Astronomy for everybody; a popular exposition of the wonders of the heavens

Preface

The present work grew out of articles contributed to McClure's Magazine a few years since on the Unsolved Problems of Astronomy, Total Eclipses of the Sun, and other subjects. The interest shown in these articles suggested an exposition of the main facts of astronomy in the same style. The result of the attempt is now submitted to the courteous consideration of the reader.

The writer who attempts to set forth the facts of astronomj" without any use of technical language finds himself in the dilemma of being obliged either to convey only a very imperfect idea of the subject, or to enter upon explanations of force and motion which his reader ma3^ find tedious. In grappling with this difficulty the author has followed a middle course, trying to present the subject in such a way as to be intelligible and interesting to every reader, and entering into technical explanations onh'- when necessary to the clear understanding of such matters as the measure of time, the changes of the seasons, the varying positions of the constellations, and the aspects of the planets. It is hoped that the reader who does not wish to master these subjects will find enough to iaterest him in the descriptions and illustrations of celestial scenery to which the bulk of the Avork is devoted.

The author is indebted to Mr. Secretary Langley, of the Smithsonian Institution, for the use of the picture which forms the frontispiece.

Simon Newcomb.

Washington, October, 1902.

Part I. The Celestial Motion's Page

I. A View of the Universe 3

What tlie universe is 6

A model of the universe = 7

n. Aspects of the Heavens 9

Apparent daily revolution of the stars o 12

Changes in the motions as we journey south 15

rH. Eelation of Time and Longitude 19

Local time 21

Standard time 21

Where the day changes o 23

rV. How the Position of a Heavenly Body is Defined ^5

Circles of the celestial sphere 27

V. The Annttal Motion of the Earth and its Eesults « . . 31

The sun's apparent path in the sky 32

The ecliptic 33

The equinoxes and solstices 37

The seasons 39

Eelations between real and apparent motions summed

up 39

The year and the precession of the equinoxes 41

PART IL ASTRONOMICAL INSTRUMENTS

I. The Eefracting Telescope 47

The lenses of a telescope 48

The image of a distant object 51

Power and defects of a telescope 53

Mounting of the telescope 55

The making of telescopes 59

Fraunhof er and Alvan Clark 61 vili ■ CONTENTS

PAttK

n. The Eeflecting Telescope 67

in. The Photographic Telescope „ 71

IV. The Spectroscope 73

Nature and wave length of light 74

The spectrum 75

How the stars are analysed • 76

V. Other Astronomical Instruments 79

The meridian circle and clock 80

How the position of stars are determined. 81

Part III. The Sun, Earth, And Moon

I. An Introductory Glance at the Solar System « . 87

II. The Sun 91

General description 91

Rotation of the sun 93

Density and gravity 94

Spots on the sun 95

TheFaculae 98

The prominences and chromosphere 99

How the sun is made up 101

The source of the sun's heat. 103

III. The Earth 107

Measuring the earth 107

The earth's interior 108

Its gravity and density Ill

Variations of latitude 114

The atmosphere 116

IV. The Moon 119

Distance of the moon 120

Revolution and phases 120

The surface of the moon 123

Is there air or water on the moon ? 127

Rotation of the moon 128

How the moon produces the tides 129

V. Eclipses of the Moon 133

The nodes of the moon's orbit 134

Eclipse seasons » . . 134

How an eclipse of the moon looks 136

CONTENTS ix

PAGE

VI. Eclipses of the Sun ^ 139

