Astronomy for schools; upon the basis of Mons. Arago's lectures at the Royal observatory of Paris ...

Front matter

r'ft'.^i

Ml

MM

Appendix

BY R. W. HASKINS, A. M.

NEW YORK: robinson, pratt & co., 63 wall st. buffalo: a. w. wilgus. 1841.

Entered according to act of Congress, in the year 18-11, by ROSWELL W. HASKINS,

in the Clerk's Office of the Northern District of New- York.

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Wilgus' Pre.-s. Ruflalo.

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Preface

The basis of the following work, as announced in the title page, re Arago's Lectures at the Observatory of Paris. Many changes and digressions from that author have, of course, been necessary, in preparing a work lor the schools of the United States, and suited to the wants, and the peculiar paucity of knowledge, upon this particular science, among the greater part of our American youth.

It has often been matter of surprise, and still more frequently of poignant regret, that Astronomy, although the most ancient and the most perfect of all the sciences, should never have been generally introduced into the list of subjects embraced by the first degrees of publick instruction, among us. When the same fact has been complainingly averred in Europe, it has been answered that this science has been, through all former time, exclusively confined to the learned, because the books, upon it, have been so loaded with difficulties as to render access to the science impossible to any others than such as were deeply versed in Mathematicks.

In the lectures of Arago, referred to, above, it was the assumed task of that eminent Astronomer to prepare a work suited to general use, by giving it an elementary form, without sacrificing any thing essential to a general knowledge of the science; and whatever of success may have crowned his efforts, in this, the present work will be found to embrace.

The almost entire absence of the study of this science, from our common schools, and the constant and daily increasing demand for an Astronomy suitable to be used as a Text Book in these schools, are matters of perfect familiarity to all such as have given any considerable attention to the subject of publick instruction, and its progress, in the schools in question. 1*

VI PREFACE.

The rising character of these schools is a proud and distinguishing feature of the present era. The middle aged, of the present day, well remember the first general introduction of the study of geography into our publick schools: now, perhaps, no corner of our land, however obscure, can be found where the schools do not embrace this study, even in laborious detail.

The study of geography has prepared the way for that of Astronomy, of which, indeed, it is, in some sort, a subordinate branch; and which can be well understood only in combination with a knowledge of the mechanism of the heavens.

In the preparation of this work the constant endeavour has been to meet the demand of our common schools, for a Text Book in Astronomy; and to give it general application by suiting it to the capacity of all those who have embraced, in their course of studies, geography, and common arithmetick. For this reason no mathematical problems, whatever, have been inserted; and while it is confidently believed that the work may be fully comprehended by pupils no farther advanced than to embrace the studies above mentioned, it is as confidently hoped that it will be found to include all that is essential to the general pupil in the science.

We have said this science is ancient. It is so: originating in the fields, and among peasants, at a period so early as to be now unknown, it has successively passed from men thus untaught, to minds the most exalted and sublime. Imposing, by the grandeur of its objects; curious, by its peculiar means of investigation; and astonishing by the number and character of its discoveries, it marks, perhaps, more pointedly than any other science, the extent to which human intelligence is capable of fathoming the secrets and the resources of Nature. But, though ancient, in its rude outline, yet perfected astronomy is of very modern date. Though many of the great truths of the science were disclosed by the immortal spirits of previous days, yet it is only about one hundred and twenty years since the true planetary adjustments and motions of the Solar System were taught in the colleges and learned societies of Europe. And yet, such was the rapid march of the science, after that period, that "at the close of the last century there did not remain a single phenomenon, in the celestial motions, that was not explained, on the principle of gravitation."

PREFACE. VH

This minute knowledge, thus perfected only some forty years since, was, of course, confined to the learned; nor is it supposed that others than such will ever attempt to become masters of these abstruse demonstrations. But the spirit of this age, no less than the salutary advantages of the knowledge itself, demands that the results thus discovered, and the general laws by which these results are produced, should become part of the instruction imparted by our schools, to the youth for whose benefit they are maintained. Such knowledge will be a valuable substitute for the lunar, stellary and cometary superstitions, the remnants of that malady of the human mind, ancient astrology, which are so widely diffused, and which can only be ejected from the mind by a substitution of true knowledge in their stead. To this end no study so directly conduces as Astronomy. "The objects which it treats of," says Playfair, "cannot fail to impart to it a degree of their own magnificence and splendour; while their distance, their magnitude, the steadiness and regularity of their movements, deeply impress the imagination, and afford a noble exercise to the understanding. Add to this, that the history of Astronomy is that w r hich is best marked out in the progress of human knowledge. Through the darkness of the early ages, we perceive the truths of this science shining, as it were, by their own light, and scattering some rays around them, that serve to discover a few definite objects, amid the confusion of ancient tradition — a few fixed points amid the uncertainty of Greek, Egyptian, or even Hindoo mythology. ,,

