SECT. I.
An Explanation of the Circles of the Sphere, and of some Astronomical Terms arising therefrom.
The Circles of the Sphere.
In order 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 the Circles 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 the Equator, and the Ecliptic, with their Secundaries and Parallels. These fixed circles are usually delineated upon the surface of the Globes.
The moveable circles are only the Horizon, its Secundaries and Parallels: 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.
I. Of the Equinoctial.
The Equator, or Equinoctial.
1. The Equator, or the Equinoctial, 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 the Line.
Northern and Southern Hemispheres.
The Axis of the World.
Poles of the World, or of 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 the Northern and Southern Hemispheres. The axis of this circle, is called the Axis of the World, or the Earth’s Axis, because the Earth revolves about it (from West to East) in 24 hours. The extreme of this axis are called the Poles of the World, whereof that which lies in the Northern Hemisphere, is called the North Pole, and the other is called the South 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, called Degrees, each degree into 60 equal parts, called Minutes, each minute into 60 Seconds, &c. But besides this division into degrees, the equinoctial is also divided into 24 equal parts, or Hours, each hour into 60 Minutes, each minute into 60 Seconds, &c. so that one hour is equal to 15 degrees, each minute of time is equal to 15 minutes of a degree, &c.
Hour Circles or Circles of Ascension, also called Meridians.
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, called Hour Circles; and the Circles 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, called Meridians.
The Brass Meridian.
The Meridians are 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 these Meridians, crossing the equinoctial 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 the Brass Meridian, and sometimes only the Meridian, it serving for this purpose to all the points upon either Globe.
The Hour 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 the Hour Circle.
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,
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.
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.
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.
Arctic Circle.
Arctic Pole.
Antarctic Circle.
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.
Ecliptic.
Equinoctial.
Solstitial Points.
Colures.
Equinoctial Colure.
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.
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, | Pisces. |
| 7. ♎ | 8. ♏ | 9. ♐ | 10. ♑ | 11. ♒ | 12. ♓ |
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 ♍.
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.
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.
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,
Longitude of any Point in the Heavens.
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,
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.
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.
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.
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.
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.
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.
Zenith.
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.
Vertical Circles.
Meridian.
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.
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.
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.
Almacanthers.
Altitudes.
Meridian Altitude.
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.
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.
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.
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.
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.
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,
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.
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.
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.
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.
Hours, &c.
Equinoxes.
Vernal and Autumnal Equinox.
Solstices.
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.
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.
Babylonish Hours.
Italian Hours.
The Babylonians began their day at Sun-rising, and reckoned 24 hours ’till he rose again! This way of computation we call the Babylonish Hours. In several parts of Germany they count their hours from Sun-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 the Italian 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.
The Jews and the Romans formerly divided the artificial days and nights each into 12 equal parts; these are termed the Jewish Hours, and are of different lengths, according to the seasons of the year; a Jewish Hour in summer being longer than one in winter, and a night-hour shorter. This method of computation is now in use among the Turks, and the hours are stiled the first hour, second hour, &c. of the day or night; so that Mid-day always falls on the sixth hour of the day. These hours are also called Planetary Hours, because in every hour one of the seven Planets were suppose to preside over the World, and so take it by turns. The first hour after Sun-rising on Sunday was allotted to the Sun; the next to Venus, the third to Mercury; and the rest in order to the Moon, Saturn, Jupiter, and Mars. 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.
A Week is 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 by Christians is Sunday, 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. The Jews observed Saturday, or the seventh day of the week, for their sabbath, or day of rest, being that appointed in the fourth commandment under the Law. The Turks perform their religious ceremonies on Friday.
A Month.
Periodical and Synodical Month.
A Month is properly a certain space of time measured by the Moon in his course round the Earth. A Lunar Month is either Periodical or Synodical. A Periodical Month is 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 a Synodical Month is the time betwixt one new Moon, and the next new Moon, which is commonly about 29½ days. But a Civil 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.
A Year Sydereal and Tropical.