Central, total, and annular eclipses 140

Beanty of a total eclipse 141

Ancient eclipses 143

Prediction of eclipses 144

The sun's appendages » 145

The corona 147

Part IV. The Planets And Their Satellites

I. Orbits and Aspects of the Planets 151

Distances of the planets 154

Bode's law 154

Kepler's laws 155

II. The Planet Mercury 157

Surface and rotation of Mercury 159

Observations of Schroter, Herschel, Schiaparelli, and

Lowell , 160

Phases of Mercury 161

Transits of Mercury . . „ 162

m. The Planet Venus » 167

The morning and evening star 167

Eotation of Venus 169

Atmosphere of Venus 172

Has Venus a satellite ? 174

Transits of Venus . , 175

IV. The Planet Mars 177

Distance, dates of opposition, etc 178

Surface and rotation of Mars 179

The canals of Mars 180

Probable nature of the channels 184

The atmosphere of Mars 185

Supposed winter snowfall near the poles 187

The satellites of Mars 188

Y. The Group of Minor Planets 191

Discovery of Ceres 191

Hunting asteroids , 192

Orbits of the asteroids 194

Groupings of the orbits 195

The most curious of the asteroids 198

X CONTENTS

PAGE

VI. Jupiter and its Satellites 201

Aspect of Jupiter 201

Surface 202

Constitution 205

Eotation 206

Eesemblance of Jupiter to the sun : 206

The satellites of Jupiter 208

VII. Saturn and its System 213

Aspects of Saturn , 213

Satellites of Saturn 214

Varying aspects of Saturn's rings 215

What the rings are 219

System of Saturn's satellites 220

Physical constitution of Saturn 224

VIII. Uranus and its Satellites 225

Discovery of Uranus 225

Old observations 226

Constitution of the planet 227

Its satellites 227

IX. Neptune and its Satellite 231

History of the discovery of Neptune 282

Satellite of Neptune 235

X. How the Heavens are Measured 237

Parallax 237

Measurement by the motion of light 239

Measurement by the sun's gravitation 240

Eesults of measurements of the sun's distance 242

XI. Gravitation and the Weighing of the Planets 243

Accuracy of astronomical predictions based on the

theory of gravitation 244

Part V. Comets And Meteohig Bodies

I. Comets 255

Description of a comet 255

Orbits of comets ; 257

Halley's comet 260

Comets which have disappeared 262

CONTENTS xi

PAGE

Encke's comet 264

Capture of comets by Jupiter o . . . 265

Wlience come comets ? 266

Nature of comets 274

II. Meteoric Bodies 277

Meteors 277

Cause of meteors 278

Meteoric showers 279

Connection of comets and meteors , . . . , 281

The zodiacal light 283

The impulsion of light 286

I. General Eeview '. 291

Stars and nebulae :. 293

Spectra of the stars , 293

Density and heat of the stars 296

II. Aspect of the Sky 299

The Milky Way.'. ". 299

---- Brightness of the stars , 300

Number of stars 301

'Colours 303

Collection into constellations 303

III. Description of the Constellations 305

To find the sidereal time 306

The northern constellations 307

The autumnal constellations , 309

The winter constellations , ; . -. ... 313

The spring constellations ^ 316

The summer constellations -. ; . . ' 317

TV. The Distances of the Stars ,. . . 321

V. The Motions op the Stars. , . . ." 325

VI. Variable and Compound Stars 329

List of Illustrations

PAGE

Total Eclipse of the Sun of May 29, 1900. Photographed by the

party of the Smithsonian Institution Frontispiece

The Celestial Sphere as it appears to us 13

The Northern Sky and the Pole Star 16

Circles of the Celestial Sphere 27

The Sun Crossing the Equator about March Twentieth 33

The Orbit of the Earth and the Zodiac 33

How the Obliquity of the Ecliptic Produces the Changes of Seasons 35 Apparent Motion of the Sun along the Ecliptic in Spring and

