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

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Hence you see that without going up to the ball or having a string from it, or in any other way making direct communication with it, we have been able to ascertain how far up in the air the ball is actually hung. This simple illustration explains the principle of the method by which astronomers are able to learn the distances of the different celestial bodies from the earth. You must think of the sun, the moon, and the stars as globes supported in some manner over our heads, and we seek to discover their distances from measurements of angles made at the ends of a base-line.

Fig.7.—This would be our Base-line when finding the Sun’s Distance.

Of course, astronomers must choose two stations which are far more widely separated than are those in our little experiment. In fact, the greater the interval between the two stations, the better. Astronomers require a much longer distance than from one side of this room to the other, or from one side of London to the other side. If it were merely a balloon at which we were looking, then, when one observer at one side of London and another at the opposite side shaped their cards carefully, we should be able to tell the height of the balloon very easily. But as the sun is so much further off than any balloon could ever be, we must separate the observers much more widely. Even the breadth of England would not be enough, so we have to make them separate more and more until they are as widely divided as it is possible for any two people on this earth to be. One astronomer takes up his position at A (Fig.7), and the other at the opposite side at B, so that they can both see the sun. They are obliged to use a much more accurate way of measuring the angles than by cutting out cards with pairs of scissors; and as the astronomer at A is not able to see his friend at B, it becomes no easy matter to measure the angles accurately. However, we shall not now trouble ourselves about such difficulties. It may suffice for the present to know that the angles are measured by delicate and very accurate instruments used by astronomers. They will not, indeed, make a little sketch such as sufficed for our purpose. They make a calculation which is a much more accurate way of effecting true measurement. The astronomers know the size of the earth, and thus they know how many thousands of miles lie between the two stations where the observations are made. This distance means in their calculation just what the length of the table did in our sketch. From each end of the line they set off an angle just as we did, and the astronomer must use the principle of similar triangles which he finds in Euclid, just we had to do. At last, when they have calculated the sides of their triangle, they obtain the distance of the sun.