Part 3

The motion of Comets in the Heavens, according to the best observations hitherto made, seem to be regulated by the same immutable law that rules the Planets; for their orbits are elliptical, like those of the Planets, but vastly narrower, or more excentric. Yet they have not all the same direction with the Planets, who move from West to East, for some of the Comets move from East to West; and their orbits have different inclinations to the Earth’s orbit; some inclining Northwardly, others Southwardly, much more than any of the Planetary orbits do.

Altho’ both the Comets and the Planets move in elliptic orbits, yet their motions seem to be vastly different: For the excentricities of the Planet’s orbits are so small, that they differ but little from circles; but the excentricities of the Comets are so very great, that the motions of some of them seem to be almost in right lines, tending directly towards the Sun.

Now, since the orbits of the Comets are so extremely excentric, their motions, when they are in their _Perihelia_, or nearest distance from the sun, must be much swifter than when they are in their _Aphelia_, or farthest distance from him; which is the reason why the Comets make so short a stay in our system; and when they disappear, are so long in returning.

The figures of the Comets are observed to be very different; some of them send forth small beams, like hair, every way round them; others are seen with a long fiery tail, which is always opposite to the Sun. Their magnitudes are also very different, but in what proportion they exceed each other, it is as yet uncertain. Nor is it probable, that their numbers are yet known, for they have not been observed with due care, nor their theories discovered, but of late years. The ancients were divided in their opinions concerning them; some imagined that they were only a kind of _Meteors_ kindled in our atmosphere, and were there again dissipated; others took them to be some ominous prodigies: But modern discoveries prove, that they are Worlds subject to the same laws of motion as the Planets are; and they must be very hard and durable bodies, else they could not bear the vast heat that some of them, when they are in their _Perihelia_, receive from the Sun, without being utterly consumed. The great Comet which appeared in the year 1680, was within ¹/₆ part of the Sun’s diameter from his surface; and therefore its heat must be prodigiously intense beyond imagination. And when it is at its greatest distance from the Sun, the cold must be as rigid.

SECT. II.

_Of the_ FIXED STARS.

[Sidenote: The fixed Stars are at immense distance from us.]

The fixed Stars are those bright and shining bodies, which in a clear night appear to us every where dispersed through the boundless regions of space. They are term’d fix’d, because they are found to keep the same immutable distance one from another in all ages, without having any of the motions observed in the Planets. The fixed Stars are all placed at such immense distances from us, that the best of telescopes represent them no bigger than points, without having any apparent diameters.

[Sidenote: The fixed Stars are luminous bodies like the Sun.]

It is evident from hence, that all the Stars are luminous bodies, and shine with their own proper and native light, else they could not be seen at such a great distance. For the _Satellites_ of _Jupiter_ and _Saturn_, tho’ they appear under considerable angles through good telescopes, yet are altogether invisible to the naked eye.

[Sidenote: The distance from us to the Sun is nothing in comparison of the vast distance of the fixed Stars.]

Although the distance betwixt us and the Sun is vastly large, when compared to the diameter of the Earth, yet it is nothing when compared with the prodigious distance of the fixed Stars; for the whole diameter of the Earth’s annual orbit, appears from the nearest fixed Star no bigger than a point, and the fixed Stars are at least 100,000 times farther from us than we are from the Sun; as may be demonstrated from the observation of those who have endeavoured to find the Parallax of the Earth’s annual Orb, or the angle under which the Earth’s orbit appears from the fixed Stars.

[Sidenote: As to appearance, the Earth may be consider’d as being the center of the Heavens.]

Hence it follows, that tho’ we approach nearer to some fixed Stars at one time of the year than we do at the opposite, and that by the whole length of the diameter of the Earth’s orbit; yet this distance being so small in comparison with the distance of the fixed Stars, their magnitudes or positions cannot thereby be sensibly altered; therefore we may always, without error, suppose ourselves to be in the same center of the Heavens, since we always have the same visible prospect of the Stars without any alteration.

[Sidenote: The fixed Stars are Suns.]

If a spectator was placed as near to any fixed Star, as we are to the Sun, he would there observe a body as big, and every way like, as the Sun appears to us: and our Sun would appear to him no bigger than a fixed Star: and undoubtedly he would reckon the Sun as one of them in numbering the Stars. Wherefore since the Sun differeth nothing from a fixed Star, the fixed Stars may be reckoned so many Suns.

[Sidenote: The fixed Stars are at vast distance from each other.]

It is not reasonable to suppose that all the fixed Stars are placed at the same distance from us; but it is more probable that they are every where interspersed thro’ the vast indefinite space of the universe; and that there may be as great a distance betwixt any two of them, as there is betwixt our Sun and the nearest fixed Star. Hence it follows, why they appear to us of different magnitudes, not because they really are so, but because they are at different distances from us; those that are nearest excelling in brightness and lustre those that are most remote, who give a fainter light, and appear smaller to the eye.