Summer 36

Apparent Motion of the Sun from March till September 37

Precession of the Equinoxes 43

Section of the Object-glass of a Telescope 50

Axes on which a Telescope turns , ....... 57

Great Telescope of the Yerkes Observatory 65

Section of a Newtonian Eeflecting Telescope 69

Wave Length of Light 74

Arrangement of the Colours of the Spectrum 75

A Meridian Instrument 80

Appearance of a Sun-spot 96

Frequency of Sun-spots in Different Latitudes on the Sun ...... 97

Eevolution of the Moon Eound the Earth , 121

Mountainous Surface of the Moon , 124

Showing how the Moon would Move if it did not Eotate on its

Axis 129

How the Moon Produces Two Tides in a Day 131

The Moon in the Shadow of the Earth 133 xiv LIST OF ILLUSTRATIONS

PAGE

Passage of the Moon through the Earth's Shadow 136

The Shadow of the Moon Thrown on the Earth during a Total

Eclipse of the Sun 139

The Moon Passing Centrally over the Sun during an Annular

Eclipse 140

Orbits of the Four Inner Planets ■ 152

Conjunctions of Mercury with the Sun 158

Elongations of Mercury 159

Phases of Venus in Different Points of its Orbit 168

Effect of the Atmosphere of Venus during the Transit of 1882. . 172 Map of Mars and its Canals as drawn at the Lowell Observatory 181 Drawings of Lacus Solis on Mars, by Messrs. Campbell and

Hussey 188

Separation of the Minor Planets into Groups 195

Distribution of the Orbits of the Minor Planets 196

Telescopic Views of Jupiter, one with the Shadow of a Satellite

Crossing the Planet 204

Perpendicular View of the Eings of Saturn. 216

Showing how the Direction of the Plane of Saturn's Rings remains Unchanged 217

Disa^jpearance of the Rings of Saturn, according to Barnard,

when seen edgewise 218

Orbits of Titan and Hyperion, showing their relation , . . . 222

Measure of the Distance of an Inaccessible Object by Triangula-

tion 237

Parabolic Orbit of a Comet 257

Donati's Comet, as drawn by G. P. Bond 268

Head of Donati's Comet, drawn by G. P. Bond 270

Great Comet of 1859, drawn by G. P. Bond 272

The Zodiacal Light in February and March 284

Ursa Major, or The Dipper 307

Ursa Minor 308

Cassiopeia 308

J^yra, the Harp 311

LIST OF ILLUSTRATIONS xv

PAGE

The Hyades 313

The Pleiades, as seen with the naked eye 313

Telescopic View of the Pleiades, with Names of the Brighter Stars 314

Orion 316

The Northern Crown 317

Aquila 318

Delphinns, the Dolphin 318

The Great Cluster of Hercules, photographed at the Lick Observatory 319

Scorpius, the Scorpion 320

Measurement of the Parallax of a Star , 322

Arcturus and the Surrounding Stars in Constellation Bootes 328

PART I A View of the Universe

Let us enter upon our subject by taking a general view of this universe in which we live, fancying ourselves looking at it from a point without its limits. Far away, indeed, is the point we must choose. To give a conception of the distance, let us measure it by the motion of light. This agent, darting through 186,000 miles in every second, would make the circuit of the earth several times between two ticks of a watch. The standpoint which we choose will probably be well situated if we take it at a distance through which light would travel in 100,000 years. So far as we know, we should at this point find ourselves in utter darkness, a black and starless sky surrounding us on all sides. But, in one direction, we should see a large patch of feeble light spreading over a considerable part of the heavens like a faint cloud or the first glimmer of a dawn. Possibly there might be other such patches in different directions, but of these we know nothing. The one which we have mentioned, and which we call the universe, is that which we are to inspect. We therefore fly toward it — how fast we need not say. To reach it in a month we should have to go a million times as fast as light. As we approach, it continually spreads out over more of the black sky, which it at length half covers, the region behind us being still entirely black.

Before reaching this stage we begin to see points of light glimmering here and there in the mass. Continuing our course, these points become more numerous, and seem to move past us and disappear behind us in the distance, wliile new ones continually come into view in front, as the passengers on a railway train see landscape and houses flit by them. These are stars, wliich, when we get well in among them, stud the whole heavens as we see them do at night. We might pass through the whole cloud at the enormous speed we have fancied, without seeing anything but stars and, perhaps, a few great nebulous masses of foggy light scattered here and there among them.

But instead of doing this, let us select one particular star and slacken our speed to make a closer inspection of it. This one is rather a small star; but as we approach it, it seems to our eyes to grow brighter. In time it shines like Venus. Then it casts a shadow; then we can read by its light; then it begins to dazzle our eyes. It looks like a little sun. It is the Sun !

