surface of the Globes.
The moveable circles are only theHorizon, itsSecundariesandParallels: These are represented by the wooden frame, and the brass ring, wherein the Globe is hung, and a thin plate of brass to be screwed in a proper place, upon the said ring, as occasion requires.
TheEquator, orEquinoctial.
1. TheEquator, or theEquinoctial, is that great circle in the Heavens, in whose plane the Earth performs her diurnal motion round her axis; or it is that great circle, parallel to which the whole Heavens seem to turn round the Earth from East to West in 24 Hours.
Note, The Equator and the Equinoctial are generally synonymous terms; but sometimes the Equator particularly signifies that great circle upon the surface of the Earth, which coincides with the Equinoctial in the Heavens. This circle is also by Mariners commonly called theLine.
NorthernandSouthern Hemispheres.
The Axis of the World.
Poles of the World, orof the Equator.
The equinoctial divides the globe of the Earth, and also the whole Heavens into two equal parts, North and South, which are called theNorthernandSouthern Hemispheres. The axis of this circle, is called theAxis of the World, or theEarth’s Axis, because the Earth revolves about it (from West to East) in 24 hours. The extreme of this axis are called thePoles of the World, whereof that which lies in the Northern Hemisphere, is called theNorth Pole, and the other is called theSouth Pole. The equinoctial circle is always delineated upon the surface of each globe, with its name at length expressed; the axis of this circle, or the Earth’s axis, is only an imaginary line in the Heavens, but on the globes it is expressed by the wires about which they really turn. The Poles of the world, are the two points upon the surface of the globe through which these wires pass; the North Pole is that which hath the little brass circle, with a moveable index placed round it; and the other opposite to it is the South Pole. The Northern Hemisphere is that wherein the North Pole is placed, and the opposite one is the Southern Hemisphere.
The astronomers divide all circles into 360 equal parts, calledDegrees, each degree into 60 equal parts, calledMinutes, each minute into 60Seconds, &c. But besides this division into degrees, the equinoctial is also divided into 24 equal parts, orHours, each hour into 60Minutes, each minute into 60Seconds, &c.so that one hour is equal to 15 degrees, each minute of time is equal to 15 minutes of a degree,&c.
Hour CirclesorCircles of Ascension, also calledMeridians.
2. All circles conceived to pass through the Poles of the world, intersecting the equinoctial at right angles, are, with respect to any point in the Heavens, calledHour Circles; and theCircles of Ascension, because the ascension of the Heavenly bodies, from a certain point, are by them determined.
These circles are also, with regard to places upon Earth, calledMeridians.
TheBrass Meridian.
TheMeridiansare commonly drawn upon the Terrestrial Globe thro’ every 15 degrees of the equinoctial, thereby making an Hour difference betwixt the places through which they pass. On the Celestial Globe there are commonly drawn but two of theseMeridians, crossing theequinoctial in four points equidistant from one another, thereby dividing it into four quadrants; but the intermediate ones are here supplied, and also upon the Terrestrial Globe, by the brass circle on which they are hung, which, is therefore called theBrass Meridian, and sometimes only theMeridian, it serving for this purpose to all the points upon either Globe.
TheHour Circle.
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 theHour Circle.
Parallels of Declination.
3. All circles parallel to the equinoctial are, with respect to any point in the Heavens, calledParallels of Declination. So that,
Declination NorthandSouth.
4. TheDeclination of any Pointin the Heavens (as of theSun, afixed Star, or the like) is an arch of the meridian passing through that point, and intercepted betwixt it and the equator; and if the saidpoint be to the (Northward/Southward) of the equator, it is called (North/South)Declination.
TropicsandPolar Circles.
Of the parallels of declination, four are eminently distinguished by particular names,viz.The twoTropics, and the twoPolar Circles.
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 theTropic of Cancer, and the Southern one, theTropic 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.
Arctic Circle.Arctic Pole.Antarctic Circle.Antarctic Pole.