[Sidenote: The distribution of the Stars into 6 classes.]

[Sidenote: Of _Telescopical Stars_.]

The astronomers distribute the Stars into several orders or classes; those that are nearest to us, and appear brightest to the eye, are called Stars of the first magnitude; those that are nearest to them in brightness and lustre, are called Stars of the second magnitude; those of the third class, are stiled Stars of the third magnitude; and so on, until we come to the Stars of the sixth magnitude, which are the smallest that can be discerned by the naked eye. There are infinite numbers of smaller Stars, that can be seen through telescopes; but these are not reduced to any of the six orders, and are only called _Telescopical Stars_. It may be here observed, that tho’ the astronomers have reduced all the Stars that are visible to the naked eye, into some one or other of these classes, yet we are not to conclude from thence that all the Stars answer exactly to some or other of these orders; but there may be in reality as many orders of the Stars, as they are in number, few of them appearing exactly of the same bigness and lustre.

[Sidenote: The Stars digested into constellations]

The ancient astronomers, that they might distinguish the Stars, in regard to their situation and position to each other, divided the whole starry firmament into Several _Asterisms_, or systems of Stars, consisting of those that are near to one another. These _Asterisms_ are called _Constellations_, and are digested into the forms of some animals; as Men, Lyons, Bears, Serpents, _&c._ or to the images of some known things; as, of a Crown, a Harp, a Triangle, _&c._

[Sidenote: _Zodiac._]

The starry firmament was divided by the ancients into 48 images, or constellations; twelve of which they placed in that part of the Heavens wherein are the planes of the Planetary orbits; which part is called the _Zodiac_, because most of the constellations placed therein resemble some living creature. The two regions of the Heavens that are on each side of the _Zodiac_, are called the North and South parts of the Heavens.

[Sidenote: Constellations within the Zodiac.]

The constellations within the _Zodiac_ are, 1. _Aries_, the _Ram_; 2. _Taurus_, the _Bull_; 3. _Gemini_, the _Twins_; 4. _Cancer_, the _Crab_; 5. _Leo_, the _Lion_; 6. _Virgo_, the _Virgin_; 7. _Libra_, the _Balance_; 8. _Scorpio_, the _Scorpion_; 9. _Sagittarius_, the _Archer_; 10. _Capricornus_, the _Goat_; 11. _Aquarius_, the _Water-Bearer_; and, 12. _Pisces_, the _Fishes_.

[Sidenote: Northern constellations.]

The constellations on the North side of the _Zodiac_ are Twenty-one, _viz._ the _Little Bear_; the _Great Bear_; the _Dragon_; _Cepheus_, a king of _Ethiopia_; _Bootes_, the keeper of the _Bear_; the _Northern Crown_; _Hercules_ with his Club, watching the _Dragon_; the _Harp_; the _Swan_; _Cassiopeia_; _Persius_; _Andromeda_; the _Triangle_; _Auriga_; _Pegasus_, or the _Flying Horse_; _Equuleus_; the _Dolphin_; the _Arrow_; the _Eagle_; _Serpentarius_; and the _Serpent_.

[Sidenote: Southern constellations.]

The constellations noted by the ancients on the South side of the _Zodiac_, were fifteen, _viz._ the _Whale_; the river _Eridanus_; the _Hare_; _Orion_; the _Great Dog_; _Little Dog_; the Ship _Argo_; _Hydra_; the _Centaur_; the _Cup_; the _Crow_; the _Wolf_; the _Altar_; the _Southern Crown_; and the _Southern Fish_. To these have been lately added the following, _viz._ The _Phœnix_; the _Crane_; the _Peacock_; the _Indian_; the _Bird of Paradise_; the _Southern Triangle_; the _Fly_; _Cameleon_; the _Flying Fish_; _Toucan_, or the _American Goose_; the _Water Serpent_, and the _Sword Fish_. The ancients placed those particular constellations or figures in the Heavens, either to commemorate the deeds of some great man, or some notable exploit or action; or else took them from the fables of their religion, _&c_. And the modern astronomers do still retain them, to avoid the confusion that would arise by making new ones, when they compare the modern observations with the old ones.

[Sidenote: _Unformed Stars._]

Some of the principal Stars have particular names given them, as _Syrius_, _Arcturus_, _&c._ There are also several Stars that are not reduced into constellations, and these are called _Unformed Stars_.

[Sidenote: The _Galaxy_, or _Milky Way_.]

