The star Thuban was once the pole-star. In the year B.C. 2300 it was ten times nearer the pole than Polaris is now. In the year A.D. 2100 the pole will be within 30' of Polaris; in A.D. 7500, it will be at a of Cepheus; in A.D. 13,500, within 7° of Vega; in A.D. 15,700, at the star in the tongue of Draco; in A.D. 23,000, at Thuban; in A.D. 28,000, back to Polaris. This indicates no change in the position of the dome [Page 199] of stars, but a change in the direction of the axis of the earth pointing to these various places as the cycles pass. As the earth goes round its orbit, the axis, maintaining nearly the same direction, really points to every part of a circle near the north star as large as the earth's orbit, that is, 185,000,000 miles in diameter. But, as already shown, that circle is too small to be discernible at our distance. The wide circle of the pole through the ages is really made up of the interlaced curves of the annual curves continued through 25,870 years. The stem of the spinning top wavers, describes a circle, and finally falls; the axis of the spinning earth wavers, describes a circle of nearly 28,000 years, and never falls. The star g Draconis, also called Etanin, is famous in modern astronomy, because observations on this star led to the discovery of the _aberration of light_. If we held a glass tube perpendicularly out of the window of a car at rest, when the rain was falling straight down, we could see the drops pass directly through. Put the car in motion, and the drops would seem to start toward us, and the top of the tube must be bent forward, or the drops entering would strike on the backside of the tube carried toward them. So our telescopes are bent forward on the moving earth, to enable the entered light to reach the eye-piece. Hence the star does not appear just where it is. As the earth moves faster in some parts of its orbit than others, this aberration is sometimes greater than at others. It is fortunate that light moves with a uniform velocity, or this difficult, problem would be still further complicated. The displacement of a star from this course is about 20".43.