Part 1

Transcriber’s Note

This book uses a number of astronomical symbols, including signs of the Zodiac (♈, ♉, ♊, ♋, ♌, ♍, ♎, ♏, ♐, ♑, ♒, ♓), symbols for planets (☿, ♀, ⊕, ♂, ♃, ♄) and for the sun and moon (☉, 🌑︎). If these characters do not display correctly, you may have to use an alternative font, such as Arial Unicode MS or DejaVu.

When italics were used in the original book, the corresponding text has been surrounded by _underscores_. Mixed fractions have been displayed with a hyphen between whole number and fraction for clarity. Superscripted characters are preceded by ^ and when more than one character is superscripted, they are surrounded by {}.

Some corrections have been made to the printed text. These are listed in a second transcriber’s note at the end of the text.

ASTRONOMY

EXPLAINED UPON Sir ISAAC NEWTON’s PRINCIPLES,

AND MADE EASY TO THOSE WHO HAVE NOT STUDIED

MATHEMATICS.

By JAMES FERGUSON.

HEB. XI. 3. _The Worlds were framed by the Word of_ GOD. JOB XXVI. 13. _By his Spirit he hath garnished the Heavens._

THE SECOND EDITION.

_LONDON_:

Printed for, and sold by the AUTHOR, at the GLOBE, opposite _Cecil-Street_ in the _Strand_. MDCCLVII.

TO

THE RIGHT HONOURABLE

_GEORGE_ EARL of MACCLESFIELD,

VISCOUNT _PARKER_ of EWELME in OXFORDSHIRE,

AND

BARON of MACCLESFIELD in CHESHIRE;

PRESIDENT of the ROYAL SOCIETY of _LONDON_,

MEMBER of the ROYAL ACADEMY OF SCIENCES at _PARIS_,

OF THE

IMPERIAL ACADEMY OF SCIENCES at _Petersburg_,

AND ONE OF THE

TRUSTEES of the BRITISH MUSEUM;

DISTINGUISHED

By his GENEROUS ZEAL for promoting every BRANCH of USEFUL KNOWLEDGE;

THIS

TREATISE of ASTRONOMY

IS INSCRIBED,

With the MOST PROFOUND RESPECT,

By HIS LORDSHIP’s

MOST OBLIGED,

And

MOST HUMBLE SERVANT,

_JAMES FERGUSON_.

THE

CONTENTS.

CHAP. I.

_Of Astronomy in general_ Page 1

CHAP. II.

_A brief Description of the_ SOLAR SYSTEM 5

CHAP. III.

_The_ COPERNICAN _or_ SOLAR SYSTEM _demonstrated to be 31 true_

CHAP. IV.

_The Phenomena of the Heavens as seen from different 39 parts of the Earth_

CHAP. V.

_The Phenomena of the Heavens as seen from different 45 parts of the Solar System_

CHAP. VI.

_The_ Ptolemean _System refuted. The Motions and Phases 50 of Mercury and Venus explained_

CHAP. VII.

_The physical Causes of the Motions of the Planets. The 54 Excentricities of their Orbits. The times in which the Action of Gravity would bring them to the Sun._ ARCHIMEDES’S _ideal Problem for moving the Earth. The world not eternal_

CHAP. VIII.

_Of Light. It’s proportional quantities on the 62 different Planets. It’s Refractions in Water and Air. The Atmosphere, it’s Weight and Properties. The Horizontal Moon_

CHAP. IX.

_The Method of finding the Distances of the Sun, Moon 73 and Planets_

CHAP. X.

_The Circles of the Globe described. The different 78 lengths of days and nights, and the vicissitude of Seasons, explained. The explanation of the Phenomena of Saturn’s Ring concluded_

CHAP. XI.

_The Method of finding the Longitude by the Eclipses of 87 Jupiter’s Satellites: The amazing velocity of Light demonstrated by these Eclipses_

CHAP. XII.

_Of Solar and Sidereal Time_ 93

CHAP. XIII.

_Of the Equation of Time_ 97

CHAP. XIV.

_Of the Precession of the Equinoxes_ 108

CHAP. XV.

