Nov. 15, 1883] 



NATURE 



69 



the earth or sea may for simplicity's sake be considered as a 

 plane bounded by the circle where the earth and sky seem to 

 meet. This is known as the circle of the horizon. To repre- 



FlG.'24 — Diagram to show how the inclination of the horiion of London will 

 t0 change 



sent this, a ;piece of paper may be put over London on our 

 globe (see Fig. 24), and London may be brought to the top. 

 \Vhen that has been done, remembering that the stars are placed 

 at so infinite a distance, the horizon which cuts'the centre of the 



35. — Diagram to show how the inclination of the horizon of : 

 the equator changes in one direction only. 



earth, and which is called the true horizon, may be considered 

 as being practically the same thing as the small sensible horizon 

 of London, represented by our piece of paper, when at the 



top of the globe, because the two planes will be parallel. 

 For, wheiher a star be seen from the equator or from London, 

 owing to its tremendous distance it will appear to occupy the 

 fame position in space. Now let the globe be made to rotate, 

 then the inclination of the plane of the horizon of any place, 

 of our horizon of London for instance, is continually changing 

 duringthe rotation (Fig. 24). An exception, however, must be made 

 with regard to the poles of the earth. At these two points the 

 inclination will be constant during the whole of the rotation. 



If now a point on the equator be brought to the top of the 

 globe, it «ill be seen, as the globe is rotated, that the observer's 

 horizon rapidly comes at right angles to its first position (see 

 Fig. 25). This will show that the conditions of observation at 

 different parts of the earth's surface are very different, and this 

 whether it be the earth or the stars which move. 



Let us now consider with a little greater detail the conditions 

 which prevail in the latitude of London. Let London be again 

 brought to the top of the globe. Let o (Fig. 26) represent 

 an observer in the middle of the horizon, s w N E. Let z be 

 the zenith, which, of course, would be reached by a line starting 

 from the centre of the earth, and passing straight up through the 

 middle of the place of observation, s' is a star, and we want to 

 define its position. How can this be done? Imagine first a line 

 drawn from the observer to the zenith. Imagine next another line 

 going from the observer to the star, or, what is the fame thing, 

 from the centre of the earth to the star. Then the angle inclosed 

 by these two lines will give us the angular distance of that star 



Fig. a6. — ^Observing condition at London. 



from the zenith, or similarly we may take the angle included 

 between imaginary lines -joining observer with horizon and star, 

 and thus obtain the star's altitude. 



Again, its position may be stated not only viith regard to the 

 zenith and to the horizon, but to some other point, say the north 

 point. In that case a Hue or plane, ZEW, is imagined passing 

 from the zenith through tlie observer, and the distance between 

 E and N will give the star's angular distance from I he north point 

 of the horizon. Again, suppose it be desired to define the star's 

 positii n with reference, not to the zenith, but with reference to 

 the pole of the heavens, that point where the eaith's axis il 

 prolonged into space would cut the skies. In that case since 

 p in our diagram marks the position of the p de, a line Ps' u ill 

 give what is called the polar distance of the star ; and lastly, if 

 the angular distance of the star from the equator of the 1 eavens 

 be requiied, since the prolongation of Ps' would cut the equator, 

 the distance from s' to the point of intersection will give the 

 angular distance of the star from the equator ; in oti.er words 

 its declination. 



We have taken London, but of course each place on the earth 

 has its sphere of observation with its zenith and the north, east, 

 south, and west points. With regard to the axes of the earth 

 and the heavens, they b< th possess north and south points, and 

 in the heavens as in the earth, the equator lies midway between 

 them. J. Norman Lockyer 



( To be continued. ) 



OUR ASTRONOMICAL COLUMN 

 The Observatory, Chicago.— Prof. G. W. Hough has 

 issued his annual Report to the Board of Directors of the 

 Chicago Astronomical Society, detailing the proceedings in the 