6. ThePolar Circlesare 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 theArctic Circle, fromArctos, a constellation situatedin the Heavens near that Place; whence also this Pole is sometimes called theArctic Pole. The other Polar circle, which is situated near the South Pole; is called theAntarctic Circle, because its position is contrary to the other; and the South Pole is sometimes called theAntarctic Pole.
The tropics and the Polar circles have each their names expressed upon the Globes.
Ecliptic.Equinoctial.Solstitial Points.Colures.Equinoctial Colure.Solstitial Colure.
7. TheEclipticis 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 theEquinoctial Points; and the two points in the ecliptic that are at the greatest distance from the equinoctial points, are called theSolstitial Points. The two meridians passing through those points, are, by way of eminence, calledColures; whereof that which passeth thro’ the equinoctial points, is called theEquinoctial Colure; and that which is at right angles toit, passing through the Solstitial Points, is called theSolstitial Colure.
The Ecliptic divided into signs.
The ecliptic is divided into 12 equal parts, calledSigns, 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.
Northern Signs.
The first six of these are called theNorthern 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 ♍.
Southern Signs.
The latter six are called theSouthern 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 theSign of Aries, is not the same with theConstellationAries; the latter is a system of Stars digested into the figure of aRam, but the sign ofAriesis only 30 degrees of the ecliptic, counted from the equinoctial point ♈, (which is reckoned the first point in the ecliptic) to the beginning ofTaurus: 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 ofAries, 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 constellationArieshas now got into the sign ♉, and so of the rest. So thatPiscesis now got into the Sign of ♈; this slow motion in the Heavens is called thePrecession of the Equinoctial Points.
Poles of the Ecliptic.
ThePoles of the Eclipticare 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 calledPolar.
Circles of Longitude.
8. All great circles passing through the Poles of the ecliptic, and consequently intersecting it at right angles, are calledCircles of Longitude: So that,
Longitude of any Point in the Heavens.
Place of a Star.
9. TheLongitudeof anyPointin the Heavens (as aStarorPlanet,&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 thePlaceof that Star or Planet.
Note, TheSunnever goes out of the ecliptic, and it is not usual to say the Sun’s longitude, but we commonly express it theSun’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 calledParallels of Latitude: So that,
Latitude of a Star, &c.
11. TheLatitudeof 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 theTerrestrial Globe, none of the circles of longitude are described; and upon theCelestial, they are commonly drawn thro’ the beginning of everySign; 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. Theparallels 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.
Horizon.
12. TheHorizonis 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, theSensible, and theRational.
Sensible Horizon.
TheSensible, orApparent 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 onTerra 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 theeye 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.
Rational Horizon.
TheRational, or trueHorizon, 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.
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 theCardinal Points, and are termed theEast,West,North, andSouth. TheEastis that point of the Horizonwhere 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 theWestis that point of the Horizon which is directly opposite to the East, or where the Sun Sets when he is in the Equinoctial. TheSouthis 90 degrees distant from the East and West, and is toward that part of the Heavens wherein the Sun always appears to us inGreat-Britainat Noon; and theNorthis 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.
Points of the Compass.
Besides the aforementioned divisions of the Horizon into degrees.Marinersdivide it into 32 equal parts, which they call thePoints 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.
Zenith.
Nadir.
The center of the Horizon is the place of observation, and the Poles of it are one exactly over our heads, called theZenith; and the other exactly under our feet, called theNadir.
Vertical Circles.
Meridian.
Azimuth.
13. All circles conceived to pass thro’ the Zenith and Nadir, are calledVertical Circles, orAzimuths. Of these circles, that which passeth thro’ the North and South points of the Horizon, is called theMeridian; so that when any object is upon the Meridian, it then bears either due South, or due North from us; and theAzimuthof 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 theEasternandWestern Hemispheres, and the Poles of it are theEastandWestpoints of theHorizon. All the heavenly objects are, during one half of their continuance above the horizon, in the EasternHemisphere, and for the other half in the Western; so that whenever the Sun arrives upon the upper part of the meridian, it is thenNoon, orMid-day, which is the reason why this circle is called the meridian; and when he comes to the lower part, it is thenMidnight.