_The Moon’s Surface mountainous: Her Phases described: 124 Her Path, and the Paths of Jupiter’s Moons delineated: The proportions of the Diameters of their Orbits, and those of Saturn’s Moons to each other; and to the Diameter of the Sun_

CHAP. XVI.

_The Phenomena of the Harvest-Moon explained by a 136 common Globe: The Years in which the Harvest-Moons are least and most beneficial, from 1751 to 1861. The long duration of Moon-light at the Poles in Winter Page_

CHAP. XVII.

_Of the ebbing and flowing of the Sea_ 147

CHAP. XVIII.

_Of Eclipses: Their Number and Period. A large 156 Catalogue of Ancient and Modern Eclipses_

CHAP. XIX.

_The Calculation of New and Full Moons and Eclipses. 189 The geometrical Construction of Solar and Lunar Eclipses. The examination of ancient Eclipses_

CHAP. XX.

_Of the fixed Stars_ 230

CHAP. XXI.

_Of the Division of Time. A perpetual Table of New 248 Moons. The Times of the Birth and Death of_ CHRIST. _A Table of remarkable Æras or Events_

CHAP. XXII.

_A Description of the Astronomical Machinery serving to 260 explain and illustrate the foregoing part of this Treatise_

_ERRATA._

_In the Table facing Page 31, the Sun’s quantity of matter should be 227500. Page 40, l. last, for_ infinite _read_ indefinite. _Page 97, l. 20, for_ this _read_ the next. _Page 164, l. 2 from the bottom, for_ without any acceleration _read_ as above, without any acceleration. _Page 199, l. 16 for_ XIV _read_ XV. _Page 238, l. 16, for_ 40 _read_ 406. _Page 240, l. 15 from the bottom, for_ Tifri _read_ Tisri, _Page 249 l. 13; from the bottom for_ XVII _read_ V.

ASTRONOMY

EXPLAINED UPON

Sir ISAAC NEWTON’s PRINCIPLES.

CHAP. I.

_Of Astronomy in general._

[Sidenote: The general use of Astronomy.]

1. Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, the situation and extent of the countries and kingdoms upon it ascertained, trade and commerce carried on to the remotest parts of the world, and the various products of several countries distributed for the health, comfort, and conveniency of its inhabitants; but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above the low contracted prejudices of the vulgar, and our understandings clearly convinced, and affected with the conviction, of the existence, wisdom, power, goodness, and superintendency of the SUPREME BEING! So that without an hyperbole,

“_An undevout Astronomer is mad_[1].”

2. From this branch of knowledge we also learn by what means or laws the Almighty carries on, and continues the admirable harmony, order, and connexion observable throughout the planetary system; and are led by very powerful arguments to form the pleasing deduction, that minds capable of such deep researches not only derive their origin from that adorable Being, but are also incited to aspire after a more perfect knowledge of his nature, and a stricter conformity to his will.

[Sidenote: The Earth but a point as seen from the Sun.]

3. By Astronomy we discover that the Earth is at so great a distance from the Sun, that if seen from thence it would appear no bigger than a point; although it’s circumference is known to be 25,020 miles. Yet that distance is so small, compared with the distance of the Fixed Stars, that if the Orbit in which the Earth moves round the Sun were solid, and seen from the nearest Star, it would likewise appear no bigger than a point, although it is at least 162 millions of miles in diameter. For the Earth in going round the Sun is 162 millions of miles nearer to some of the Stars at one time of the year than at another; and yet their apparent magnitudes, situations, and distances from one another still remain the same; and a telescope which magnifies above 200 times does not sensibly magnify them: which proves them to be at least 400 thousand times farther from us than we are from the Sun.

[Sidenote: The Stars are Suns.]

4. It is not to be imagined that all the Stars are placed in one concave surface, so as to be equally distant from us; but that they are scattered at immense distances from one another through unlimited space. So that there may be as great a distance between any two neighbouring Stars, as between our Sun and those which are nearest to him. Therefore an Observer, who is nearest any fixed Star, will look upon it alone as a real Sun; and consider the rest as so many shining points, placed at equal distances from him in the Firmament.

