A high-school astronomy: in which the descriptive, physical, and practical are combined ...

Front matter

HIGH-SCHOOL ASTRONOMY:

IN WHICH THE

DESCRIPTIVE, PHYSICAL, AND PRACTICAL

Preface

The design of this work is to furnish a suitable text-book of Astronomy for academies and seminaries of learning. Though substantially a revised edition of the " Elementary Astronomy" so extensive and important have been the additions and improvements, as to justify the adoption of a new title, and to warrant the hope that it will not only be found eminently suited to its purpose, but that it may now go on for years without further revision or alteration.

For juvenile learners, the " Primary Astronomy" may be preferred ; and for advanced classes, who wish to study the constellations in connection with Mythology, the " Geography of the Heavens" should be chosen in preference to all others ; but for all ordinary students, this intermediate work will be found sufficiently elementary on the one hand, and sufficiently extended on the other.

The work is now divided in three parts. After an Introduction, which consists of Preliminary Observations and Definitions, and occupies twenty pages, Part First is devoted to the Solar System — the sun, planets, comets, eclipses, tides, &c. ; Part Second relates to the Sidereal Heavens — the fixed stars, constellations, clusters, and nebulae; and Part Third to Practical Astronomy — the structure and use of instruments, refraction, parallax, &c. This department, so seldom introduced into text-books for schools, will be found especially interesting and valuable.

Besides embracing all the late discoveries in astronomy, under a strictly philosophical classification, the work is now thoroughly illustrated, by the introduction of diagrams into the pages, in connection with the text; and the adaptation throughout to the use of the blackboard, during recitation, cannot fail to be appreciated by every practical teacher.

The variety of type affords an agreeable relief to the eye of the student, and at the same time distinguishes the main text (which ought, in all cases, to be thoroughly understood before it is passed) from the less important matter, the more careful study of which may be left for a review. The suggestive topical questions at the bottom of the page complete the design.

On the whole, the work is believed to be a decided improvement upon the works heretofore in use in this department of study ; and as such it is offered to the professional teachers of the country.

New York, Jan. 1, 1853.

ASTRONOMICAL WORKS

In the Authors Library, and more or less consulted in the compilation of the following pages :

A Cycle of Celestial Objects, for the use of Naval, Military, and Private Astronomers, &c. By Capt. Wm. Henry Smyth, &c. 2 vols. 8vo. London, 1344.

An Introduction to Astronomy, in a Series of Letters from a Preceptor to his Pupil, &c. By John Bonnycastle, Professor of Mathematics, &c. 1 vol. Svo. London, 1822.

An Introduction to the True Astronomy ; or, Astronomical Lectnres read in the Astronomical School of the University of Oxford. By John Keill, M. D., F. R. S-, &c. I vol. 8vo. Dublin, 1793.

Astronomy Explained, upon Sir Isaac Newton's principles, &c«, &c. By Jambs Ferguson, F. R. S. 1 vol. 4to. London, 1764.

Tlie Elements of Physical and Geometrical Astronomy. By David Gregory, M. D., late Sullivan Professor of Astronomy at Oxford, &c. 2 vols. 8vo. London, 1726.

Astronomy, in Five Books. By Roger Long, D. D., F. R. S., &c, University of Cambridge. 2 vols. 4to. Cambridge (Eng.), 1742.

Astronomia Carolina, <fec, by Thomas Street ; and A Series of Observations on ths Planets, chiefly the Moon, &c, by Dr. Edmund Halley. 1 vol. 4to. London, 1716.

Astronomical Lectures, read in the Public School at Cambridge (Eng). By William Whiston, M. A., Professor of Mathematics, <fec. 1 vol. Svo. London, 1728.

Tlie Wonders of the Heavens ; apopular view of Astronomy, &c. By Dunoan Bradford. 1 vol. royal 4to. New York, 1843.

