An elementary class book on astronomy

Front matter

Book iLigJ^.

Copyright H?

COPYRIGHT DEPOS1E

Preface

Ev£RY one strives to adapt means to ends, and when the author prepared his large work on Astronomy, he had no other end in view than to teach Astronomy to such as may be competent to the task and fully prepared to learn it. His first aim was to produce a book of the right tone and character, without any regard to the number of persons who might be prepared to use it. That effort was entirely successful, but the book is not adapted to the great mass of pupils, because it requires of the learner considerable mathematical knowledge, and a corresponding discipline of mind, therefore but few persons, comparatively speaking, feel qualified to study that book. At the same time a book of like tone, character, and spirit, is demanded by teachers for the use of their more humole pupils, except that it must be on a lower mathematical plane, and this book is designed to supply that demand.

In this work we have omitted mathematical investigations almost altogether. Yet we have endeavored to retain the spirit of the University edition, and much of the plain matter of fact in that book is the same in this. Some of the more abstruse parts of the science are omitted, and some of the more simple and elementary parts are more enlarged upon in this book, than in that.

Because we have avoided mathematical investigations, and attempted to adapt our work to the common qualifications of pupils, it must not be inferred that we have therefore made an easy text book, one which requires

iii iv PREFACE.

no particular attention on the part of the reader to comprehend. Astronomy is no study for children — the subject admits of no careless reading-, and however good the book, and however well qualified the teacher, the student must take vigorous hold of the study for himself, and, in a measure, take his own way to meet with success.

Although this professes to be but a primary and elementary work, it contains more than a mere statement of astronomical facts, it deduces hidden truths from primary observations, and endeavors to draw out the logical powers of the reader and make him feel the true spirit of the science.

Science properly learned is never forgotten, but science committed to memory soon evaporates, and science cannot be obtained from books and teachers alone, — in addition to the materials, the original perceptions and reasoning powers of the learner, must come in with decided earnestness and force, — and these remarks are particularly applicable to the science of Astronomy.

We make these remarks to impress on the mind of the teacher the necessity of giving perfectly sound instruction, and not be contented with memoritor recitations, or the mere accumulation of facts.

For instance, the sun is nearer to the earth in January than in July, but this fact alone is not science, scarcely knowledge — and it would be only a dead weight to the mind to crowd it into the memory : but when it is ascertained how we know this fact, from what observation it was deduced, and what logical induction was applied, then it becomes another matter, then it is science, and thus learned, could never be lost.

Some elementary works on Astronomy put great stress on pictorial illustrations, — but at best such illustrations are little better than caricatures, and some of them give incorrect impressions ; for instance, in attempting to show the relative motions of the sun and moon in space, by a figure, the moon's motion is generally represented as describing loops, when the true motion is progressively onward, and at all times concave towards the sun.

PREFACE. V

The difficulty of giving true representations on paper, in Astronomy, is so great that the teacher should be careful so to guide the perceptions of the learner, that they be more truthful and refined than the figure can possibly be, or the learner will draw distorted if not erroneous impressions from them. For instance, if we wished to make a correct representation of the sun, earth, and moon, and made the earth but one-eighth of an inch in. diameter, the diameter of the moon's orbit must be 7J^ inches, the diameter of the moon the 32d part of an inch, the diameter of the earth's orbit 3000 inches, or 250 feet, and the diameter of the sun must cover 14 inches. These considerations show the utter impossibility of making correct astronomical representations on paper, for who would have the earth drawn out less than one-eighth of an inch in diameter, and even that small magnitude would require a sheet two hundred and fifty feet wide, on which none of the exterior planets could be drawn. For these reasons we do not place as much value on pictorial representations and astronomical maps as many do, and whenever we make use of such things, as we sometimes do, we take much care that the impressions drawn from them, are not as gross as the representations themselves.