[Sidenote: And innumerable.]

5. By the help of telescopes we discover thousands of Stars which are invisible to the naked eye; and the better our glasses are, still the more become visible: so that we can set no limits either to their number or their distances. The celebrated HUYGENS carries his thoughts so far, as to believe it not impossible that there may be Stars at such inconceivable distances, that their light has not yet reached the Earth since it’s creation; although the velocity of light be a million of times greater than the velocity of a cannon bullet, as shall be demonstrated afterwards § 197, 216: and, as Mr. ADDISON very justly observes, this thought is far from being extravagant, when we consider that the Universe is the work of infinite power, prompted by infinite goodness; having an infinite space to exert itself in; so that our imaginations can set no bounds to it.

[Sidenote: Why the Sun appears bigger than the Stars.]

6. The Sun appears very bright and large in comparison of the Fixed Stars, because we keep constantly near the Sun, in comparison of our immense distance from the Stars. For, a spectator, placed as near to any Star as we are to the Sun, would see that Star a body as large and bright as the Sun appears to us: and a spectator, as far distant from the Sun as we are from the Stars, would see the Sun as small as we see a Star, divested of all its circumvolving Planets; and would reckon it one of the Stars in numbering them.

[Sidenote: The Stars are not enlightened by the Sun.]

7. The Stars, being at such immense distances from the Sun, cannot possibly receive from him so strong a light as they seem to have; nor any brightness sufficient to make them visible to us. For the Sun’s rays must be so scattered and dissipated before they reach such remote objects, that they can never be transmitted back to our eyes, so as to render these objects visible by reflection. The Stars therefore shine with their own native and unborrowed lustre, as the Sun does; and since each particular Star, as well as the Sun, is confined to a particular portion of space, ’tis plain that the Stars are of the same nature with the Sun.

[Sidenote: They are probably surrounded by Planets.]

8. It is no ways probable that the Almighty, who always acts with infinite wisdom and does nothing in vain, should create so many glorious Suns, fit for so many important purposes, and place them at such distances from one another, without proper objects near enough to be benefited by their influences. Whoever imagines they were created only to give a faint glimmering light to the inhabitants of this Globe, must have a very superficial knowledge of Astronomy, and a mean opinion of the Divine Wisdom: since, by an infinitely less exertion of creating power, the Deity could have given our Earth much more light by one single additional Moon.

9. Instead then of one Sun and one World only in the Universe, as the unskilful in Astronomy imagine, _that_ Science discovers to us such an inconceivable number of Suns, Systems, and Worlds, dispersed through boundless Space, that if our Sun, with all the Planets, Moons, and Comets belonging to it were annihilated, they would be no more missed out of the Creation than a grain of sand from the sea-shore. The space they possess being comparatively so small, that it would scarce be a sensible blank in the Universe; although Saturn, the outermost of our planets, revolves about the Sun in an Orbit of 4884 millions of miles in circumference, and some of our Comets make excursions upwards of ten thousand millions of miles beyond Saturn’s Orbit; and yet, at that amazing distance, they are incomparably nearer to the Sun than to any of the Stars; as is evident from their keeping clear of the attractive Power of all the Stars, and returning periodically by virtue of the Sun’s attraction.

[Sidenote: The stellar Planets may be habitable.]

10. From what we know of our own System it may be reasonably concluded that all the rest are with equal wisdom contrived, situated, and provided with accommodations for rational inhabitants. Let us therefore take a survey of the System to which we belong; the only one accessible to us; and from thence we shall be the better enabled to judge of the nature and end of the other Systems of the Universe. For although there is almost an infinite variety in all the parts of the Creation which we have opportunities of examining; yet there is a general analogy running through and connecting all the parts into one scheme, one design, one whole!

[Sidenote: As our Solar Planets are.]