Perfection, of course, I have not hoped to attain; yet great labour has been bestowed, by a constant reference to first authorities, to ensure, as far as practicable, an avoidance of errour3 w T hich are known to abound in minor compilations.

Every effort, also, has been made, and it is believed successfully, to obtain, from the great Astronomical depositories of Europe, full and accurate knowledge of all the recent discoveries in the science: and the results of these efforts will be found occupying their appropriate places, in the different parts of the work to wrhich they severally pertain.

My hope is that this work may become instrumental in imparting to our youth, in the publick schools, a general knowledge of the leading truths of Astronomy: of that sublime science which

Vlll PREFACE.

has extended man's views, upon every side; and, by rendering this globe too narrow a space for the range of the human mind, has enlarged its conceptions of the extent, no less than its knowledge of the mechanism of the Universe: of that science which is identified with our daily and hourly necessities; which regulates the labours of agriculture; affords us the only measure of time that we have, or can obtain; is inseparable from accurate geographical knowledge, and is indispensable to navigation: and should it prove useful in this, then, indeed, the end for which it was undertaken will have been fully -attained.

THE AUTHOR.

Buffalo, May 10, 1841.

Index

Page-

Acromatick Telescopes, 29

Aerolites, 119

Asia, depression of land in, how produced, 2*22

Astronomers, the Chaldeans among the first, .... 41

the Phenecian, 41

the Greek, 41

Astronomy, history of, 40

carried to Greece by Thales, 41

taught by Anaxamander, 42

taught by Pythagoras, 42

sought for in Egypt, 43

taught by Aristotle, 43

" at the school of Alexandria, ... 43

" by Ptolemy, 44

" by Copernicus, 44

" by Kepler, 45

" by Galileo, 45

Astronomical terms, explanation of, 319

Alphabet, Greek, 47

Annual parallaxes, table of, 150

Authorities, 15

Atmosphere, of the, 252

height of the, 253

refraction caused by, 253

Bode's law of planetary distances, 100

supposed solution of, 100

Breeze, land, and sea, 280

Calendar, of the, 281

Celestial bodies, apparent motion of the, 67

sphere, reflections upon the motion of the, . 69

X INDEX.

Chronometer, how to be set, for finding longitude, . . 248

Comet, parts of defined, 189 of a short period, 195

of six years and three quarters, .... 195

of 1744, with six tails, 199

Halley's, strange appearance of, at the Cape of

Good Hope, . - 201

Halley's, as seen at Buffalo, 197

can the earth pass into the tail of a, . . . 210 were the dry fogs of 1783 and 1831 caused

by the tail of a, 211

has the moon been struck by a, . • . . 213

has the moon been formerly a, 213

is it possible that the earth may become a satellite of a, 214

have the lattudes of places upon the earth been

changed by a blow from a, .... 219

Comets, 69 the tails, different directions of, ... 198 are they self luminous 1 ? curious experiment to