Let us get into a position which, compared with the distances we have been travelling, is right alongside of the sun, though, expressed in our ordinary measure, it may be a thousand million miles away. Now, looking down and around us, we see eight star-like points scattered around the sun at different distances. If we watch them long enough we shall see them all in motion around the sun, completing their circuit in times ranging from three

A MEW OF THE UNIVERSE 5

months to more than 160 3'cars. They move at very different distances; tlie most distant is seventy times as far as tlie nearest.

These star-hke bodies are the planets. By careful examination we see that they differ from the stars in being opaque bodies, shining only by light borrowed from the sun.

Let us pay one of them a visit. We select the third in order from the sun. Approaching it in a direction which we may call from above, that is to say from a direction at right angles to the line drawn from it to the sun, we see it grow larger and brighter as we get nearer. When we get very near, we see it looking like a half-moon — one hemisphere being in darkness and the other illuminated by the sun's rays. As we approach 3"et nearer, the illuminated part, always growing larger to our sight, assumes a mottled appearance. Still expanding, this appearance gradually resolves itself into oceans and continents, obscured over perhaps half their surface by clouds. The surface upon which we are looking continually spreads out before us, filling more and more of the sky, until we see it to be a world. We land upon it, and here we are upon the earth.

Thus, a point which was absolutely invisible while we were flying through the celestial spaces, which became a star when we got near the sun, and an opaque globe when yet nearer, now becomes the world on which we live.

This imaginary flight makes known to us a capital fact of astronomy: The great mass of stars which stud the heavens at night are suns. To express the idea in another way, the sun is merely one of the stars. Compared with its fellows it is rather a small one, for we know of stars that emit thousands or even tens of thousands of times the light and heat of the sun. Measuring things simply by their intrinsic importance, there is nothing special to distinguish our sun from the hundreds of millions of its companions. Its importance to us and its comparative greatness in our eyes arise simply from the accident of our relation to it.

The great universe of stars which we have described looks to us from the earth just as it looked to us during our imaginary flight through it. The stars which stud our sky are the same stars which we saw on our flight. The great difference between our view of the heavens and the view from a point in the starry distances is the prominent position occupied by the sun and planets. The former is so bright that during the daytime it completely obliterates the stars. If we could cut off* the sun's rays from any very wide region, we should see the stars around the sun in the daytime as well as by night. These bodies surround us in all directions as if the earth were placed in the centre of the universe, as was supposed by the ancients.

What the Universe Is

We may connect what we have just learned about the the universe at large with what we see in the heavens. What we call the heavenly bodies are of two classes. One of these comprises the millions of stars the arrangement and appearance of which we have just described.

WHAT THE UNIVERSE IS 7

The other comprises a single star, which is for us the most important of all, and the bodies connected with it. This collection of bodies, with the sun in its centre, forms a little colony all by itself, which we call the solar system. The feature of this system which I wish first to impress on the reader's mind is its very small dimensions when compared with the distances between the stars. All around it are spaces which, so far as we yet know, are quite void through enormous distances. If we could fly across the whole breadth of the system, we should not be able to see that we were any nearer the stars in front of us, nor would the constellations look in any way different from what they do from our earth. An astronomer armed with the finest instruments would be able to detect a change only by the most exact observations, and then only in the case of the nearer stars.

A conception of the respective magnitudes and distances of the heavenly bodies, which will help the reader in conceiving of the universe as it is, may be gained by supposing us to look at a little model of it. Let us imagine that, in this model of the universe, the earth on which we dwell is represented by a grain of mustard seed. The moon will then be a particle about one fourth the diameter of the grain, placed at a distance of an inch from the earth. The sun will be represented by a large apple, placed at a distance of forty feet. Other planets, ranging in size from an invisible particle to a pea, must be imagined at distances from the sun varying from ten feet to a quarter of a mile. We must then imagine all these little objects to be slowly moving around the sun at their respective distances, in times varying from three months to 160 years. As the mustard seed performs its revolution in the course of a year we must imagine the moon to accompany it, making a revolution around it every month.