Besides the Stars visible to the naked eye, there is a very remarkable space in the Heavens, called the _Galaxy_, or _Milky Way_. This is a broad circle of a whitish hue, like milk, going quite round the whole Heavens, and consisting of an infinite number of small Stars, visible thro’ a telescope, tho’ not discernable by the naked eye, by reason of their exceeding faintness; yet with their light they combine to illustrate that part of the Heavens where they are, and to cause that shining whiteness.

The places of the fixed Stars, or their relative situations one from another, have been carefully observed by astronomers, and digested into catalogues. The first among the _Greeks_, who reduced the Stars into a catalogue, was _Hypparchus_, who, from his own observations, and of those who lived before him, inserted 1022 Stars into his catalogue, about 120 years before the Christian _Æra_: This catalogue has been since enlarged and improved by several learned men, to the number of 3000, of which there are a great many telescopical, and not to be discerned by the naked eye; and these are all ranked in the catalogue as the Stars of the seventh magnitude.

It may seem strange to some, that there are no more than this number of Stars visible to the naked eye; for sometimes in a clear night they seem to be innumerable: but this is only a deception of our sight, arising from their vehement sparkling, while we look upon them confusedly, without reducing them into any order; for there can seldom be seen above 1000 Stars in the whole Heavens with the naked eye at the same time; and if we should distinctly view them, we shall not find many but what are inserted upon a good _Celestial_ Globe.

Altho’ the number of Stars that can be discerned by the naked eye are so few, yet it is probable there are many more which are beyond the reach of our optics, for through telescopes they appear in vast multitudes, every where dispersed throughout the whole Heaven; and the better our glasses are, the more of them we still discover. The ingenious Dr. _Hook_ has observed 78 Stars in the _Pleiades_, of which the naked eye is never able to discern above 7; and in _Orion_, which has but 80 Stars in the _British_ catalogue (and some of them telescopical) there has been numbered 2000 Stars.

[Sidenote: An idea of the Universe.]

Those who think that all these glorious bodies were created for no other purpose than to give us a little dim light, must entertain a very slender idea of the Divine Wisdom; for we receive more light from the _Moon_ itself, than from all the _Stars_ put together. And since the _Planets_ are subject to the same laws of motion with our _Earth_, and some of them not only equal, but vastly exceed it in magnitude, it is not unreasonable to suppose, that they are all habitable Worlds. And since the _Fixed Stars_ are no ways behind our _Sun_, either in bigness or lustre, is it not probable, that each of them have a system of _Planetary Worlds_ turning round them, as we do round our Sun? And if we ascend as far as the smallest Star we can see, shall we not then discover innumerable more of these glorious bodies, which now are altogether invisible to us? And so _ad infinitum_, thro’ the boundless space of the universe. What a magnificient idea must this raise in us of the _Divine Being_! Who is every where, and at all times present, displaying his Divine Power, Wisdom and Goodness, amongst all his Creatures!

_The_ DESCRIPTION _and_ USE _of the_ CELESTIAL _and_ TERRESTRIAL GLOBES.

[Sidenote: _Globe_ or _Sphere_.]

A _Globe_ or _Sphere_ is a round solid body, having every part of its surface equally distant from a point within it, called its _Center_; and it may be conceived to be formed by the revolution of a semicircle round its diameter.

[Sidenote: _Great Circle._]

[Sidenote: _Hemispheres._]

Any circle passing through the center of the sphere, thereby dividing into two equal parts or segments, is called a _Great Circle_; and the segments of the sphere so divided, are called _Hemispheres_.

Every great circle has its Poles and Axis.

[Sidenote: _Poles._]

The _Poles_ of a great circle are two points on the surface of the sphere, diametrically opposite to one another, and every where equally distant from the said circle.

[Sidenote: _Axis._]

The _Axis_ of 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

[Sidenote: _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 its _Secundaries_.

[Sidenote: _Parallel_ or _lesser Circles_.]

All circles dividing the sphere into two unequal parts, are called _lesser_ or _parallel Circles_, and are usually denominated by that great circle to which they are parallel.

[Sidenote: _Terrestrial Globe._]

The Earth being globular, its outward parts, as the several _Countries_, _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 a _Terrestrial Globe_.

[Sidenote: _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 a _Celestial Globe_. These Globes are both placed in frames, with other appurtenances, as shall be described in a proper place.

[Sidenote: The principal use of the Globes.]

The principal uses of the Globes (besides their serving as _Maps_, 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.

[Sidenote: 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 immense distance 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.

SECT. I.

_An Explanation of the Circles of the Sphere, and of some Astronomical Terms arising therefrom._

[Sidenote: 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._

[Sidenote: 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_.

[Sidenote: _Northern_ and _Southern Hemispheres_.]

[Sidenote: _The Axis of the World._]

[Sidenote: _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._

[Sidenote: _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_.

[Sidenote: 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.

[Sidenote: The _Hour Circle_.]