In conclusion, we would remind the reader that the subject of Astronomy is so vast and magnificent, that it is almost as impossible to do justice to it in composition as it is in geometrical diagrams. We have made no pretensions to delineate the high mental satisfaction that a knowledge of this science imparts ; we have only attempted to guide others in attaining that knowledge, and in this particular we do not claim to have made a perfect book, — far from it, — perfection is impossible, decidedly so, when applied to a book ; and if all books were perfect, there would be little need of schools and teachers.

Section II. — Chapter I

First consideration in respect to distance, astronomically speaking, 68 — 70

Distance to the moon — deduced from parallax 76 — 78

CHAPTER II.

Solar parallax 79

How to measure the sun's apparent diameter 80 — 83

Figure of the apparent solar orbit 84

How to find the position of the longer axis of the solar orbit 85 — 86

CHAPTER III.

CHAPTER IV.

Equation of time 92

Causes which produce inequality in apparent time 93 — 95

Equation table, <fec 96 — 98

CHAPTER V.

The apparent motions of the planets 99 — 102

The sun the center of the planetary motions 103 — 104

Copernicus — and his system 105

CHAPTER VI.

The relative distances of the planets from the sun discovered.. . .108 — 110

The times of revolution deduced from observation Ill — 116

Kepler's laws 117

CHAPTER VII.

The transits' of Venus and Mercury 118

The sun's horizontal parallax determined 119 — 123

CHAPTER VIII. The real distance to the sun determined 124

The distance of each planet from the sun determined 125

The magnitudes of the planets discovered 126

CHAPTER IX. A general description of the solar system 127 — 132

A table of the asteroids 133

The velocity of light discovered 134 — 137

General directions to acquire a correct impression of distances

and magnitudes in the solar system 141 — 142

CONTENTS. i x

SECTION III. CHAPTER I. The moon — its periodical revolutions, &c 143 — 144

The lunar cycle, or golden number 145

The figure of the moon's orbit, &c% 146

Libration of the moon 147 — 151

CHAPTER II.

Limits of eclipses from the nodes. ■. 154

Periodical eclipses 156 — 160

Occultations 161

CHAPTER III.

The tides , 162

The obvious connection of the tides and the moon 162

Tides opposite to the moon explained 163 — 164

Mass of the moon determined by the tides 165

The great inequality of the tides explained 166

CHAPTER IV.

On comets, — their general description, &c 167

Comets, planetary bodies 168

Description of particular comets 169 — 172

The probable effect of a comet striking the earth 173

CHAPTER V.

On the peculiarities of the fixed stars 174 — 175

Their immense distance from the earth „ 176

The new star of 1572 176

Double and multiple stars 178

Nebulae — and the milky -way 179

Gradual changes of the stars in right ascension and declination,

caused by the motion of the equator on the ecliptic 179

Proper motion of fixed stars — how determined 180

CHAPTER VI.

Aberration — how discovered — and by whom 181

Aberration illustrated 182

Velocity of light determined by aberration 182 — 183

Aberration — a proof of the earth's annual revolution 184

Nutation — its cause and effects 185

Nutation illustrated 186

Precession of the equinoxes 188

Precession and nutation depend on the same cause, the spheroidal

form of the earth 189—190

X CONTENTS.

Problems on the terrestrial globe 191 — 196

Problems to be solved with or without the use of of the globes. . .196 — 202 Various problems for finding the latitude of the observer, by the

meridian distances of the heavenly bodies 199 — 201

Problems to be solved by the use of the celestial globe 202 — 205

How to find the time of rising and setting of any known

heavenly body 205—206

ASTRONOMY.

Introduction

Astronomy is the science which treats of the heavenly bodies, describes their appearances, determines their magnitudes, and discovers the laws that govern their motions.

Astronomy is divided into Descriptive, Physical, and Practical.

Descriptive Astronomy merely states facts and describes appearances.

Physical Astronomy explains the causes which bring about the known results. It investigates the laws which govern the celestial motions.