Popular Lectures on Science and Art, &c. By Dionysius Lardner, F. R. S., &c, &c. 2 vols. 8vo. New Tork, 1846.

Outlines of Astronomy. By Sir John F. W. Herschel, Bart, K. H., &c. 1 vol. 8vo. Philadelphia, 1849.

Phenomena and Order of the Solar System, and Views of tte Architecture of the Heavens. By J. P. Nichol, F. R. S. E., &c. 2 vols. 12mo. New York, 1842.

The Practical Astronomer, &c. By Thomas Dick, LL.D. 1 vol. 12mo. New York, 1846. Also, " Celestial Scenery," and " The Sidereal Heavens," by the same author.

Tlie Planetary and Stellar Worlds. By Prof. O. M. Mitchel. 1 vol. 12mo. New York, 1849.

An Elementary Treatise on Astronomy, &c. By William A. Norton, A. M. 1 vol, 8vo. New York, 1845.

An Introduction to Astronomy, &c. By Denison Olmsted, A. M. 1 vol. 8vo. New York, 1844. Also, Letters on Astronomy, and Life and Writings of Ebenezer Porter Mason, by the same author. 2 vols. 12 mo.

Tlie Solar System ; or, the Sun, Moon, and Stars. By J. R. Hind, Director of Mr. Bishop's Observatory, Regent's Park, London. 1 vol. 12mo. London, 1S52.

A Pictorial Display of the Astronomical Phenomena of tlie Universe, &c. By C. F, Blount. 4to. New York, 1844.

The Recent Progress of Astronomy, &c. By Elias Loomis, Professor of Mathematics. &c. 1 vol. 12mo. New York, 1850.

Annual of Scientific Discovery, &c. By David A. Wells, A. M. 1 vol. 12mo. Boston, 1852.

Tlie Sidereal Messenger ; a Monthly Journal, devoted to Astronomical Science. By O. M. Mitchel, A. M. (Now discontinued.)

Introduction

PRELIMINARY OBSERVATIONS AND DEFINITIONS.

PAG*

Ghaf. I. — Origin and History of the Science.

Ptolemaic Theory of the Structure of the Universe .... 12

The Coperniean System 13

II. — Definitions.

Solids, Surfaces, (fee 16

Spheres, Hemispheres, and Spheroids 17

Lines and Angles 19

Of Triangles 20

Circles and Ellipses *. 21

The Terrestrial Sphere . 23

The Celestial Sphere 25

First Grand Divisions of the Universe 28

Chap. I. — The Primary Planets. Classification of the Solar Bodies 29

Names of the Primary Planets 31

Explanation of Mythological Signs 82

Distances of the Planets 36

Light and Heat of the Planets 38

Magnitude of the Planets 49

Density 41

Gravitation 42

Periodic Revolutions of the Planets. 44

6 CONTENTS.

PAGE

Chap. I. — Hourly Motion of the Planets in their Orbits 45

Centripetal and Centrifugal Forces 45

Laws of Planetary Motion 46

Aspects of the Planets 48

Sidereal and Synodic Revolution 49

The Ecliptic, Zodiac, Signs, &c 50

Celestial Latitude and Longitude , 53

Mean and True Places of a Planet . . 54

Direct and Retrograde Motions 55

Morning and Evening Stars 57

Deviation of the Orbits of the Planets from the Ecliptic 58

Philosophy of Transits 60

II. — Primary Planets Continued.

Inclination of the Axis of the Planets, and its Effects. . 65

Rotation of the Planets upon their axes 69

Time 70

Equation of Time 72

Time, as affected by Longitude 76

True Figure of the Planets 77

Precession of the Equinoxes 80

III. — Telescopic Views of the Planets.

Mercury — Phases, Mountains, &c 83

Venus — Phases, Mountains, Atmosphere 84

Mars — " Continents and Seas," Color, Snow Banks 86

The Asteroids — Color, Hazy Appearance 87

Jupiter — Oblateness, Belts, Moons 88

Saturn — Oblateness, Rings, Belts, Phases, Moons 89

Uranus — a Telescopic World, Satellites 94

Neptune — purely Telescopic, Satellite 93

Herschel's Solar System in Miniature 91

IV* — Seasons of the Different Planets, <fcc.

