Star-Land: Being Talks With Young People About the Wonders of the Heavens

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.

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