Practical Astronomy includes observations, and all kinds of astronomical computations, such as the distances and magnitudes of the planets, the times of their rising, setting, and coming in conjunction, opposition, &c. &e.

By astronomical observations men can determine the position of a ship on sea, and this branch of astronomy is called Nautical Astronomy, and thus geography and astronomy are combined, and no one can fully understand geography without some aid from astronomy.

Astronomy is the most ancient of all sciences, for the people could not avoid observing the successive returns of day and

Define Astronomy. How is Astronomy divided ? What is Descriptive Astronomy ? "What is Physical ? What is Practical ? Are Geography and Astronomy connected ? How ? Is Astronomy a modern science ? Why is it allowed to be the most ancient science ?

night, and of summer and winter. They could not fail to perceive that short days corresponded to winter, and long days to summer ; and thus the operations of agriculture, were in a measure connected with astronomy.

DEFINITION OF TERMS.

Every science has its technicalities and conventional terms ; and astronomy is by no means an exception to the general rule ; and as it will prepare the way for a clearer understanding of our subject, we now give a short list of some of the technical terms, which must be used in our composition.

Horizon. — Every person, wherever he may be, conceives himself to be in the center of a circle ; and the circumference of that circle is where the earth and sky apparently meet. That circle is called the horizon; it is completely visible, however, only on a smooth sea ; on land it is more or less broken and obscured by mountains and hills.

The mathematical and astronomical horizon is a plane passing through the center of the earth, and a plumb line from the observer is perpendicular to it.

No two places have the same horizon.

Sphere. — A sphere, as all well know, is a perfect bail, and the surface of the heavens appears to all as a perfect concave sphere. Around every sphere in any direction is three hundred and sixty degrees, written thus 360°.

Poles. — The center of any circle round a sphere, which is also on the sphere, is called the pole of that circle; and every circle round a sphere has its poles 90° distant from the circle.

Zenith. — The zenith of any place is the point directly over head ; and the nadir is directly opposite to the zenith, or under our feet. The zenith and nadir are the poles to the horizon.

Meridian. — When the sun (or other celestial object) in its diurnal course attains its highest possible elevation above the

What is meant by the horizon? Is the horizon every \rhere visible? Why ? What is the true astronomical horizon ? Explain what is meant by a Sphere ? roles ? Degrees ? Zenith ? Meridian ?

INTRODUCTION. 13

horizon, it is said to be on the meridian, and the direction to that object just at that moment, is said to be south or north, according to the locality of the observer : hence, a meridian is an imaginary line, north and south from any point or place, whether it is conceived to run along the earth or through the heavens. If the meridian is conceived to divide both the earth and the heavens, it is then considered as a plane, and is spoken of as the plane of the meridian.

Altitude. — The altitude of any celestial object is its perpendicular distance from the horizon, measured in degrees, there being 90° from the horizon to the zenith.

The altitude and zenith distance of a body always make 90°.

Verticals. — All lines passing from the zenith, perpendicular to the horizon, are called Verticals, or Vertical Circles. The one passing at right angles to the meridian, and striking the horizon at the east and west points, is called the Prime Vertical.

Azimuth. — The angular position of a body from the meridian, measured on the circle of the horizon, is called its Azimuth.

The angular position, measured from its prime vertical, is called its Amplitude.

The sum of the azimuth and amplitude must always make 90 degrees.

When the days and nights are equal all over the earth, which is observed to be the fact about the 20th of March and the 23d of September of each year ; — then the sun in its diurnal motion appears to describe a great circle about the earth, which is called the equaior.

Then the sun (loosely speaking) appears to rise directly in the east and set directly in the west, as seen from all places.

The Earth's Equator. — The Earth' s Equator is a great circle, east and west, and equi-distant from the poles, dividing the earth into two hemispheres, a northern and a southern.