Cause of the Seasons — Mercury, Earth 97

Venus, Mars, Jupiter 98

Saturn and Uranus 99

Discovery of the different Planets 100

V. — Secondary Planets — The Moon.

Character and Number of the Secondaries 102

The Moon's Distance, Shape, Position of Orbit, tfcc 103

CONTENTS.

PJUM

Chap. V.- -Magnitude, Density, Revolution Eastward 105

Form of Lunar Orbit 107

Cause of the Moon's Changes 109

Natural appearance — Same side always toward us. . . . Ill

Moon's Li orations — in Latitude and Longitude 112

Telescopic Appearance of the Moon — Lunar Mountains 113 Finding the Longitude by the Moon's place 115

VI. — Eclipses of the Sun and Moon.

Philosophy of both 116

Law of Shadows 117

Why not two Eclipses every Lunar Month 118

Why Solar pass eastward over the Sun, and Lunar westward over the Moon 119

Ecliptic Limits — Umbra and Penumbra 120

Why all Central Eclipses not total 122

Ml

VII.-^-Satellites of the Exterior Planets.

Satellites of Jupiter — Distances, Periods, <fec 124

Eclipses of Jupiter's Moons — Immersions and Emersions 126

Moons of Saturn — Why seldom eclipsed 127

Satellite of Neptune 129

VIII. — Nature and Cause of Tides.

Description of Tides, Causes, <fcc 130

Spring and Neap Tides 134

IX. — Of Comets.

Name, Parts, Orbits, <fcc 136

Magnitudes, Velocity, Temperature, Periods 140

Numbers, Physical Natures, <fec 143

X.— The Sun.

'True Figure, Spots • . 146

Physical Constitution, Temperature 151

Zodiacal Light 153

Sun's Proper Motion in Space 155

XL — Miscellaneous Remarks upon the Solar System.

Nebular Theory of its Origin 156

Were the Asteroids originally one Planet ? 159

Are the Planets inhabited by rational beings ? 161

CONTENTS.

Part Second. The Sidereal Heavens

PAGB

Chap. I. — The Fixed Stars.

Classification of the Stars 1C6

Number of the Stars 168

Distances of the Stars 170

II. — Description of the Constellations.

Nature, Origin, Classification 172

Visible in October, November, and December 174

" January, February, and March 177

" April, May, and June 180

" July, August, and September 183

III. — Double, Variable, and Temporary Stars, &c.

Stars Optically and Physically Double 187

Binary Systems 189

Variable or Periodical Stars 194

Temporary Stars — New and Lost 196

IV. — Clusters of Stars and Nebulae.

Pleiades, Hyades, <kc 199

Nebulse — Resolvable, Irresolvable, Annular, &c 201

Planetary, Stellar, <fcc 203

Star Dust, Milky Way 206

Part Third. Practical Astronomy

Chap. I. — Properties of Light.

Refraction of Light 211

Atmospherical Refraction „ 214

Refraction by Glass Lenses , 216

11 . — Telescopes.

Refracting Telescopes 221

Reflecting Telescopes 231

Transit Instrument 235

Mural Circle 236

Parallax 237

Meteors and Meteoric Stones 239

INTRODUCTION.

PRELIMINARY OBSERVATIONS AND DEFINITIONS.

Chapter I. Origin And History Of The Science

1. Science is knowledge systematically arranged, so as to be conveniently taught, easily learned, and readily applied.

2. Astronomy is the science of the heavenly bodies — the Sun, Moon, Planets, Comets, and Fixed Stars.

The term astronomy is from the Greek astron, a star, and nomos, a law ; and signifies the laws or science of the stars.