Prime Vertical.
The vertical circle passing thro’ the East and West points of the horizon, is called thePrime Vertical, orCircleofEastandWest, 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.
Amplitude.
That degree in the horizon wherein any object rises or sets from the East or West points, is called theAmplitude; which for rising is calledAmplitude Ortive, andOccasivefor setting; which must be also denominated whether it be Northerly or Southerly.
It may be observed, that theAmplitudeandAzimuthare 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, thebearing 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, itsAmplitudeis 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, itsAzimuthis 80 degrees from the South, or 100 degrees from the North, both which expressions signify the same.
Almacanthers.Altitudes.Meridian Altitude.Zenith Distance.
14. All circles drawn parallel to the horizon, in the upper hemisphere, are calledAlmacanthers, orParallels of Altitude: So that theAltitudeof 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 theMeridian Altitude. The complement of the altitude, or what it wants of 90 degrees, is called theZenith Distance.
The horizon (by which we mean the rational) is represented by the uppersurface 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 theEcliptic, drawn upon the globe itself.
TheVertical Circles, and theParallels 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.
Quadrant of Altitude.
The fiducial edge thereof representing theVertical Circles, and theDegreesupon it, describing the Parallels of Altitude. This thin plate is called theQuadrant 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 areteor 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 axisaccordingly; 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 beobserved, that theEquinoctial, theEcliptic, and theHorizon, with their Secundaries and parallels, are all alike; and altering their position, may be made to serve for one another. Thus, if thePoles of the Worldbe brought into theZenithandNadir, theEquinoctialwill coincide with theHorizon, theMeridianswill be the same with theVertical Circles, and the parallels ofDeclinationwill be the parallels ofAltitude. After the same manner, if shifting the position, we bring theEclipticto coincide with theHorizon, the circles ofLongitudewill be theVertical Circles, and the parallels ofLatitudeandAltitudewill coincide.
The horizon and the equator may be either parallel, perpendicular, or oblique to each other.
Parallel Sphere.
15. AParallel Sphereis 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.
Right Sphere.
16. ARightorDirect Sphereis that position where the equator isperpendicular to the horizon, the inhabitants whereof are those who live under the equinoctial.
Oblique Sphere.
17. AnOblique Sphereis when the equinoctial and the horizon make oblique angles with each other, which every where happens but under the equator and the poles.
DiurnalandNocturnal Arch.
The arch of any parallel or declination, which stands above the horizon is called theDiurnal Arch; and the remaining part of it, which is below the horizon, is called theNocturnal 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,
Right Ascension.
18. TheRight Ascensionof theSun,Moon, or anyStar, &c.isan arch of the equator contained betwixt the beginning of ♈, and that point of the equinoctial which rises with them in aRight Sphere, or which comes to the meridian with them in an oblique sphere.
Oblique Ascension.
19.Oblique Ascension, orDescension, is an arch of the equinoctial intercepted between the beginning of ♈, and thatPointof theEquatorwhich rises or sets with any point in the Heavens in an oblique sphere.
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.
The parts that time is distinguished into, areDays,Hours,Weeks,Months, andYears.
A Day is either natural or artificial.
NaturalandArtificial Day.
ANatural Dayis 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: AnArtificial Dayis the time betwixt the Sun’s rising and setting; to which is opposed theNight, that is, the time the Sun is hid under the horizon.
Hours, &c.
Equinoxes.
VernalandAutumnal Equinox.
Solstices.
SummerandWinter Solstices.