What is the altitude of any celestial object ? How is altitude measured ? What are vertical circles ? Through what point do all vertical circles pass ? What is meant by Azimuth ? What by Amplitude ? What do you under stand by the Earth's Equator ?

H ELEMENTARY ASTRONOMY.

The Celestial Equator is the plane of the earth's equator conceived to extend into the heavens.

When the sun, or any other heavenly body, meets the celestial equator, it is said to be in the Equinox, and the equatorial line in the heavens is called the Equinoctial.

Latitude. — The latitude of any place on the earth, is its distance from the equator, measured in degrees on the meridian, either north or south.

If the measure is toward the north, it is north latitude ; if toward the south, south latitude.

The distance from the equator to the poles is 90 degrees — one-fourth of a circle ; and we shall know the circumference of the whole earth whenever we can find the absolute length of one degree on its surface.

Co-Latitude. — Co-latitude is the distance, in degrees, of any place from the nearest pole.

The latitude and co-latitude (complement of the latitude) must, of course, always make 90 degrees.

Parallels of latitude are small circles on the surface of the earth, parallel to the equator.

Every point, in such a circle, has the same latitude.

Longitude. — The longitude of a place, on the surface of the earth, is the inclination of its meridian to some other meridian which may be chosen to reckon from. English astronomers and geographers take the meridian which runs through Greenwich Observatory, as the zero meridian.

Other nations generally take the meridian of their principal observatories, or that of the capital of their country, as the first meridian ; but this is national vanity, and creates only trouble and confusion : it is important that the whole world

What do you understand by the Celestial Equator ? "What is latitude on the earth ? How is it measured ? How many degrees are there from the equator to the pole ? If the earth were larger, would there be more degrees from the equator to the pole? What is longitude? What meridian is it reckoned from ? Why that meridian ? Could it have been from anv other ?

INTRODUCTION. 15

should agree on someone meridian, from which to reckon longitude ; but as nature has designated no particular one, it is not wonderful that different nations have chosen different lines.

In this work, we shall adopt the meridian of Greenwich as the zero line of longitude, because most of the globes and maps, and all the important astronomical tables, are adapted to that meridian, and we see nothing to be gained by changing it.

Declination. — Declination refers only to the celestial equator, and is a leaning or declining, north or south of that line, and it is similar to latitude on the earth.

Solsticial points. — The points, in the heavens, north and south, where the sun has its greatest declination, are the solsticial points.

The northern point we call the Summer Solstice, and the southern point the Winter Solstice; the first is in longitude 90°, the second in longitude 270°.

As latitude is reckoned north and south, so longitude is reckoned east and west ; but it would add greatly to systematic regularity, and tend much to avoid confusion and ambiguity in computations, were this mode of expression abandoned, and longitude invariably reckoned westward, from to 360 degrees.

Declination in the heavens is similar to latitude on the earth. If a person were in 20° north latitude when the sun's declination was 20° north, the sun at noon would then be in his zenith, or pass directly over his head.

Ecliptic. — The ecliptic is a great circle in the heavens, along which the sun appears to pass in a year, extending from about 23£ degrees of south declination to about 23|- degrees of north declination in the opposite longitude of the heavens. This circle is called the ecliptic, because all eclipses, both of the sun and moon, take place when the moon is either in or near it.

What is decimation ? What is the declination of the north pole ? What are the solsticial points ? In what latitude are they ? What circle is the sun always in ? Why is that circle called the ecliptic ?

Equator and Ecliptic. — The celestial equator and the ecliptic are two great circles, in the heavens, which intersect each other (at the present day) by an angle of about 23° 27' 32".

The sun, in its apparent annual motion, runs round the heavens, crosses the equator from the south to the north on the 20th of March of each year, and re-crosses from the north to the south on the 23d of September.

The point on the ecliptic where the sun mee.ts the celestial equator in the spring, is taken as the zero point from which to reckon longitude, in astronomy, eastward along the ecliptic. This point is called the Vernal Equinox.