On this scale a plan of the whole solar system can be laid down in a field half a mile square. Outside of this field we should find a tract broader than the whole continent of America without a visible object in it unless perhaps comets scattered around its border. Far beyond the limits of the American continent we should find the nearest star, which, hke our sun, might be represented by a large apple. At still greater distances, in every direction, would be other stars, but, in the general average, they would be separated from each other as widely as the nearest star is from the sun. A region of the little model as large as the whole earth might contain only two or three stars.

We see from this how, in a flight through the universe, like the one we' have imagined, we might overlook such an insignificant little body as our earth, even if we made a careful search for it. We should be like a person flying through the Mississippi Valley, looking for a grain of mustard seed wliich he knew was hidden somewhere on the American continent. Even the bright shining apple representing the sun might be overlooked unless we happened to pass quite near it.

II Aspects of the Heavens

The immensity of the distances wLich separate us from the heavenly bodies makes it impossible for us to form a distinct conception of the true scale of the universe, and very difficult to conceive of the heavenly bodies in their actual relations to us. If, on looking at a body in the sky, there were any way of estimating its distance, and if our eyes were so keen that we could see the minutest features on the surface of the planets and stars, the true structure of the universe would have been obvious from the time that men began to study the heavens. A little reflection will make it obvious that if we could mount above the earth to a distance of, say. ten thousand times its diameter, so that it would no longer have any perceptible size, it would look to us, in the light of the sun, like a star in the sky. The ancients had no conception of distances like this, and so supposed that the heavenly bodies werej> as they appeared, of a constitution totally different from that of the earth. We ourselves, looking at the heavens, are unable to conceive of the stars being millions of times farther than the planets. All look as if spread out on one sky at the same distance. We have to learn their actual arrangement and distances by reason.

It is from the impossibility of conceiving these enormous differences in the distances of objects on the earth and the heavens, that the real difficulty of forming a mental picture of them in their true relation arises. I shall ask the reader's careful attention in an attempt to present these relations in the simplest way, so as to connect things as they are with things as we see them.

I^et us suppose the earth taken away from under our feet, leaving us hanging in mid space. We should then see the heavenly bodies — sun, moon, planets, and stars — surrounding us in every direction, up and down, east and w^est, north and south. The eye would rest on nothing else. As we have just explained, all these objects would seem to us to be at the same distance.

A great collection of points scattered in every direction at an equal distance from one central point, must all lie upon the inner surface of a hollow^ sphere. It follows that, in the case supposed, the heavenly bodies will appear to us as if set in a sphere in the centre of which we appear to be placed. Since one of the final objects of astronom}" is to learn the directions of the heavenly bodies from us, this apparent sphere is talked about in astronomy as if it were a reality. It is called the celestial sphere. In the case we have supposed, with the earth out of the way, all the heavenly bodies on this sphere would at any moment seem at rest. The stars would remain apparently at rest day after day and week after w^eek. It is true that, by watching the planets, we should in a few days or weeks, as the case might be, see their slow motion around the sun, but this would not be perceptible at once. Our first impression would be that the

ASPECTS OF THE HEAVENS 11

sphere was made of some solid, crystalline substance, and that the heavenly bodies were fastened to its inner surface. The ancients had this notion, which they brought yet nearer the truth by fancying a number of these spheres fitting inside of each other to represent the different distances of the heavenly bodies.

With this conception well in mind, let us bring the earth back under our feet. Now we have to make a draft upon the reader's power of conception. Considered in its relation to the magnitude of the heavens, the earth is a mere point ; 3^et, when we bring it into place, its surface cuts off one half of the universe from our view, j ust as an apple would cut off the view of one side of a room from an insect crawling upon it. That half of the celestial sphere which, being above the horizon, remains visible is called the visible hemisphere; the half below, the view of which is cut off by the earth, is called the invisible hemisphere. Of course we could see the latter by travelling around the earth.

