Part 1

STAR-LAND

_BEING TALKS WITH YOUNG PEOPLE ABOUT THE WONDERS OF THE HEAVENS_

BY

SIR ROBERT STAWELL BALL, F.R.S.

LOWNDEAN PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF CAMBRIDGE AUTHOR OF “THE STORY OF THE HEAVENS,” ETC.

Illustrated

NEW AND REVISED EDITION

BOSTON, U.S.A., AND LONDON GINN & COMPANY, PUBLISHERS The Athenæum Press 1899

ENTERED AT STATIONERS’ HALL

COPYRIGHT, 1899, BY CASSELL & COMPANY, LIMITED

ALL RIGHTS RESERVED

To THOSE YOUNG FRIENDS WHO HAVE ATTENDED MY CHRISTMAS LECTURES THIS LITTLE BOOK IS DEDICATED

PREFACE TO FIRST EDITION.

It has long been the custom at the Royal Institution of Great Britain to provide each Christmastide a course of Lectures specially addressed to a juvenile audience.

On two occasions, namely, in 1881 and in 1887, the Managers entrusted this honorable duty to me. The second course was in the main a repetition of the first; and on my notes and recollections of both the present little volume has been founded.

I am indebted to my friends Rev. MAXWELL CLOSE, Mr. ARTHUR RAMBAUT, and Dr. JOHN TODHUNTER for their kindness in reading the proofs.

ROBERT S. BALL.

OBSERVATORY, CO. DUBLIN, _Oct. 22, 1889_.

CONTENTS.

LECTURE I.

THE SUN. PAGE

The Heat and Brightness of the Sun--Further Benefits that we receive from the Sun--The Distance of the Sun--How Astronomers measure the Distances of the Heavenly Bodies--The Apparent Smallness of Distant Objects--The Shape and Size of the Sun-- The Spots on the Sun--Appearances seen during a Total Eclipse of the Sun--Night and Day--The Daily Rotation of the Earth-- The Annual Motion of the Earth round the Sun--The Changes of the Seasons--Sunshine at the North Pole 1

LECTURE II.

THE MOON.

The Phases of our Attendant the Moon--The Size of the Moon--How Eclipses are produced--Effect of the Moon’s Distance on its Appearance--A Talk about Telescopes--How the Telescope aids us in Viewing the Moon--Telescopic Views of the Lunar Scenery--On the Origin of the Lunar Craters--The Movements of the Moon--On the Possibility of Life in the Moon 74

LECTURE III.

THE INNER PLANETS.

Mercury, Venus, and Mars--How to make a Drawing of our System-- The Planet Mercury--The Planet Venus--The Transit of Venus-- Venus as a World--The Planet Mars and his Movements--The Ellipse--The Discoveries made by Tycho and Kepler--The Discoveries made by Newton--The Geography of Mars--The Satellites of Mars--How the Telescope aids in Viewing Faint Objects--The Asteroids, or Small Planets 134

LECTURE IV.

THE GIANT PLANETS.

Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of Jupiter--Saturn--The Nature of the Rings--William Herschel-- The Discovery of Uranus--The Satellites of Uranus--The Discovery of Neptune 212

LECTURE V.

COMETS AND SHOOTING STARS.

The Movements of a Comet--Encke’s Comet--The Great Comet of Halley--How the Telegraph is used for Comets--The Parabola-- The Materials of a Comet--Meteors--What becomes of the Shooting Stars--Grand Meteors--The Great November Showers--Other Great Showers--Meteorites 255

LECTURE VI.

STARS.

We try to make a Map--The Stars are Suns--The Numbers of the Stars--The Clusters of Stars--The Rank of the Earth as a Globe in Space--The Distances of the Stars--The Brightness and Color of Stars--Double Stars--How we find what the Stars are made of--The Nebulæ--What the Nebulæ are made of--Photographing the Nebulæ--Conclusion 318

CONCLUDING CHAPTER.

HOW TO NAME THE STARS. 381

STAR-LAND.