From the same point eastward along the equator is reckoned right ascension, and it is counted from degrees to 360 degrees, or from hours to 24 hours ; 15 degrees of arc corresponding to one hour in time.

Zodiac. — Ancient astronomers defined the zodiac to be a space in the heavens sixteen degrees wide, eight degrees on each side of the ecliptic, and quite round the sphere : the ecliptic was therefore the center of the zodiac. The ecliptic, or zodiac, was divided into twelve signs, called signs of the zodiac, each sign was therefore 30° in extent.

The first sign, commencing at the vernal equinox, is called Aries, and the character denoting it is written thus r jf'.

The sun enters the 12 signs as follows :

Aries (°f) on the 20th of March ; Taurus (\j) on the 19th of April ; Gemini ( J[) on the 20th of May ; Cancer (6§) on the 21st of June ; Leo (<Q) on the 22d of July ; and Virgo{W) on the 22d of August.

The foregoing are called northern signs, because the sun must have north declination while the sun is in them.

The following are designated as the southern signs of the

Does the ecliptic intersect the equator? At what angle ? "What point in the heavens is called the Vernal Equinox ? What is the Zodiac ? What ia meant by the signs of the Zodiac ? When does the sun enter the first sign of the Zodiac ?

INTRODUCTION. 17

zodiac, because the sun must have south declination while he is in them :

The sun enters Libra (lqj) on the 23d of September; Scorpio (T)1 ) on the 22d of October ; Sagittarius (^[) on the 22d of November ; Capvicomus (/5) on ^ ne ^lst °f December ; Aquarius (*») on the 20th of January ; and Pisces ()-() on the 19th of February. Passing through this last sign the sun again enters (^p) on the 20th of March, to perform the revolution over again, and thus it goes on year by year.

The zodiac and signs of the zodiac being but the offspring of astrology and heathen mythology, they are entirely discarded by modern astronomers ; yet they still linger in country almanacs and in many school books, and it is with reluctance that we even mention them. They are of no use, even as points of reference, and they embrace no scientific principle whatever.

Conjunction. — When two celestial bodies have the same longitude, they are said to be in conjunction.

When two celestial bodies have the same right ascension, that is, come to the meridian at the same time, they are said to be in conjunction in right ascension.

Opposition. — When two celestial bodies have a difference of longitude of 1 80 degrees, they are said to be in opposition.

Direct. — Direct, in astronomy, is a motion to the eastward among the stars.

Retrograde. — Retrograde is a motion to the westward among the stars. Stationary means apparently so in respect to the stars. Other terms not here mentioned will be explained as we use them.

Why are some of the signs called northern, and others southern signs ? Is the distinction of signs necessary ? What is meant by conjunction ? By opposition ? What by direct ? Retrograde ?

Chapter II. Preliminary Observations

To commence the study of astronomy, we must observe and call to mind the real appearance of the heavens.

Take such a station, any clear night, as will command an extensive view of that apparent, concave hemisphere above us, which we call the sky, and fix well in the mind the directions of north, south, east, and west.

At first, let us suppose the observer to be somewhere in the United States, or somewhere in the northern hemisphere, about 40 degrees from the equator.

Soon he will perceive a variation in the position of the stars : those at the east of him will apparent!} 7 rise ; those at the west will appear to sink lower, or fall below the horizon ; those at the south, and near his zenith, will apparently move westward ; and those at the north of him, which he may see about half way between the horizon and the zenith, will appear stationary.

Let such observations be continued during all the hours of the night, and for several nights, and the observer cannot fail to be convinced that not only all the stars, but the sun, moon, and planets, appear to perform revolutions, in about twenty-four hours, round a fixed point ; and that fixed point, as appears to us (in the middle and northern part of the United States), is about midway between the northern horizon and the zenith.