3. Astronomy is divided into Descriptive, Physical, and Practical.

Descriptive Astronomy includes the mere facts of the science, irrespective of the causes of the phenomena observed, or of the means by which the facts were ascertained.

Physical Astronomy explains the causes of the various phenomena observed, as of Day and Night, the Seasons, Eclipses, Tides, &c.

Practical Astronomy relates to the means for acquiring astronomical knowledge by the use of instruments, and by mathematical calculations.

These three departments have arisen, one after the other, in the order in which they are here stated. At first a feA\* facts and phenomena were observed, but the causes were unknown. Next some of the causes were investigated one by one; and, finally, instruments were invented for measuring distances, altitudes, &c. ; data for calculations were obtained; and thus arose the department of Practical Astronomy.

1. Define the term Science.

2. What is Astronomy' 1 . (From what is the term derived ?)

3. How is astronomy divided ? Descriptive? Physical? Practical? (Stats the ortier n which these departments have arisen.)

i*

4. Astronomy has long been regarded as the most sublime of the sciences, eminently calculated to illustrate the wisdom, power, and goodness of God; to elevate and expand the human mind, and to fill it with exalted views of the Creator —

" The glorious Architect who built the skies."

% "The greatest men of all ages have pronounced this science to be the most sublime and surpassing of all that can be tested by human genius, and to be worthy of a life of study." — Smyth's Celestial Cycle.

2. " 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! 11 — Ferguson.

3. So remarkably does this science exhibit the glory and majesty of God, by its astounding revelations of His works, that it almost necessarily tends to fill the mind with awe and reverence. It was in view of this tendency that the poet Young said,

" An undevout astronomer is mad.'"

4. To the moral influence of the contemplation of the heavens, we have frequent reference in the sacred Scriptures. " The heavens declare the glory of God; and the firmament showeth his handy-work." (Psalm xix. 1.) " When I consider thy heavens, the work of thy fingers; the moon and stars, which thou hast ordained; what is man. that thou art mindful of him? and the son of man, that thou visitest him? 11 (Psalm viii. 3, 4.)

5. Astronomy is probably the most ancient of all the sciences. Some of the Chaldean observations date as far back as 2,250 years before Christ, or only 98 years after the Flood ! Laplace speaks confidently of Chinese observations 1,100 b. c. ; and Mr. Bailly, an English astronomer, fixes the time of a conjunction of Mars, Jupiter, Saturn, and Mercury, mentioned in Chinese records, at 2,449 years before Christ.

1. The ancient Chinese astronomers and mathematicians were held to a fearful responsibility for the correctness of their calculations. In the reign of the Emperor Choukang, his two chief astronomers, Ho and Hi, were condemned to death for neglecting to announce the precise time of a solar eclipse, which took place B. C. 2,169.

2. The Holy Scriptures, some parts of which are very ancient, contain several allusions to the science of astronomy. In the first chapter of Genesis we have an account of the creation of the Sun, Moon, and Stars. "And God said, Let there be lights in the firmament of the heaven, to divide the day from the night, and let them be for signs, and for seasons, and for days and years. And let them be for lights in the firmament of the heaven, to give light upon the earth : and it was so. And God made two great lights ; the greater Mght to rule the day, and the lesser light to rule the night: he made the stars also." Verses 14-16.

3. In the book of Job, written 1,500 years before Christ, we read of several constellations that bear the same names now that they did three thousand years ago. "Which maketh Arcturus, Orion, and Pleiades, and the chambers of the south." (ix. 9.) Again : 1 Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion ? Canst thou bring forth Mazzaroth in his season? or canst thou guide Arcturus with his sons?" (Chap, xxxviii. 31, 32.)

