Fig. 35.—Different Kinds of Solar Eclipse.
An eclipse of the sun is the simpler occurrence, so we shall describe it first. It happens when the moon comes between the earth and the sun. Look at our little astronomers shown in [Fig. 34]. A boy and a girl are both gazing at the sun, when the moon comes between. To the boy the moon appears to take a great bite out of the sun, so that it looks like the left-hand picture in [Fig. 35]. (I have drawn a line from the end of the telescope in [Fig. 34], which shows how much of the sun is cut off.) This would be called a partial eclipse of the sun. The almanac will sometimes describe the eclipses as visible in London, or visible at Greenwich; but that need not be taken so literally as was supposed by a Kensington gentleman, who, on noticing that the almanac said an eclipse was to be visible in London, called a cab and drove into the city to look for it. His almanac had not mentioned that it would be visible from his own house. You may usually take for granted that when an eclipse is said to be visible from London or Greenwich, it will be more or less visible all over England. Most of these eclipses are only partial, and though they are interesting to watch they do not teach us much. By far the most wonderful kind of eclipse is that in which the whole of the bright part of the sun is blotted out. Then, indeed, we do see wonders. But such eclipses are rare, and even when they do occur they only last a very few minutes. The sights that are displayed are so interesting that astronomers often travel thousands of miles to reach a suitable locality for making observations.
The girl in [Fig. 34] is placed in the best possible position for seeing the eclipse. There you find her right in the line of the sun and moon; and I think you will agree that she cannot see any part of the sun, for the moon is altogether in the way. I have drawn two dotted lines, one at each side. All that she can see beyond the moon must lie outside these dotted lines, and she will be in the dark as long as the moon stays in the way. When the eclipse is complete, comparative darkness steals over the land. The birds are deceived, and fly home to the trees to roost. The owls and the bats, thinking their time has arrived, venture forth on their nocturnal business. Even flowers close their petals, only to open a few minutes later when the sun again bursts forth. Other flowers that give forth their fragrance at night are also sweetly perceptible so long as the sun remains obscured. An unruly cow, accustomed to break into a meadow at night, was found there after an eclipse was over; while I learn from the same authority that a man rushed over in great excitement to see what his chickens were doing, but came back much disappointed on finding them pecking away as if nothing had happened.
It will sometimes happen that the moon is so placed that the edge of the sun can be seen all round it. A case of the kind is shown in the right-hand picture of [Fig. 35]. It is called an annular, or ring-shaped eclipse.
The eclipses of which we have been speaking are, of course, only to be seen during the day when the sun must be up. The lunar eclipses, which are visible at night, are due to the interposition of the earth between the sun and the moon. The sun is at night-time under our feet at the other side of the earth, and the earth throws a long shadow upwards. If the moon enter into this shadow, it is plain that the sunlight is partly or wholly cut off, and since the moon shines by no light of her own, but only by light borrowed from the sun, it follows that when she is buried in the shadow all the direct light is intercepted, and she must lose her brilliancy. Thus we obtain what is called a lunar eclipse. It is total if the moon be entirely in the shadow. The eclipse is partial if the moon be only partly in the shadow. The lunar eclipse is visible to everybody on the dark hemisphere of the earth if the clouds will keep out of the way, so that usually a great many more people can see a lunar eclipse than a solar eclipse, which is only visible from a limited part of the earth. It thus happens that the lunar eclipse is the more familiar spectacle of the two.
When the moon is entirely in the shadow, one might naturally think that it would become totally invisible. This is not always the case. It is a curious fact that in the depth of a total eclipse the moon is often still visible, for she glows with a copper-colored light, which is bright enough to render some of the chief marks on her surface distinctly discernible.
EFFECT OF THE MOON’S DISTANCE ON ITS APPEARANCE.
We are now about to take a good look at the moon and examine the different objects which are marked upon it. There is a peculiar interest attached to this particular orb, because it is much the nearest of all the heavenly bodies to our globe, and therefore the one that we can see the best. Every other object—sun, star, or planet—is hundreds, or perhaps thousands, of times as far off as the moon. It is right that we should desire to learn all we can about the bodies in space. We know that the earth is a great ball, and we see that there are many other such bodies. Some of them are much larger, and some of them are smaller than the globe on which we dwell; some of them are dark bodies like the earth, and among them the moon is one. Is it not reasonable that we should make special efforts to find out all we can about this interesting neighbor?
Though the moon is so close to us relatively to other objects in space, yet when we express its distance in the ordinary methods of measurement it is a very long way off—about 240,000 miles—a length nearly as great as that of all the railways in the world put together. An express train which runs forty miles an hour would travel 240 miles in six hours, and the whole distance to the moon would be accomplished in 6000 hours, so that travelling by night and day incessantly you would accomplish the journey in 250 days. To take another illustration, if you wrapped a thread ten times round the equator of the earth, it would be long enough to stretch from the earth to the moon. Or suppose a cannon could be made sufficiently strong to be fired with a report loud enough to be audible 240,000 miles away. The sound would only be heard at that distance a fortnight after the discharge had taken place.
The moon is too far for us to examine the particular features on its surface by the unaided eye. Suppose that there was a mighty city like London on the moon, with great buildings and teeming millions of people, and you went out on a fine night to take a look at our neighbor. What do you think you would be able to see of the great lunar metropolis? Would you be able to see its streets full of omnibuses, or even its great buildings? Would you see St. Paul’s and Westminster—the great parks and the river? Of all these things your unaided eye would show you almost nothing. I can give you a little illustration. Suppose that you made a tiny model of London; imagine this little structure all complete, so that the streets, the buildings, the bridges, the railways, the parks, and the Thames were placed in their true proportions; suppose that the miniature city was so small that it could stand on a penny postage stamp, surely everything would look very insignificant, even if you had the model in your hand and looked at it with the aid of a magnifying glass. But suppose it were put on the other side of the table or on the other side of the room, or the other side of the street. Even St. Paul’s Cathedral itself would have ceased to be distinguishable; but yet the distance is not nearly great enough. You would have to put the little model a quarter of a mile away before it would be in the right position to illustrate the appearance of a lunar London to the unaided eye.