remain the same relatively with itself. This is a point of great importance. You will remark that all these conclusions are derived from observations of the stars; that observations of the Polar Star are used to determine the Pole round which the heavens appear to revolve; or on the more rational supposition, to determine the point of the heavens towards which the axis of the rotation of the earth is supposed to be directed; summer or winter the Polar Star is the Polar Star still, and guides our observations; summer or winter the axis of the earth is directed towards that same part of the heavens where the Polar Star is seen. Therefore if the earth does revolve round the sun, we must suppose it to revolve in such a manner, not that its North Pole is always inclined towards the sun, but that it is sometimes inclined away from the sun; in point of fact, the motion will be imitated by the motion of the earth in the simple orrery,[1] represented in Figure 25.
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Fig. 25.
- ↑ In Figure 25, A represents a block to be screwed to the table; B, a grooved pulley fixed firmly to A, and having no motion whatever; C, a stand supporting a lamp D, which represents the sun; E, an arm which turns round the axis of B, running upon a small roller F; G, a grooved pulley of the same size as B, which turns in E and carries the axis of the earth H at the inclination 23½ degrees to the vertical. A band is passed round B and G. Then, when the earth is turned round the sun, its axis moves always parallel to itself.