determine this question, 203

have they a sensible influence upon the course of

the seasons? 205

can one of them strike the earth? .... 206

has one of them ever struck the earth? . . 208

Colours, complementary, 39

some persons unable to distinguish, ... 40

Continents, western coasts of coldest, 272

Cycle, solar, 287

Days and seasons, of the, 264

Day, sidereal, 70

Densities of the sun and planets, table of the, .... 155

Distances of the sun and planets, table of the, .... 154

Distance passed over by each planet, per minute, . • . 157

Discovery curious, explained, 221

Earth, figure of the, 167

dimensions of the, 168

diurnal rotation of the, 171

cause of the shape of the, 177

annual motion of the, 178

INDEX. XI

Earth, two motions of the, 270

temperature of the, "271

mean tempeature of, at the equator, . . . 272

Ecliptick, diminution of the inclination of, 188

Eclipses, of 224

of the moon, 224

of the sun, 227

" total one of June, 1806, ... 231

ancient ones, their aids to chronology, . . 232

Ellipse, 49

how to describe one, the foci and the transverse

axis being given, 53

how to describe, the axes, only, being given, 53

Eye. conformation of the, 35

Equinoxes, precession of the, 185

Gulf stream, how caused, 243

Heavens, aspect of the, varies with the position of the observer, 73

Heat, has the earth primitive? 273

Inclination of orbits to the ecliptick, table of the, . . . 157 axes of sun and planets to their respective orbits, table of the, 157

Inequalities, secular and periodiek, 183

lunar and terrestrial, 183

Juno, 135 satellites of, 138

Laws of Kepler, 158

Latitude and Longitude, determination of, 243

Lenses, of, 23

Light, general laws of the reflection of, 17

reflection of, by plane mirrors, 19

concave " 21

convex " 22

general laws of the refraction of, ... . 22

dispersion of, 23

calorifick and chymical rays of, ... . 23 decomposed, in rays of different colours, when

refracted, 28

progressive motion of, 175

abberration of, 180

Longitude, finding of, difficult, 245

Xll INDEX.

Longitude, method of finding, by chronometer, and astronomical tables, 246

interesting account of instance of finding, 247 tables for finding published some years before

their date, and why, 249

various meridians from which to compute, . 249

table of the lengths of the degrees of, . . 251

Lunar variations, 184

Mars, 131

Meridian line, how found, 74

Masses of the sun and planets, table of the, 155

how known, 165

Milky-Way, 86

Meton, cycle of, 283

Medium resisting, 293

Moon, new, • . 68 physical constitution of, ...*.. Ill rays of the, have neither heating nor chymical

properties, 114

superstitions respecting, refuted, . . . 114

the horizontal, 254

" figure of, 258

the Harvest and the Hunter's, 258

the full, at the polar circles, 263

" at the poles, 264

Objects, apparent size of, modified by the supposed distance at

which they are situated, 27

Parallax, annual, 96

Planets, primary, 48

secondary, 48

names of, 49

orbits of, 49

distances of, &c 98

inferiour, 126

superiour, , , . . . 131

telescopick, 134

stationary and retrograde appearances of, illustrated, 180

Plants, a species of vegetable thermometer, .... 274

Preface, 5

INDEX. Xll 1

Rays, luminous, sensation produced by, has a sensible duration, 39

temperature of, bow measured, .... 106

Revolutions of the planets, table of tbe, 156

Rotation, npon their axes, of tbe sun and planets, . . . 156 satellites of, ....... 142

rings of, 143

" appearance of in the different signs, . 144

" measures of the, 145

*' of new discoveries respecting. . . 147

shape of, mistaken by Herschel, .... 149

Satellites of Jupiter, table of the, 157

Scintillation of the fixed stars explained, 72

Stars, how designated in maps and upon globes, .... 47,

uniform motion of ihe, 70

positions of, how determined, 76

fixed, of the, 81

" motion detected in some of these, . . 81

" number of greatly overrated, ... 82

li constellations of the, names of, &c. . 83

variable and new, . 90

fixed, distance of, 95

" recent discoveries respecting, ... 97

" lost, 98

Sun, physical constitution of, . 102

spots on, • . . 103

motion of, 68

Teachers, note to, 16

Telescopes, refracting, 29

reflecting, ... 32

front view, . • 34

cross wires for, how made, 35

magnificent one, 31

large and perfect refracting ones unknown in

England, 31

Tides, of the, 231

circumstances of the phenomena of the, . 241 absence of, in the Baltick and Mediterranean accounted for, 242

Time, sidereal, how found, 80

equation of, 288

Trade winds, 275

XIV INDEX.

errours in the history of the discovery of, corrected, 151

satellites of, 153

" peculiarities of the, 153

Variable winds, 279 transit of, 99

Velocity of falling bodies at the suriace of the sun and planets,

table of the, 156

Vision, act of, how performed, 37

distinct, at unequal distances, how produced, effect upon, by operation for cataract, . . 38

Volumes of the sun and planets, table of the, .... 155

Week, days of the, 286

Year, the Egyptian and Perdan, 282

the great Canicular, 282

Zodiack, signs of the, 58

' ' etymological explanation of, ... 59

AUTHORITIES.