TheNatural Dayis divided into 24Hours, each hour into 60Minutes, each minute into 60Seconds, &c.TheArtificial Daysare 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 ofMarch, and the 23d ofSeptember; at those times the Sun rises at six and sets at six to all the inhabitants of the Earth. These days are called theEquinoxes, orEquinoctial Days; the first of which, or when the Sun is in the first point ofAries, is called theVernal Equinox, and the latter is called theAutumnal 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 theSolstitial Points♋ or ♑. At those timesthe 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 ofJune, and the 22d ofDecember, which days are sometimes called theSolstices; the first of which we call theSummer Solstice, and the latter theWinter Solstice.
The different beginning of the day.
All nations do not begin their day, and reckon their hours alike. InGreat-Britain.France, andSpain, and in most places inEurope, 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 theAstronomers(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.
Babylonish Hours.
Italian Hours.
TheBabyloniansbegan their day at Sun-rising, and reckoned 24 hours ’till he rose again! This way of computation we call theBabylonish Hours. In several parts ofGermanythey count their hours fromSun-setting, calling the first hour after the Sun has set, the first hour,&c.’till he sets the next day, which they call the 24th hour: These are commonly called theItalian Hours. According to both these ways of computation, their hours are commonly either a little greater or less than the ¹/₂₄ part of a natural day, in proportion as the Sun rises or sets sooner or later in the succeeding days. They have also this inconvenience, that their mid-day and midnight happen on different hours, according to the seasons of the year.
Jewish Hours.
Planetary Hours.
TheJewsand theRomansformerly divided the artificial days and nights each into 12 equal parts; these are termed theJewish Hours, and are of different lengths, according to the seasons of the year; aJewish Hourin summer being longer than one in winter, and a night-hour shorter. This method of computation is now in use among theTurks, and the hours are stiled thefirst hour,second hour, &c.of the day or night; so thatMid-dayalways falls on the sixth hour of the day. These hours are also calledPlanetary Hours, because in every hour one of the seven Planets were suppose to preside over theWorld, and so take it by turns. The first hour after Sun-rising onSundaywas allotted to theSun; the next toVenus, the third toMercury; and the rest in order to theMoon,Saturn,Jupiter, andMars. By this means on the first hour of the next day, the Moon presided, and so gave the name to that day; and so seven days by this method had names given them from the Planets that were supposed to govern on the first hour.
A Week.
AWeekis a system of seven days, in which each day is distinguished by a different name. In most countries these days are called after the names of the seven Planets, as above noted. All nations that have any notion of religion, lay apart one day in seven for public worship; the day solemnized byChristiansisSunday, or the first day of the week, being that on which our saviour rose from the grave, on which the apostles afterwards used more particularly to assemble together to perform divine worship. TheJewsobservedSaturday, or the seventh day of the week, for their sabbath, or day of rest, being that appointed in the fourth commandment under the Law. TheTurksperform their religious ceremonies onFriday.
A Month.
PeriodicalandSynodical Month.
AMonthis properly a certain space of time measured by the Moon in his course round the Earth. ALunar Monthis eitherPeriodicalorSynodical. APeriodical Monthis that space of time the Moon takes to perform her course from one point in the ecliptic ’till she arrives to the same again, which is 27 days, and some odd hours; and aSynodical Monthis the time betwixt one new Moon, and the next new Moon, which is commonly about 29½ days. But aCivil Month, is different from these, and consists of a certain number of days, fewer or more, according to the laws and customs of the country where they are observed.
AYear SyderealandTropical.
The compleatest period of time is aYear, in which all the variety of seasons return, and afterwards begin anew. AYearis eitherAstronomicalorCivil. AnAstronomical Yearis either aSyderealwherein the Sun departing from a fixed Star, returns to it again; orTropical, which is the space of time the Sun takes to perform his course from any point of the ecliptic, ’till he returns to it again.
ATropical Yearconsists of 365 days, 5 hours, and 49 minutes; this is the time in which all the seasons compleatly returns, which is a small matter less than a Sydereal Year.