4. How astronomy regarded ? (Smyth ? Ferguson ? Young ? Scriptures ?)

5. What of antiquity of astronomy *? Chaldean and Chinese observations? ("Resoonsibility of Chinese astronomers ? Ancient Scriptural allusions ?)

EARLY ASTRONOMERS. 11

6. The first astronomers were shepherds and herdsmen, who were led to this study by observing the movements of the sun, moon, and stars, while watching their flocks from year to year in the open fields.

ANCIENT ASTRONOMEK8 OBSERVING THE HEAVENS.

7. Thales, one of the seven wise men of Greece, was the first regular teacher of Astronomy, b. c. 600. The next was Anaximander, a disciple of Thales, who succeeded him as head of the school at Miletus, b. c. 548. He asserted the true figure of the earth, and seems to have had some idea of its daily revolution.

Anaximander is supposed to have been the first who constructed globes and maps. Ho taught that the moon shines by reflection, and in several other respects advanced beyond the knowledge imparted by his distinguished tutor.

8. Pythagoras, another Greek philosopher, who founded the school of Croton, b. c. 500, greatly enlarged the science. He first gave form to the vague ideas that the stin was in the center of the planetary orbits, that the earth floated unsupported in space, and that the distant stars were worlds, and probably inhabited.

"It was Pythagoras," says Smyth, "who taught, in fact, the system which now immortalizes the name of Copernicus." But he adds that his teachings were but "the conjectures of a sagacious mind, not possessed of the evidence requisite to give stability to its opinions." Pythagoras is said to have perished from hunger, in his old age.

6. Who were the first astronomers ? How led to this study ?

7. Who first regular teacher of this science ? How early ? Who next ?— and when ? What correct notions did he seem to entertain ? (For what else distinguished ?)

8. Who next after Anaximander ? What advances did he make in this study f (What does Smyth say of his teachings ? What said of his death ?)

9. Ptolemy, an Egyptian philosopher, taught astronomy in the second century of the Christian era. He adopted the theory that the earth was located in the center of the universe, that it was perfectly at rest, and that the sun, moon, and stars actually revolved around it, from east to west, as they appear to do, every twenty-four hours. This system is called, after its author, the Ptolemaic Theory \

PTOLEMAIC THEORY OF THE STRUCTURE OF THE UNIVERSE.

1. Ptolemy supposed the earth to he in the center of a system of crystalline arches, or hollow spheres, arranged one within the other, as represented in the cut. It is thought by some that he understood the spherical figure of the earth, and the cut is constructed upon this supposition. Ptolemy farther supposed that the sun, moon, and stars were fixed in these crystalline spheres, at different distances from our globe ; that the Moon was in the first, Mercury in the second, Venus in the third? the Sun in the fourth, Mars,

9. Who was Ptolemy ? — and when did he flourish ? Describe his theory, (How locate sun, moon, &c, ? What absurdity did it involve, as it respects

COPERNICAN SYSTEM. 13

Jupiter, and Saturn in the next three, and the fixed stars in the eighth. The aneients had no knowledge of Uranus or Neptune. This ponderous machinery was supposed to revolve from east to west around the earth, carrying with it the sunj moon, and stars, every twenty-four hours ; and the spheres being crystal, the distant stars were visible through them.

2. If the sun was designed to enlighten and warm the different sides of our globe, the Ptolemaic method of effecting this object is most unreasonable. To carry the sun around the earth, to warm and enlighten its different sides, instead of having the earth turn first one side and then the other to the sun, by a revolution on its axis, would be like carrying a fire around a person who was cold, and wished to be warmed, instead of his turning himself to the fire as he pleased.