The following are among the authorities which have been critically consulted during the preparation of this work:

Lapif) 119 du .Monde;

Pontecoulant, Theorie Analytique du Systeme du

" " " du moyen age,

" " M " moderne,

Bailly, " " " ancienne,

" »« •• moderne;

lonnier, Observations Astronomique; Humboldt. Voyage aux regions e;minoxiales du nou-

veau continent; Struve, Dorpat Catalogue of Double and Multiple

Je, Bibliograpbie Astronomique; ["ems; - hebdomadaires des Seances de FAcae.ie des Sciences; ?phical Transactions of the Royal Society, i Ion;

le American Academy; Transactions of the American Philosophical Soe

Association for the Advancemei.

NOTE TO TEACHERS.

The etymological explanation of the several names of the signs of the zodiuck has often been attempted; and there is no lack of theories and suggestions, in the books, upon their probable origin and signification. Reference has been made, for this purpose, to the fables of Castor and Pollax, the abduction of Europa, the Argonautick expedition, the passage of Venus and Cupid over the Euphrates — in short, to whatever mythological fable could be found, in the prolifick catalogue of these, that savoured of symbolical representations which might serve as their explanations.

The true meaning of these several names having finally been discovered, through the successful researches of the Institute of Egypt, at Paris, these are inserted, at page 59, and onward; but they differ so widely from what has hitherto been taught, upon this subject, that it has been deemed advisable to gke them in the several languages through which they have come to us, as well as in Englis h the better to show their authenticity. Another reason for this has been found in the highly figurative language employed, which often admits of only a very loose and general rendering in English, that could scarcely fail, if alone, to prove unsatisfactory to those whose business it is to communicate instruction.

This explanation is made as an apology for the appearance of Coptick, Arabick, Greek and Latin words and phrases, in an elementary book; but it is hoped all cause of objection is removed by the fact shown, namely, that these are in no sense intended for the exercises of the pupil, but only for the satisfaction of those to whom he is accustomed to look for elucidation.

ASTRONOMY.

Chapter First. Op Astronomical Instruments

Before entering upon the domain of Astronomy, strictly speaking, it is important to know the instruments which opticks has placed at the service of this science, and to the aid of which it owes so much of its perfection: instruments of which the power has augmented, the sphere of activity of our organs so as, in some degree, to bring the mechanism of the universe within our view. The study of these instruments will form the object of this chapter.

The construction of reflecting telescopes depends upon the laws of the reflection of light, and that of refracting telescopes upon those of its refraction. Let us first examine these two important properties of the luminous fluid.

GENERAL LAWS OF THE REFLECTION OF LIGHT.

If we cause to fall, obliquely, a pencil of solar rays upon a polished surface, we may observe the following phenomena:

1st. A part of the luminous rays are reflected in a certain direction, and if we place the eye in the line of 2 this direction, wc see an image of the sun, in the prolongation of the reflected rays:

2d. The point where the incident, or approaching rays encounter the polished surface is visible in all directions; but it appears incomparably less luminous, from any other point of view than when observed in the line of the reflected rays — the only view which gives a regular image of the sun:

3d. A portion of the incident light is not reflected, but passes, according to known laws, through the reflecting body, if it is transparent; while if it is opaque the same quantity of light is absorbed.

Here, then, we have three distinct phenomena: one portion of the incident light is reflected regularly, in a particular direction; another part is reflected indifferently, in all directions, and disseminated as if by a body not polished, while the remainder either passes through the reflecting body, or is absorbed by it.

But what is the direction followed by that portion of light which is regularly reflected? We find:

1st. That the incident and reflected rays are comprised in the same plane, perpendicular to the reflecting surface; and,

2d. That the incident and reflected rays always form equal angles with the reflecting surface; or, that the angle of reflection and the angle of incidence are equal.

Such are the two general laws of reflection; and they very readily explain to us the formation of images, in this way.

