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

by Robert S. Ball

This telescope has a building to itself, which stands on the lawn in front of the house. The site is open and elevated, so as to command an extensive prospect of the heavens. You will see in [Fig. 36] a picture of the structure. It is circular in form and is entered by the little porch. The most peculiar feature of an edifice intended to contain this kind of telescope is its roof, or Dome, as we call it. It is of a hemispherical shape with a projecting rim at the bottom. But no one would go to the trouble and expense of making a round dome like that over the Observatory if it were not necessary for a particular purpose. The dome is very unlike ordinary roofs, not only in appearance, but also because it can turn round. In the next figure you will see a section through the building, and the wheels are exposed by which the dome is carried. These wheels run easily on rails, so that when the attendant pulls the rope which you see in his hands, he turns round a large pulley, and that operates a little cogwheel which works into a rack, and thus makes the dome revolve. The roof is built of timber, covered with copper; it weighs more than six tons, but the machinery is so nicely adjusted, that a child four years old can easily set the whole in motion. The object of all this machinery is seen when we learn that there is only one opening in the dome. It is covered by the shutter shown over the doorway in [Fig. 36]. When opened to the top, it gives a long and wide aperture, through which the astronomer can look out at the heavens. Of course the dome has to be turned until the opening has been brought to face the required aspect. The big telescope can thus be directed to any object above the horizon. You see a gentleman using the telescope ([Fig. 37]), and this shows that the great instrument is nearly three times as long as the astronomer himself! No doubt the telescope seems to be composed of a good many different parts, but the essential portions of the instrument are comparatively few and simple. At the upper end is the object glass, which consists of two lenses, one of flint glass and the other of crown glass. Both of these must be of exceptional purity, and the shape to be given to the lenses is a matter of the utmost importance. It is in the making of this pair of glasses that the skill of the optician has to be specially put forth. So valuable indeed is an object glass which fulfils all the requirements, that it is by far the most costly part of the instrument. There are no glasses in the interior of the tube until you come to the end where the observer is looking in. This is closed by an eyepiece consisting of a lens, or a pair of lenses. There are usually many different eyepieces for a telescope, and they contain lenses of varied powers, to be used according to the state of the atmosphere, or to the particular kinds of observation in progress.

If you point a big telescope to the sky, and see therein the sun or the moon or any of the stars, you will speedily find that the objects pass away out of view. Remember our earth is constantly turning round, and bears, of course, the Observatory with it, so that though the telescope be rightly pointed to the heavens at one moment, by the next it will have been turned aside. To you who are using the telescope, the appearance produced is as if the heavenly bodies were themselves moving. We can counteract this inconvenience. The telescope is supported on a pedestal, which is built on masonry, that goes down through the floor to its foundation on the solid rock beneath. In the iron casing at the top of the pedestal you will see a little window, and inside is clockwork driven by a heavy weight. This clockwork turns the whole telescope round in the opposite direction to that in which the earth is moving. The consequence is that the telescope remains constantly pointed to the same part of the heavens.

Fig. 38.—The Yerkes Telescope, University of Chicago.

This instrument is no doubt a large one, but of late years many much greater have been built. The most powerful telescope that has ever been erected is the great Yerkes instrument belonging to the University of Chicago, of which a picture is shown in [Fig. 38]. The object glass is 40 inches across.

HOW THE TELESCOPE AIDS US IN VIEWING THE MOON.

Those who are in charge of an observatory are often visited by persons who, coming to see the wonders of the heavens, and finding instruments of such great proportions, not unnaturally expect the views they are to obtain of the celestial bodies shall be of corresponding magnificence. So they are, no doubt, but then it frequently happens that the pictures which even the greatest telescope can display will fall far short of the ideal pictures which the visitors have conjured up in their own imaginations, so that they are often sadly disappointed. Especially is this true with regard to the moon. I have seen people who, when they had a view of the moon through a great telescope, were surprised not to find vast ranges of mountains which looked to them as big as the Alps, or mighty deserts, over which the eye could roam for thousands of miles. They have sometimes expected to behold stupendous volcanoes that not only were, but that looked to be as big as Vesuvius. Others seem to have thought they ought to see the moon with such clearness that the fields were to be quite visible, and some would not have been much astonished if they had observed houses and farmyards, and, perhaps, even cocks and hens.

There are different ways of estimating the apparent dimensions of an object, but the size the moon appears to me to have in a great telescope may be illustrated by taking an orange in your hand and looking at the innumerable little marks and spots on its surface. The amount of detail that the eye will show on the orange is about equal to the amount of detail that a good telescope will show on the moon. A desert on the moon, which really is a hundred miles across, will then correspond to a mark about an eighth of an inch in diameter on the orange. Some of you may ask what is gained by the use of a telescope, for the moon looks to us as large as a plate with the unaided eye, and now we hear it only looks as big as an orange in the telescope. But where is the plate with which you compare your moon supposed to be held? It is surely not in your hand. It is imagined to be up in the sky, a very long way off. Though an orange is much smaller than a plate, yet you will be able to see many more details in the orange by taking it in your hand than you could see on a plate which was at the other side of the street.

Fig. 39.—The Advantage of using a Telescope.