LECTURE I.

THE SUN.

The Heat and Brightness of the Sun--Further Benefits that we receive from the Sun--The Distance of the Sun--How Astronomers measure the Distances of the Heavenly Bodies--The Apparent Smallness of Distant Objects--The Shape and Size of the Sun--The Spots on the Sun--Appearances seen during a Total Eclipse of the Sun--Night and Day--The Daily Rotation of the Earth--The Annual Motion of the Earth round the Sun--The Changes of the Seasons--Sunshine at the North Pole.

THE HEAT AND BRIGHTNESS OF THE SUN.

We can all feel that the sun is very hot, and we know that it is very big and a long way off. Let us first talk about the heat from the sun. On a cold day it is pleasant to go into a room with a good fire, and everybody knows that the nearer we go to the fire, the more strongly we feel the heat. The boy who is at the far end of the room may be shivering with cold, while those close to the fire are as hot as they find to be pleasant. If we could draw much nearer to the sun than we actually are, we should find the heat greatly increased. Indeed, if we went close enough, the temperature would rise so much that we could not endure it; we should be roasted. On the other hand, we should certainly be frozen to death if we were transported much further away from the sun than we are now. We are able to live comfortably, because our bodies are just arranged to suit the warmth which the sun sends to that distance from it at which the earth is actually placed.

Suppose you were able to endure any degree of heat, and that you had some way of setting out on a voyage to the sun. Take with you a wax candle, a leaden bullet, a penny, a poker, and a flint. Soon after you have started you find the warmth from the sun increasing, and the candle begins to get soft and melt away. Still, on you go, and you notice that the leaden bullet gets hotter and hotter, until it becomes too hot to touch, until at last the lead has melted, as the wax had previously done. However, you are still a very long way from the sun, and you have the penny, the poker, and the flint remaining. As you approach closer to the luminary the heat is ever increasing, and at last you notice that the penny is beginning to get red-hot; go still nearer, and it melts away, and follows the example of the bullet and the candle. If you still press onwards, you find that the iron poker, which was red-hot when the penny melted, begins to get brighter and brighter, till at last it is brilliantly white, and becomes so dazzling that you can hardly bear to look at it; then melting commences, and the poker is changed into liquid like the penny, the lead, and the wax. Yet a little nearer you may carry the flint, which is now glowing with the same fervor which fused the poker, but even the flint itself will have to yield at last and become, not merely a liquid like water, but a vapor like steam.

You will ask, how do we learn all this? As nobody could ever make such a journey, how can we feel certain that the sun is so excessively hot? I know that what I say is true for various reasons, but I will only mention one, which is derived from an experiment with the burning-glass, that most boys have often tried.

We may use one of those large lenses that are intended for magnifying photographs. But almost any kind of lens will do, except it be too flat, as those in spectacles generally are. On a fine sunny day in summer, you turn the burning-glass to the sun, and by holding a piece of paper at the proper distance a bright spot will be obtained (Fig. 1). At that spot there is intense heat, by which a match can be lighted, gunpowder exploded, or the paper itself kindled. The broad lens collects together the rays from the sun that fall upon it, and concentrates them in one spot, which consequently becomes hot and bright. If we merely used a flat piece of glass the sunbeams would go straight through; they would not be gathered together, and they would not be strong enough to burn the paper. The lens, you see, is not flat; its faces are curved, and they thus acquire the power of bending in rays of light or heat, so as to unite their effect on that one point which we call the _focus_. When a great number of rays are thus collected on the same spot, each of them contributes a little warmth.

Some ingenious person has turned this principle to an odd use, by arranging a burning-glass over a cannon in such a way that just when noon arrived the spot of light should reach the touch-hole of the cannon and fire it off. Thus the sun itself is made to announce the middle of the day (Fig. 2).