Having this state of things well in mind, we must make another draft on the reader's attention. We know that the earth is not at rest, but revolves unceasingly around an axis passing through its centre. The natural result of this is an apparent rotation of the celestial sphere in the opposite direction. The earth rotates from west toward east ; hence the sphere seems to rotate from east toward west. This real revolution of the earth, with the apparent revolution of the stars which it causes, is called the diurnal motion, because it is completed in a day.

Appaj'ent Daily BcroJuiion of the Stars

Our next problem is to sliow the connection between the very simple conception of the rotation of the earth and the more complicated appearance presented by the apparent diurnal motion of the heavenly bodies which it brings about. The latter varies with the latitude of the observer upon the earth's surface. Let us begin with its appearance in our middle northern latitudes.

For this purpose we may in imagination build a hollow globe representing the celestial sphere. We may make it as large as a Ferris wheel, but one of thirty or forty feet in diameter would answer our purpose. Let Figure 1 be an inside view of this globe, mounted on two pivots, P and Q, so that it can turn round on them diagonally. In the middle, at O, we have a horizontal platform, NS, on which we sit. The constellations are marked on the inside of the globe, covering the whole surface, but those on the lower half are hidden from view by the platform. This platform, as is evident, represents the horizon.

The globe is now made to turn on its pivots. What will happen? We shall see the stars near the pivot P revolving around the latter as the globe turns. The stars on a certain circle KN will graze the edges of the platform, as they pass below P. Those jet farther from P will dip below the platform to a greater or less extent, according to their distance from P. Stars near the circle EF, halfway between P and Q, will perform half their course above, and half below the platform. Finally,

REVOLUTION OF THE STARS

IS stars within the circle ST will never rise ahove the level of the platform at all, and will remain invisible to us.

To our eves the celestial sphere is such a globe as this, of infinite dimensions. It seems to us to be continually

Fig. 1. — The Celestial Sphere as it appears to us.

revolving round a certain point in the sky as a pivot, making one revolution in nearl}^ a day, and carrying the sun, moon, and stars wdth it. The stars preserve their relative positions as if fastened to the revolving celestial sphere. That is to say, if we take a photograph of them at any hour of the night, the same photograph will show their appearance at any other hour, if we only hold it in the right position.

The pivot corresponding to P is called the north celestial pole. To dwellers in middle northern latitudes, where most of us live, it is in the northern sky, nearly midway between the zenith and the northern horizon. The farther south we live, the nearer it is to the horizon, its altitude above the latter being equal to the latitude of the place where the observer stands. Quite near it is the pole star, which we shall hereafter show how to locate. To ordinary observation, the pole star seems never to move from its position. In our time it is little more than a degree from the pole, a quantity with which we need not now concern ourselves.

Opposite the north celestial pole, and therefore as far below our horizon as the north one is above it, lies the south celestial pole.

An obvious fact is that the diurnal motion as we see it in our latitude is oblique. When the sun rises in the east it does not seem to go straight up from the horizon, but moves over toward the south at a more or less acute angle with the horizon. So when it sets, its motion relative to the horizon is again oblique.

Now, imagine that we take a pair of compasses long enough to reach the sky. We put one point on the sky at the north celestial pole, and the other point far enough from it to touch the horizon below the pole. Keeping the first point at the pole we draw a complete circle on the celestial sphere with the other point. This

REVOLUTION OF THE STARS 15

circle just touches the north horizon at its lowest point and, in our northern latitudes, extends to near the zenith at its highest point. The stars within this circle never set, but only seem to perform a daily course around the pole. For this reason this circle is called the circle of perpetual apparition.

The stars farther south rise and set, but perform less and less of their daily course above our horizon, till we reach the south point of it, where they barely show themselves.

Stars yet farther south never rise at all in our latitudes. They are contained within the circle of perpetual occultation, which surrounds and is centred on the south celestial pole, as the circle of perpetual apparition is centred on the north one.