Page:Popular Astronomy - Airy - 1881.djvu/92

78 Everybody who knows the character of the seasons at the different parts of the earth, will know that this representation is entirely in conformity with their respective changes. Therefore, we must make the following supposition: that if the earth does turn round the sun, its axis of rotation remains parallel to itself, having no reference whatever to the sun, whilst the earth is going round the sun; or, to express it in other words—that the earth has the power of preserving its axis of rotation in the same position all the time.

Now, it is remarkable, as a mechanical fact, that nothing is so permanent in nature as the axis of rotation of anything which is rapidly whirled. We have examples of this in every-day practice. The first is the motion of a boy's hoop. You will think perhaps that this illustration is of a rather superficial character, but it is of great importance. What keeps the hoop from falling? It is its rotation. I cannot now enter upon an explanation of this, which is one of the most complicated subjects in mechanics. Another thing pertinent to the question before us, is the motion of a quoit. Everybody who ever threw a quoit knows, that to make it preserve its position as it goes through the air, it is necessary to give it a whirling motion. It will be seen, that while whirling it preserves its plane in the same position, whatever that position may be, and however it may be inclined to the direction in which the quoit travels. Now, this has greater analogy with the motion of the earth than anything else. Another admirable illustration is the motion of a spinning top I hope I shall not be thought derogating from the dignity of science by giving such an illustration. The greatest mathematician of the last century, the celebrated Euler, has