Another application of the burning-glass is to obtain a record of the number of hours of sunshine in each day. You will understand the apparatus from Fig. 3; the lens is here replaced by a glass globe, which acts as a burning-glass. As the sun moves over the sky the bright spot of light also moves, and therefore burns its track on a sheet of paper marked with lines corresponding to the hours. When the sun is hidden by clouds the burning ceases, so by preserving each day the piece of paper, we have an unerring tell-tale, which shows us during what hours the sun was shining brightly, and the hours during which he was hidden. You see, the burning-glass is not merely a toy, it can be made useful in helping us to learn something about the weather.

Another experiment with the burning-glass will also teach us something. Take a candle, and from its flame you can get a bright point at the focus. It may fall upon your hand, but you can hardly feel it, and you will readily believe that the focus is not nearly so hot as the candle. Even when a burning-glass is held in front of a bright fire there is comparatively little heat in the focus. By using a lens to condense the beams from an electric lamp, Professor Tyndall has shown how to light a piece of paper, and to produce many other effects. But, nevertheless, the focus is not nearly so hot as the arc between the two glowing carbons. You might move your finger through the focus without much inconvenience, but I would not recommend you to trust your finger between the poles of the electric light itself. The temperature obtained at the focus of a burning-glass seems thus to be always less than that prevailing at the source of heat itself. This principle will be equally true when we turn a burning-glass to the sun, and hence we know that the sun must be hotter than any heat which can be obtained by the biggest burning-glass on the brightest of summer days. But burning-glasses a yard wide have been made, and astonishing heat effects have been produced. Steel has thus been melted by the sunbeams, and so have other substances which even our greatest furnaces cannot fuse. Therefore the sun must have a higher temperature than that of molten steel; higher, indeed, than any temperature we can produce on the earth.

I have tried to prove to you that the sun is very hot; but it would be well to see what arguments might be used on the other side. Indeed, it is by considering objections that we often learn. So I shall tell you of a difficulty that was once raised when I was endeavoring to explain the heat of the sun to an intelligent man. “I am sure,” said my friend, “that you must be quite wrong. You said that the nearer you got to the sun the hotter it would be; but I know this to be a mistake. When tourists go to Switzerland, they sometimes climb very high mountains. But the top of a mountain, of course, is nearer the sun than below; and so, if the sun were really hot, the climber should have found it much warmer on the top of the mountain than at its base. But every one knows that there is abundant ice and snow on lofty Alpine summits, while down below in the valleys there may be at the same time excessively warm weather. Does it not therefore seem that the nearer we go to the sun the colder it is, and the further we are from the sun the warmer it is?”

But my friend was quite wrong in his argument. The coldness of the mountain tops depends upon something which he had not taken into account. There is something else besides the sun which helps to make us so warm and comfortable. This other essential thing is more or less deficient at great heights. You know that we live by breathing air, and we find air wherever we go, over land and sea, all round the earth. Those who ascend in balloons are borne upwards by the air, and thus we can show that air extends for miles and miles over our heads, though it becomes lighter and thinner the loftier the elevation.

We not only utilize the air for breathing, but it is also of indispensable service to us in another way. It acts as a blanket to keep the earth warm; indeed, we ought rather to describe the air as a pile of blankets one over the other. These air blankets enable the earth to preserve the heat received from the sunbeams by preventing it from escaping back again into space. Thus warmth is maintained, and our globe is rendered habitable. You see then, that for our comfort we require not only the sun to give us the heat, but also the set of blankets to keep it when we have got it. If we threw off the blankets we should be uncomfortable, though the sun were as bright as before. A man who goes to the top of a mountain at mid-day does approach the sun to some extent, and, so far as this goes, he ought no doubt to feel warmer, but the gain is far too small to be thought of. Even at the top of Mont Blanc the increase in heat due to the approach to the sun would be only one ten-millionth part of the whole. This would be utterly inappreciable; even a thermometer would not be delicate enough to show it. On the other hand, by ascending to the top of the mountain, the climber has got above the lower regions of the air; he has not, it is true, reached even halfway to the upper surface--that is still very far over his head--but the higher layers of the atmosphere are so very thin that they form most indifferent blankets. The Alpine climber on the top of the mountain has thus thrown off the best portion of his blankets, and receives a chill; while the gain of heat arising from his closer approach to the sun is imperceptible. Perhaps you will now be able to understand why eternal snow rests on the summits of the great mountains. They are chilled because they have not so many air blankets as the snug valleys beneath.