11. And then, to an attentive considerer, it will appear highly probable, that the Planets of our System, together with their attendants called Satellites or Moons, are much of the same nature with our Earth, and destined for the like purposes. For, they are solid opaque Globes, capable of supporting animals and vegetables. Some of them are bigger, some less, and some much about the size of our Earth. They all circulate round the Sun, as the Earth does, in a shorter or longer time according to their respective distances from him: and have, where it would not be inconvenient, regular returns of summer and winter, spring and autumn. They have warmer and colder climates, as the various productions of our Earth require: and, in such as afford a possibility of discovering it, we observe a regular motion round their Axes like that of our Earth, causing an alternate return of day and night; which is necessary for labour, rest, and vegetation, and that all parts of their surfaces may be exposed to the rays of the Sun.

[Sidenote: The farthest from the Sun have most Moons to enlighten their nights.]

12. Such of the Planets as are farthest from the Sun, and therefore enjoy least of his light, have that deficiency made up by several Moons, which constantly accompany, and revolve about them, as our Moon revolves about the Earth. The remotest Planet has, over and above, a broad Ring encompassing it; which like a lucid Zone in the Heavens reflects the Sun’s light very copiously on that Planet: so that if the remoter Planets have the Sun’s light fainter by day than we, they have an addition made to it morning and evening by one or more of their Moons, and a greater quantity of light in the night-time.

[Sidenote: Our Moon mountainous like the Earth.]

13. On the surface of the Moon, because it is nearer us than any other of the celestial Bodies are, we discover a nearer resemblance of our Earth. For, by the assistance of telescopes we observe the Moon to be full of high mountains, large valleys, and deep cavities. These similarities leave us no room to doubt but that all the Planets and Moons in the System are designed as commodious habitations for creatures endowed with capacities of knowing and adoring their beneficent Creator.

14. Since the Fixed Stars are prodigious spheres of fire, like our Sun, and at inconceivable distances from one another, as well as from us, it is reasonable to conclude they are made for the same purposes that the Sun is; each to bestow light, heat, and vegetation on a certain number of inhabited Planets, kept by gravitation within the sphere of it’s activity.

[Sidenote: Numberless Suns and Worlds.]

15. What an august! what an amazing conception, if human imagination can conceive it, does this give of the works of the Creator! Thousands of thousands of Suns, multiplied without end, and ranged all around us, at immense distances from each other, attended by ten thousand times ten thousand Worlds, all in rapid motion, yet calm, regular, and harmonious, invariably keeping the paths prescribed them; and these Worlds peopled with myriads of intelligent beings, formed for endless progression in perfection and felicity.

16. If so much power, wisdom, goodness, and magnificence is displayed in the material Creation, which is the least considerable part of the Universe, how great, how wise, how good must HE be, who made and governs the Whole!

CHAP. II.

_A brief Description of the_ SOLAR SYSTEM.

[Sidenote: PLATE I. Fig. 1.

The Solar System.]

17. The Planets and Comets which move round the Sun as their center, constitute the Solar System. Those Planets which are nearer the Sun not only finish their circuits sooner, but likewise move faster in their respective Orbits than those which are more remote from him. Their motions are all performed from west to east, in Orbits nearly circular. Their names, distances, bulks, and periodical revolutions, are as follows.

[Sidenote: The Sun.]

18. The SUN ☉, an immense globe of fire, is placed near the common center, or rather in the lower[2] focus, of the Orbits of all the Planets and Comets[3]; and turns round his axis in 25 days 6 hours, as is evident by the motion of spots seen on his surface. His diameter is computed to be 763,000 miles; and, by the various attractions of the circumvolving Planets, he is agitated by a small motion round the center of gravity of the System. All the Planets, as seen from him, move the same way, and according to the order of Signs in the graduated Circle ♈ ♉ ♎ ♋ &c. which represents the great Ecliptic in the Heavens: but, as seen from any one Planet, the rest appear sometimes to go backward, sometimes forward, and sometimes to stand still; not in circles nor ellipses, but in[4] looped curves which never return into themselves. The Comets come from all parts of the Heavens, and move in all sorts of directions.

[Sidenote: PLATE I. Fig. I. The Sun.

The Axes of the Planets, what.]

