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

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A body rapidly rotating about an axis has a tendency to preserve the direction of that axis, and powerfully resists any attempt to change it. Our earth is spinning in this fashion. It is true that the rotation is, in one sense, a slow one, for it requires almost an entire day for each rotation. But when we remember the dimensions of our earth, we shall modify this notion. We have already stated that any place on the equator has to travel more than one thousand miles each hour in order to accomplish the journey within the required time. So far, therefore, the earth moves like a rifle-bullet, and the direction of its axis remains constant.

In the course of the great voyage between summer and winter, the earth travels from one side of the sun to the opposite side, and in doing so it still continues to spin about an axis parallel to the original direction. See the consequences which follow. The sun illuminates half the earth, and in the left position in Fig.27, representing summer, the North Pole is turned over towards the sun, and lies in the bright half of the earth. There is continual day at the North Pole, and night is unknown there at this time of year, because the turning of the earth about its axis will not bring the Pole nor the regions near the Pole into the dark hemisphere. Thus it is that the Arctic regions enjoy perpetual day at this season. Look now at the position of England when the northern hemisphere is tilted towards the sun, and is consequently enjoying the full splendor of midsummer. As the earth turns round, England will gradually cross the boundary between light and shade, and will enter upon the darkened hemisphere. Then there will be night in England, but you will see from the figure that the day is much longer than the night, and hence it is that we enjoy the fine long days in summer.