The brightness of the sun is among the most wonderful things in nature, and there are three points that I ask you to remember, and then indeed you will agree with Milton, that the sun is “with surpassing glory crowned.” First think of the beauty and brilliancy of a lovely day in June. Then remember that all this flood of light comes from a single lamp at a most tremendous distance; and thirdly, recollect that the sun is not like a bull’s-eye lantern, concentrating all his light _specially_ for our benefit, but that he diffuses it equally around, and that we do not get on this earth the two-thousand-millionth part of what he gives out so plenteously! When we think of the brightness of day, of the distance from which the light has come, though Nature has not adjusted any vast lenses to direct the light specially in our direction, we begin to comprehend the sun’s true magnificence.

FURTHER BENEFITS THAT WE RECEIVE FROM THE SUN.

I want to show you how great should be the extent of our gratitude to the sun. Of course, on a bright summer’s day, when we are revelling in the genial warmth and enjoying the gladness of sunshine, it needs no words to convince us of the utility and of the beneficence of sunbeams. So we will not take midsummer. Let us take midwinter. Take this very Christmas season when the days are short and cheerless, the nights are long and dark and cold. We might be tempted to think that the sun had well-nigh forgotten us. It is true he only seems to pay us very occasional visits, and between fogs and clouds we in England see but little of him; but, visible or invisible, the sun incessantly tends us, and provides for our welfare in ways that perhaps we do not always remember.

Let me give an illustration of what I mean. You will go back this dull and cold afternoon to the happy home where your Christmas holidays are being enjoyed. It will be quite dark ere you get there, for the sun in these wintry days sets so very early. You will gather around a cheerful fire. The curtains will be drawn, the lamps will be lighted, and the disagreeable weather outside will be forgotten in the pleasant warmth and light within. Five o’clock has arrived, the pretty wicker table has been placed near mamma’s chair; on it are the cups and saucers and the fancy teapot. Under the table is a little shelf, with some tempting cakes and a tender muffin. Two or three welcome friends have joined the little group, and a delightful half-hour is sure to follow.

But you may say, “What have tea and muffins, lamps and fireplaces to do with the sun? Are they not all mere artificial devices, as far removed as possible from the sunbeams or the natural beauties which sunbeams create?” Well, not so far, perhaps, as you may think. Let us see.

Poke up the fire, and while it is throwing forth that delicious warmth, and charming but flickering light, we will try to discover where that light and heat have come from. No doubt they have come from the coal, but then, whence came the coal? It came from the mine, where brave colliers hewed it out deep under the ground, and then it was hoisted to the surface by steam engines. Our inquiry must not stop here, for another question immediately arises, as to how this wonderful fuel came into the earth? When we examine coal carefully, by using the microscope to see its structure, we find that it is not like a stone; it is composed of trees and other plants, the leaves and stems of which can be sometimes recognized. Indeed, the fossil trunks and roots of the great trees are occasionally conspicuous in the coal-pit. It is quite plain that these are only the remains of a vegetation which was formerly growing and flourishing, and on further inquiry we learn that coal must have been produced in the following manner:--

Once upon a time a great forest flourished. The sun shone down on this forest, and it was watered by genial showers, while insects and other creatures sported in its shades. It is true that the trees and plants were not like those we now see about us. They were more like ferns and mare’s-tails and gigantic club-mosses. In the fulness of time they died, and fell, and decayed, and others sprang up to meet the like end. Thus it happened that, in course of ages, the remains of leaves, and fruits, and trunks accumulated over the soil. The forest was situated near the seashore, and then a remarkable change took place--the land began slowly to sink. You need not think that this is impossible. Land has often been known to change its level gradually. In fact, a sinking process is slowly going on now in many places on the earth, while the land is rising in other localities. As the forest gradually sank lower and lower, the sea-water began to inundate it, and all the trees perished until, at last, deep water submerged the surface which had once been covered by a fine forest. At the bottom of this sea lay the decaying vegetation.