19. Having mentioned the Sun’s turning round his axis, and as there will be frequent occasion to speak of the like motion of the Earth and other Planets, it is proper here to inform the young _Tyro_ in Astronomy, that neither the Sun nor Planets have material axes to turn upon, and support them, as in the little imperfect Machines contrived to represent them. For the axis of a Planet is a line conceived to be drawn through it’s center, about which it revolves as on a real axis. The extremities of this line, terminating in opposite points of the Planet’s surface, are called its _Poles_. That which points towards the _northern_ part of the Heavens is called the _North Pole_; and the other, pointing towards the _southern_ part, is called the _South Pole_. A bowl whirled from one’s hand into the open air turns round such a line within itself, whilst it moves forward; and such are the lines we mean, when we speak of the Axes of the Heavenly bodies.

[Sidenote: Their Orbits are not in the same plane with the Ecliptic.

PLATE I.

Their Nodes.

Where situated.]

20. Let us suppose the Earth’s Orbit to be a thin, even, solid plane; cutting the Sun through the center, and extended out as far as the Starry Heavens, where it will mark the great Circle called the _Ecliptic_. This Circle we suppose to be divided into 12 equal parts, called _Signs_; each Sign into 30 equal parts, called _Degrees_; each Degree into 60 equal parts, called _Minutes_; and every Minute into 60 equal parts, called _Seconds_: so that a Second is the 60th part of a Minute; a Minute the 60th part of a Degree; and a Degree the 360th part of a Circle, or 30th part of a Sign. The Planes of the Orbits of all the other Planets likewise cut the Sun in halves; but extended to the Heavens, form Circles different from one another, and from the Ecliptic; one half of each being on the north side, and the other on the south side of it. Consequently the Orbit of each Planet crosses the Ecliptic in two opposite points, which are called the Planet’s _Nodes_. These Nodes are all in different parts of the Ecliptic; and therefore, if the planetary Tracks remained visible in the Heavens, they would in some measure resemble the different rutts of waggon-wheels crossing one another in different parts, but never going far asunder. That Node, or Intersection of the Orbit of any Planet with the Earth’s Orbit, from which the Planet ascends northward above the Ecliptic, is called the _Ascending Node_ of the Planet; and the other, which is directly opposite thereto, is called it’s _Descending Node_. Saturn’s Ascending Node is in 21 deg. 13 min. of Cancer ♋, Jupiter’s in 7 deg. 29 min. of the same Sign, Mars’s in 17 deg. 17 min. of Taurus ♉, Venus’s in 13 deg. 59 min. of Gemini ♊, and Mercury’s in 14 deg. 43 min. of Taurus. Here we consider the Earth’s Orbit as the standard, and the Orbits of all the other Planets as oblique to it.

[Sidenote: The Planets Orbits, what.]

21. When we speak of the Planets Orbits, all that is meant is their Paths through the open and unresisting Space in which they move; and are kept in, by the attractive power of the Sun, and the projectile force impressed upon them at first: between which power and force there is so exact an adjustment, that without any solid Orbits to confine the Planets, they keep their courses, and at the end of every revolution find the points from whence they first set out, much more truly than can be imitated in the best machines made by human art.

[Sidenote: Mercury.

Fig. I.

May be inhabited.

PLATE I.]

22. MERCURY, the nearest Planet to the Sun, goes round him (as in the circle marked ☿) in 87 days 23 hours of our time nearly; which is the length of his year. But, being seldom seen, and no spots appearing on his surface or disc, the time of his rotation on his axis, or the length of his days and nights, is as yet unknown. His distance from the Sun is computed to be 32 millions of miles, and his diameter 2600. In his course, round the Sun, he moves at the rate of 95 thousand miles every hour. His light and heat from the Sun are almost seven times as great as ours; and the Sun appears to him almost seven times as large as to us. The great heat on this Planet is no argument against it’s being inhabited; since the Almighty could as easily suit the bodies and constitutions of it’s inhabitants to the heat of their dwelling, as he has done ours to the temperature of our Earth. And it is very probable that the people there have such an opinion of us, as we have of the inhabitants of Jupiter and Saturn; namely, that we must be intolerably cold, and have very little light at so great a distance from the Sun.

[Sidenote: Has like phases with the Moon.]