The compleatest period of time is a Year, in which all the variety of seasons return, and afterwards begin anew. A Year is either Astronomical or Civil. An Astronomical Year is either a Sydereal wherein the Sun departing from a fixed Star, returns to it again; or Tropical, 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.
A Tropical Year consists 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.
Egyptian Year.
The Civil Year is the same with the Political established with the laws of a country; and is either moveable or immoveable. The moveable year consists of 365 days, being less than the tropical year by almost six hours, and is called the Egyptian Year, because observed in that Country.
The Romans divided the year into 12 kalendar months, to which they gave particular names, and are still retained by most of the European nations, viz. January, February, March, April, May, June, July, August, September, October, November, and December. The number of days in each month may be known by the following verses:
Thirty Days hath September, April, June, and November; February hath Twenty-eight alone, And all the rest have Thirty-one.
The year is also divided into four quarters or seasons, viz. Spring, Summer, Autumn, and Winter. These quarters are properly made when the Sun enters into the equinoctial and solstitial points of the ecliptic; but in civil uses they are differently reckoned, according to the customs of several countries. In England, we commonly reckon the first day of January to be the first in the year, which is therefore vulgarly called New-Year’s-Day; but in political and ecclesiastical affairs, the year is reckoned to commence on Lady-day which is the 25th of March; and from thence to Midsummer-day, which is the 24th of June, is reckoned the first quarter; from Midsummer-day to Michaelmas-day, which is the 29th of September, is the second quarter; the third quarter is reckoned from Michaelmas-day to Christmas-day, which is the 25th of December; and from Christmas-day to Lady-day, is reckoned the last quarter in the year. In common affairs, a quarter is reckoned from a certain day to the same in the fourth month following. Sometimes a month is reckoned four weeks, or 28 days, and so a quarter 12 weeks. To all the inhabitants in the (Northern/Southern) Hemisphere, their Midsummer is properly when the Sun is in the tropic of (Cancer,/Capricorn,) and their Midwinter at the opposite time of the year; but those who live under the equinoctial have two winters, &c. when the Sun is in either tropic; tho’ indeed properly, there is no season that may be called winter in those parts of the world.
Bissextile, or Leap-Year.
The Egyptian year of 365 days being less than the true solar year, by almost six hours, it follows, that four such years are less than four solar years by a whole day; and therefore in 365 times four years, that is, in 1460 years, the beginning of the years move through all the seasons. To remedy this inconveniency, Julius Cæsar (considering that the six hours, which remain at the end of every year, will in four years make a natural day) ordered that every fourth year should have an intercalary day, which therefore consists of 366 days; the day added was put in the month of February, by postponing St. Matthias’s day, which in common years fall on the 24th, to the 25th of the said month, all the fixed feasts in the year from thenceforwards falling a week-day later than otherwise they would. According to the Roman way of reckoning, the 24th of February was the sixth of the kalends of March, and it was ordered that for this year there should be two sixths, or that the sixth of the kalends of March should be twice repeated; upon which account the year was called Bissextile, which we now call the Leap-Year.
To find whether the year of our Lord be leap-year, or the first, second, or third after; divide it by four, and the remainder, if there be any, shews how many years it is after leap-year; but if there be no remainder, then that year is leap-year: Or, you may omit the hundreds and scores, and divide the residue by 4, Examp. 1757, omitting the hundreds and the twenties, I divide the residue 17, by 4, and the remainder 1, shews it to be the first after leap-year.
Julian Account or the Old Style.
This method of reckoning the year, viz. making the common year to consist of 365 days, and every fourth year to have 366 days, is now used in Great-Britain and Ireland, and some of the Northern parts of Europe, and is called the Julian Account, or the Old Style. But the time appointed by Julius Cæsar for the length of a solar year is too much; for the Sun finishes his course in the ecliptic, in 365 days, 5 hours, and 49 minutes, which is 11 minutes less than the civil year; and therefore he again begins his circuit 11 minutes before the civil year is ended; and so much being gained every year, amounts in 131 years, to a whole day. So that if the Sun in any year entered the equinox upon the 20th of March at noon, after the space of 131 years, he’ll enter the same point on the same hour, on the 19th of March. And therefore the exquinoxes will not always fall on the same day of the month, but by degrees will move towards the beginning of the year.