3. The Ptolemaic theory would require a motion inconceivably rapid in all the heavenly bodies. As the sun is ninety-five millions of miles from the earth, the entire diameter of his sphere would be one hundred and ninety millions of miles, and its circumference about six hundred millions. Divide this distance by twenty-four — the number of hours in a day — and it gives twenty-five million miles an hour, or sixty-nine thousand four hundred and forty -four miles per second, as the velocity of the sun ! This theory would require a still more rapid motion in the fixed stars. It would require the nearest of these to move at the rate of nearly fourteen thousand millions of miles per second, or seventy thousand times as swift as light, in order to accomplish their daily course. But with all these difficulties in its way, the Ptolemaic theory was generally believed till about the middle of the sixteenth century, or three hundred years ago.

THE COPERNICAN SYSTEM.

10. About the year 1510, the ancient theory of Pythagoras was revived and improved by Copernicus, a Prussian astronomer, and has since been called, after him, the Copernican System.

1. The investigations of Copernicus were conducted between the years 1507 and 1530. In the latter year he finished his tables of the planets, and his great work, TJie Revolution of the Celestial Orbs ; but he did not venture to publish his views till thirteen years after, or 1543, when he received a copy of it only a few hours before his death, and consequently never read it in print. It contains the old philosophy, interspersed with his own* original and acute conceptions, and was received under very considerable opposition. — Smyth, vol. 1, p. 38.

2. Copernicus is generally regarded as the discoverer of the system which bears his name, but this is a popular error. There is abundant proof, notwithstanding the loss of his writings, that Pythagoras understood the leading features of what is now called the Copernican Theory.

11. The first prominent feature of the Copernican system is, that the earth is a sphere or globe, inhabited on all sides.

The evidence that the earth is a sphere or globe may be arranged and stated as follows :

1. Admitting that the sun, moon, and stars are worlds, the fact that they are round, as we see them to be, affords ground for the presumption, at least, that the earth also is round

2. Water falling from the clouds is gathered into little globes or drops ; and melted lead poured from the summit of a high tower assumes the form of globes, which, when cooled, are called shot. And the same law would cause a larger mass of fluid matter, if left undisturbed in space, to assume the same shape. But the Bible teaches that the

light and heat? What in respect to the motions of the heavenly bodies? Was such a theory ever generally believed ? Till how recently ?)

10. Who was Copernicus ? For what distinguished? About what time? (What of his investigations ? His work ? Its publication ? Character ? What popular error noticed ?)

11. State the first leading feature of the Copernican theory. (What proofs of its correctness ? The first ? Second ? Third ? Fourth ? Fifth ? Sixth 2 Seventh ?)

whole earth was once in a fluid state — one vast drop — the substances now constituting the oceans and continents being indiscriminately mingled together. " And the earth was without form and void [/. c, chaotic, confused, unorganized], and darkness dwelt upon the face of the deep; and the spirit of God moved upon the face of the waters. * * * And God said. Let the waters under the heavens be gathered together unto one place, and let the dry land appear: and it was so. And God called the dry land earth, and the gathering together of the waters called he seas" — Genesis i. 2, 9, 10. Up to this time there was no " earth," either as continents or islands, neither were there any •'seas, 1 ' but all the elements were mingled together; and a mass of fluid thus dropped into space, from the hand of the Creator, would be as certain to assume the form of a globe, as the melted lead from the shot-tower, or the water from the passing cloud.

3. The apparent elevation and depression of the North Star, as we approach toward or recede from it, shows that the surface of the earth is convex, or that the earth is a globe.

4. The fact that the tops of mountains are last seen as we recede from, or first as we approach, the sea-shore, proves that the surface of the water upon which we sail is convex ; so when a ship is approaching the shore, the topmasts are always seen first, and the hull or body last. And when seamen wish to survey the horizon at sea to a great distance, in search of whale or other shipping, they " go to the mast-head," as they call it, from which point they can often discover objects that are entirely invisible from the deck of the ships.