Take, first, the plane mirror; suppose, fig. 1, S the radiant point, O the eye of the observer, and A B the plane reflector. Among all the luminous rays emanating From S, there will be one of them, as S I, which, after being reflected, by the mirror, will meet the eye at O, in the direction I 0, thus making the angle of incidence equal to the angle of reflection. If, from the radiant point S, we draw the perpendicular

OP ASTRONOMICAL INSTRUMENTS.

S A, which encounters, at A, the reflecting surface; and if we prolong this perpendicular, upon the opposite side of the mirror, by the quantity A D, equal to S A : then, from the point D, draw the line D O, to the eye; DO will be the direction of the reflected ray, and the B point I, where it cuts the surface of the mirror, will be the point of incidence. Moreover, if the luminous object and the eye are both supposed to be mathematical points, without sensible extent, the ray determined by the pre ceding rule, is the only one which can be rejected to the eye.

But the opening of the pupil, which ad- <i mits rays into the eye, is not a mathematical point; it is a space which, in man, has about ,07874 of an inch diameter, and which we may represent by L L, fig. 2.

All the reflected rays which can enter this opening, will come, then, to the retina, and thus contribute to vision. Each ray being governed by the same law, which we have here explained, it is

OP ASTRONOMICAL INSTRUMENTS.

evident that these would form a cone with a circular base, of which the apex would be D, and the base L L. To the eye penetrated by reflected rays the luminous body appears placed at the point from which the visible rays seem to diverge. Thus, the eye being placed at 0, the luminous point, seen by reflection, would appear at D: that is to say, the same distance behind, that it really is before the mirror.

If the radiant object has a defined extent, each of its points of luminous surface will contribute to the formation of its image, according to the rule which we will explain; and the sum of these partiaj images constitutes that of the entire object To illustrate this, suppose the object to be an arrow, S S', fig. 3: the image

of the base S, of the arrow, will be shown a: D: the poin:. S . in like manner at D ; and the intermediate portions of the arrow will be seen, in the rightline,DD'. Thus the entire ima g the arrow will be comprised between D O, DO: its actual size, D D . will be equal to S S'; in other words, to that of the object itself: but it will appear reversed, from right to left

The preceding explanations will suffice for resolving all questions which may arise, relating to the reflection of light and the vision of objects by plane mirrors.

A

As to curved surfaces, whatever may otherwise be their figure, for determining, in general, the apparent place, the form and the magnitude of the images which they reflect, it will suffice to conceive the reflection of each luminous ray to be made upon the plane tangent to the surface, at the point of incidence. Yet in practical use it is unnecessary to ascend to this general law, since we can never employ any other than spherical, concave or convex mirrors, these being the only ones that we can construct and polish, with accuracy; and even in these, to obtain the most perfect images, it is necessary that the luminous rays fall very near perpendicularly upon their surface. We shall therefore confine ourselves, here, to these figures, alone.

Suppose, then, a luminous point, in space, sends forth its rays upon the different parts of a spherical surface, either concave or convex, and selecting one of these rays, let us seek the direction in which it will be reflected.

pig. 4. Suppose M A M', fig. 4,

the spherical mirror, S the luminous point, and S I,

- the incident ray under con-

j^><! * sideration. From the point

R c I to the centre of the sphere

draw the line I C; then take the angle C I R, equal to C I S, and I R will be the direction of the reflected ray.

By proceeding in like manner with all the incident rays emanating from S, we shall find, either by diagram or by calculation, that the reflected rays all fall very nearly together, within a small space denominated the focus, where they form, by their concentration, an image of the point S. Experiment has confirmed the correctness of this theory. 2*

Analogous reasoning and construction show us that the image produced by a convex mirror is always ideal, and is formed behind the mirror, so that, although one may view it, by direct vision, it cannot be exhibited upon ground glass, or a screen.

GENERAL LAWS OP THE REFRACTION OF LIGHT.

We have seen in what manner luminous rays are influenced when reflected from the surface of bodies; and we are now to investigate the action to which those are subject, which traverse the substance of the bodies upon which they fall.

These last, when the angle of incidence is oblique, continue not in a right line, but are made to deviate from that direction; and it is this phenomenon that is called the refraction of liglvt.

Whenever a luminous ray passes obliquely from one medium into another, it is refracted; and the extent of this refraction, or deviation from a right line, depends upon the difference which exists between the densities and the natures of the two mediums. In all bodies not crystallized the refracted ray is simple, and follows the prolongation of the plane of incidence. It deviates more or less from a right line to the common surface of the two mediums according as it passes from the more rare to the more dense, or from the more dense to the more rare of them.