Gregorian Account, or New Style.
At the time of the Council of Nice, when the terms were settled for observing of Easter, the Vernal Equinox fell upon the 21st of March; but by its falling backwards 11 minutes every year, it was found that in Anno 1582, when the kalendar was corrected, the Sun entered the equinoctial circle on the 11th of March, having departed ten whole days from its former place in the year: and therefore Pope Gregory the XIIIth, designing to place the equinoxes in their situation with respect to the year, took these ten days out of the kalendar, and ordered that the 11th of March should be reckoned as the twenty-first: And to prevent the seasons of the year from going backwards for the future, he ordered every hundredth year, which in Julian form was to be a Bissextile, should be a common year, and consist only of 365 days; but that being too much, every fourth hundred was to remain Bissextile. This form of reckoning being established by the authority of Pope Gregory XIII. is called the Gregorian Account, or the New Style; and is observed in all the countries where the authority of the Pope is acknowledged, and likewise by several nations of the reformed religion. There being now above an hundred years past, since the reformation was made in the kalendar, the Gregorian account has accordingly got before the Julian one day more than it was in the time of its institution, the difference between these two accounts being now eleven days; so that the first day of any month, according to that way of reckoning, is the 12th of the same month, according to the New Style.
I shall conclude this section with a brief account of the Atmosphere.
Atmosphere.
The Atmosphere is that thin body of air which surrounds the Earth, in which the clouds hover, and by which in their descent they are broke into drops of rain; which sometimes, according to the warmth or coldness of air, are froze into Snow, or Hailstones. Thunder and Lightning are also made in the Atmosphere, and wind is nothing else but a percussion of the air, occasioned by its different density in different places. The benefits we receive from the atmosphere are innumerable; without air no earthly creature could live, as is plainly proved by experiments made by the Air-Pump; and the wholsomeness of a climate chiefly depends upon that of its air: If there was no atmosphere to reflect the rays of the Sun, no part of the heavens would be lucid and bright, but that wherein the Sun was placed; and if a spectator should turn his back towards the Sun, he would immediately perceive it to be quite dark, and the least Stars would be seen shining as they do in the clearest night; and the Sun immediately before his setting would shine as brisk as at noon, but in a moment, as soon as he got below the horizon, the whole hemisphere of the Earth would be involved in as great a darkness as if it were midnight.
But by means of the atmosphere it happens, that while the Sun is above the horizon, the whole face of the heavens is strongly illuminated by its rays, so as to obscure the faint light of the Stars, and render them invisible; and after Sun-setting, though we receive no direct light from him, yet we enjoy its reflected light for some time: For the atmosphere being higher than we are, is a longer time before it is withdrawn from the Sun (as if a man was to run to the top of a steeple, he might see the Sun after it had been set to those at the bottom.) The rays which the atmosphere receives from the Sun, after he is withdrawn from our sight, are by refraction faintly transmitted to us; until the Sun having got about 18 degrees below the horizon, he no longer enlightens our atmosphere, and then all that part thereof which is over us becomes dark. After the same manner in the morning, when the Sun comes within 18 degrees of our horizon, he again begins to enlighten the atmosphere, and so more and more by degrees, until he rises and makes full day.
Twilight, or Crepusculum.
This small illumination of the atmosphere, and the state of the Heavens between day and night, is called the Twilight, or the Crepusculum.
The duration of twilight is different in different climates, and in the same place at different times of the year. The beginning or ending of twilight being accurately given, we may from thence easily find the height of the atmosphere, which is not always the same. The mean height of the atmosphere is computed to be about 40 miles; but it is probable, the air may extend itself a great deal further, there being properly no other limits to it, as we can conceive, but as it continually decreases in density the farther remote it is from the Earth, in a certain ratio; which at last, as to our conception, must in a manner terminate.