It should always be borne in mind that these motions are

How can a person convince himself that seme of the stars have an apparent motion from east to west, like the sun in the day time ? Do all the stars have such an apparent motion, as seen from this place ? "What stars do not?

but apparent, the stars keeping the same positions with respect to each other, whether they are rising or falling, or north or south of the observer, and the general aspect of the heavens is the same now, as it was in the very earliest ages of astronomy, and will be the same in ages to come.

All the heavenly bodies, whether sun, moon, planets, or stars, appear to have a diurnal motion round a fixed 'point, and all those stars which are 90 degrees from that point, apparently describe a great circle. Those stars which are nearer to the fixed point than 90 degrees, describe smaller circles; and the circles are smaller and smaller as the objects are nearer and nearer the fixed point.

There is one star so near this fixed point, that the small circle it describes, in about 24 hours, is not apparent from mere inspection. To detect the apparent motion of this star, we must resort to nice observations, aided by mathematical instruments.

This fixed point, that we have several times mentioned, is the Worth Pole of the heavens, and this one star that we have just mentioned, is commonly called the North Star, or the Pole Star.

As the North Star appears stationary, to the common observer, it has always been taken as the infallible guide to direction; and every sailor of the ocean, and every wanderer of the African and Arabian deserts, has held familiar acquaintance with it.

If our observer now goes more to the southward, and makes the same observations on the apparent motions of the stars, he will find the same general results ; each individual star will describe the same circle ; but the pole, the fixed point, will be lower down, and nearer to the northern horizon ; and it will be

Those stars that do not rise and set, what motion, real or apparent, do they have ? Is any star apparently stationory ? Is there a star just at the north pole ? What use has been made of the north star ? If a person should go south from this place, "what apparent effect "would that have on the north star, as viewed by him ?

lower and lower in proportion to the distance the observer goes to the south. After the observer has gone sufficiently far, the fixed point, the pole, will no longer be up in the heavens, but down in the northern horizon ; and when the pole does appear in the horizon, the observer is at the equator, and from that line all the stars at or near the equator appear to rise up directly from the east, and go down directly to the west ; and all other stars, situated out of the equator, describe their small circles parallel to this perpendicular equatorial circle.

If the observer goes south of the equator, the north pole will sink below his horizon, and the south polar point will appear to rise up above his horizon, and it will rise more and more as he goes farther and farther south ; and if he could possibly get to the south pole on the earth, the south pole of the apparent revolving heavens would be right over his head, and the equator of the heavens would bound his horizon.

In a similar manner if an observer goes north, the north pole to him would appear to rise in the heavens; and should he continue to go north, he would finally find the pole in his zenith, and all the stars would apparently make circles round the zenith, as a center, and parallel to the horizon; and the horizon itself would be the celestial equator.

When the north pole of the heavens appears at the zenith, the observer must then be at the north pole, on the earth, or at the latitude of 90 degrees.

Any celestial body, which is north of the equator, is always visible from the north pole of the earth ; hence, the sun, which is north of the equator from the 20th of March to the 23d of September, must be constantly visible during that period, in a clear sky.

Just as the sun comes north of the equator, its diurnal progress, or rather, the progress of 24 hours, is around the horizon. When the sun's declination is 10 degrees north of the equator,

What is the apparent diurnal motion of the stars, as seen from the equator ? "What from the south pole ? What part of the heavens bounds the horizon as seen from the south pole ? What from the north pole ?

the progress of the sun, in 24 hours, as seen from the north pole, is around the horizon at an altitude of about 10 degrees ; and so on for any other degree.

From the north pole, all directions on the surface of the earth are south. North, strictly speaking, would be in a vertical direction, which would make the absolute south directly down towards the center of the earth.

We have observed that the pole of the heavens rises as we go north, and sinks toward the horizon as we go south ; and when we observe that the pole has changed its position ona degree, in relation to the horizon, we know that we must have changed place one degree on the surface of the earth.