It remains to determine the relation which exists, for each incidence, between the obliquity of the incident rays to the perpendicular, and that of the refracted rays, in order to be able to calculate one of its directions, the other being known.

This brings us to the two following laws, discovered by Descartes:

1st The incident and refracted rays are always comprised in the same plane, perpendicular to the common surface of the two mediums:

2d. The sine of the angle of refraction is to the sine of the angle of incidence in a constant ratio, under all incidences, for the same medium. This is denominated the ratio of refraction.

Refraction is always accompanied by a remarkable phenomenon. The refracted ray is decomposed, and separated into rays of different colours, of which the refrangibility increases from the red ray, where it is the least, to the violet, where it is greatest. This phenomenon is designated, the dispersion of light.

In addition to the seven prismatick colours, experiment has shown, in the refracted ray, calorifick or heating rays, of which the intensity, according to the prismatick arrangement of the colours, augments from the violet to a point beyond the red; and chymical rays, of which the intensity augments in the opposite direction; that is, it is greatest in the red ray, and least at a point beyond the violet.

OF LENSES.

When a luminous ray is received upon a prism of glass it is refracted and caused to approach nearer the base of the prism; conforming also, in other respects, to the laws we are about to explain. Now it is easy to conceive a system, an assemblage of prisms, cut and disposed in such a manner that the rays refracted by them would tend to one and the same point We realize, at once, how important it would be to be able thus to concentrate a great number of luminous rays; but the difficulty of constructing, with sufficient precision, an apparatus of this kind would have opposed great obstacles to the progress of science, if, by unexpected good fortune, it had not been discovered that the necessary principles were all combined in the spherical lens, which, indeed, is no other than an assemblage of prisms, and which may be constructed with exactness and comparative facility.

OP ASTRONOMICAL INSTRUMENTS.

Of these we distinguish several kinds: 1st. Double convex lens, or glass, fig. 5.

Fig. 5. The resemblance, in form, of this

<r -~- --^^ glass to the seed of the lentil origi-

^^- — — -^'nally gave it the name of lens, but

this name has since been indiscriminately applied to all spherical glasses.* Fig. 6.

2d. Plano-convex, fig. 6. 3d. Concavo-convex, or meniscus, fig. 7, and fig. 8.

Fig. 7. Fig. 8.

Fig. 9.

4th. Plano-concave, fig. 9.

5th. Double-concave, fig. 10. (

All these different forms of glasses may be ranked in two classes, according as the base or point of the prism is turned towards the axis of the lens; and as refraction always takes place in a direction towards the base of the prism, the former will cause to converge, and the latter to diverge the luminous rays which fall in a parallel manner upon their surfaces: hence we sometimes designate one class convergent, and the other divergent lenses, or glasses.

It is easy to see how such glasses would assist vision, by correcting the too strong or too feeble convergence of rays for the eyes of persons that are defectively long or short sighted; but our object does not require us to dwell upon this point

* Lens, Latin; Lentille, French; Lentil, English; a kind of plant bearing a pod in which are contained seeds resembling, in shape, the double convex lens, fig. 5.

OP ASTRONOMICAL INSTRUMENTS.

Fig 11.

ssssssss

Suppose an assemblage of parallel rays to fall upon a convex lens, that we may examine more closely this phenomenon. Among the incident rays, S, fig. 11, there is one which coincides with the axis of the lens, and therefore traverses it without refraction. But with the remaining rays this is not so. They all experience a refraction; and this is greater the farther the point at which they enter the lens is situated from its axis; so that the whole, after passing through the glass, are made to converge to the point F. This point is named the focus of the lens. It is evident that the greater the convexity of the lens the stronger will be the refraction, and consequently the nearer will be the focus of the lens to itself. On the other hand, if the luminous rays, after having been assembled at the focus F, are radiated back, to the lens, upon the same angle, they will, after passing through the lens, issue from the opposite surface of it in lines parallel to each other: from which it follows that luminous rays, radiating from any body placed in the focus of such a lens, and so directed as to fall upon all parts of its surface, would experience the same refraction, and issue, after traversing it, in the same parallel manner.