5. If an aqueduct is to be constructed a mile long, so as to be filled with water to the brim at every point, it must be about eight inches higher in the middle than at the ends, so as to allow the surface of the water to conform to the convex figure of the globe. We say higher, not that it needs to be higher as determined by a water level, for a water level is convex, but higher as determined by a straight line drawn from one end of the aqueduct to the other. This definite knowledge of the curvature of water, even for small distances, shows that the earth's surface is convex — or, in other words, that the earth is spherical. (The curvature from a tangent line is 8 inches tor one mile, from the point of contact ; 32 inches for two miles ; 72 inches for three miles, <fcc.)

6. When the moon falls into the shadow of the earth and is eclipsed, or, in other words, the earth gets into her sunlight, and throws its shadow upon her, the shadow is seen to be convex. We must either conclude, therefore, that the earth, which casts the shadow, is in the form of a dinner-plate, and is always kept sidewise, and the same side toward the sun (which we know is not the case) ; or that it is a globe, and casts a conical shadow, whatever its position.

7. The earth is known to be a globe, from the fact that ships are constantly sailing around it.

8. It is not certain whether Ptolemy admitted the earth to be a sphere or not. Some writers maintain that he rejected this doctrine, and others that he admitted it. In the " Primary Astronomy," page 8, the author has inserted a cut representing the Ptolemaic theory, with the earth fiat; but in this work (page 12), where the same theory is represented, the earth is shown as a globe. In all other respects, the theory represented is the same in both works ; and this is only a minor point in the system.

12. A second leading feature of the Copernican theory is, that the apparent revolution of the sun, moon, and stars westward every day, is caused by the revolution of the earth around its own axis, from west to east, every twenty-four hours.

That the heavenly bodies appear to revolve westward, is no proof that they are actually in motion. We often transfer our own motion, in imagination, to bodies that are at rest ; especially when carried swiftly forward without any apparent cause, as when one travels in a steamboat or railway car, and when for a time he forgets his own motion. " Copernicus tells us that he was first led to think that the apparent motions of the heavenly bodies, in their diurnal revolution, were owing to the real motion of the earth in the opposite direction, from observing instances of the same kind among terrestrial objects ; as when the shore seems to the mariner to recede as he rapidly sails from it, and as trees and other objects seem to glide by us, when, on riding swiftly past them, we lose the consciousness of our own motion." This remark would go to show that the revolution of the earth on its own axis was an original discovery with Copernicus.

12. State the second leading feature of the Copernican system. (Do not our own senses furnish proof that the heavenly bodies revolve westward daily ? Why not ? What remark from Copernicus ? What does it seem to imply ?)

COPERNICAN SYSTEM.

13. A third feature of the Copernican theory is, that the sun is the grand center around which the earth and all the other planets revolve.

THE COPERNICAN SYSTEM.

1. The above cut is a representation of the Copernican Theory of the Solar System. In the center is seen the sun, in a state of rest. Around him, at unequal distances, are the planets and fixed stars — the former revolving about him from west to east, or in the direction of the arrows. The white circles represent the orbits, or paths, in which the planets move around the sun. On the right is seen a comet plunging down into the system around the sun, and then departing. This is the Copernican Theory of the Solar System.

" O how unlike the complex works of man, Heaven's easy, artless, unencumber'd plan !"

2. The truth of the Copernican theory is eatablished by the most conclusive and satisfactory evidence. Eclipses of the sun and moon are calculated upon this theory, and astronomers are able to predict thereby their commencement, duration, &c, to a minute, even hundreds of years before they occur. We shall therefore assume the truth of this system without further proof, as we proceed hereafter to the study of the heavenly bodies.

13. State the third prominent feature of the theory of Copernicus. (De- Bcribe the cut. What additional evidence of the truth of this theory, as a whole

Chapter II

14. Solids, Surfaces, &c.

A Solid, or Body, is a figure having length, breadth, and thickness.

A Surface is the outside or exterior of a body, and has length and breadth only.

Surfaces are of three kinds — Plane, Concave, and Convex.

A surface may also be rough or smooth, hard or soft; the above definition having reference only to the general figure of bodies.