That which was the destruction of the growing forest, proved to be the means of preserving its remains, for, then as now, the rivers flowed into the sea, and the waters of the rivers, especially in times of flood, carried down with them clay or mud, held in suspension. Upon the floor of the ocean this material was slowly deposited; and thus a coating of mud overlay the remains of the forest. In the course of ages, these layers grew thick and heavy, and hardened into a great flat rock, while the trunks and leaves underneath were squeezed together by the weight, and packed into a solid mass which became black, and in the course of time was transformed into coal.

After ages and ages had passed by, the bed of the sea ceased to sink, and began slowly to rise. The water over the newly made layers of stone became shallower, and at last the floor was raised until it emerged from the sea. But, of course, it would not be the original ground which formed the surface of the newly uncovered land. The sheets of consolidated clay lay on the top; over the fresh surface life gradually spread, until man himself came to dwell there, while far beneath his feet the remains of the ancient vegetation were buried.

When we now dig down through the rocks we come upon the portions of trees and other plants which the lapse of time, and the influence of pressure, have turned from leaves and wood into our familiar coal.

That ancient forest grew because sunbeams abounded in those early times, and nourished a luxuriant vegetation. The heat and the light then expended so liberally by the sun were seized by the leaves of flourishing plants, and were stored away in their stems and foliage. Thus it is that the ancient sunbeams have been preserved in our coal-beds for uncounted thousands of years. When we put a lump of coal on our fire this evening, and when it sends forth a grateful warmth and cheerful light, it but reproduces for our benefit some of that store of preserved sunbeams of which our earth holds so large a treasure. Thus, the sun has contributed very materially to our comfort, for it has provided the fire to keep us warm.

The orb of day has, however, ministered further to our tea party, for has it not produced the tea itself? The tea grew a long way off, most likely in China, where the plant was matured by the warmth of the sunbeams. From China the tea-chests were brought by a sailing vessel to London; the ship performed this long voyage by the use of sails, blown by what we call wind, which is merely the passage of great volumes of air as they hurry from one part of the earth to another.

We may ask what makes the air move, for it will not rush about in this way unless there be considerable force to drive it. Here again we perceive the influence of the sun. Tracts of land are warmed by the genial sunbeams. The air receives the heat from the land, and the warm air is buoyant and ascends, while cooler air continually flows in to supply its place. To do this it has, of course, to rush across the country, and thus wind is caused. All the air currents on our earth are consequently due to the sun. You see, therefore, how greatly we are indebted to our brilliant luminary for the enjoyment of our tea-table. Not only has the sun given us the coal and the tea, but it has actually provided the means by which the tea was carried all the way from China to our own shores.

We can also trace the connection between the hot water and the sun. Of course, the water has come immediately from the kettle, and that has been taken from the fire, and the fire was produced by sunbeams. Thus we learn that it is the warmth of the sun that has made the water boil. If you visit the water-works you will see great reservoirs. In some cases they have been filled by a river, sometimes the water is pumped from a deep well in the ground, sometimes it is the surface-water caught on a mountain side. Whatever be the immediate source of our water supply, the real origin is to be sought, not in the earth beneath, but in the heavens above. All the water we use day by day has come from the clouds. It is the clouds which sent down the rain, or sometimes the snow, or the hail, and it is this water from the clouds which fills our rivers. It is this water also which sinks deep into the earth and supplies our wells, so that from whatever apparent source the water seems to have come, it is indeed the clouds which have been the real benefactors. The water in your teacup to-night was, a little while ago, in a cloud, floating far overhead in the sky.