444 



NATURE 



[September io, 1908 





5L.RI.Er/A'G VOR ARCH.EOLOGJSTS.' 

 II. 



Horizons — Earlhly and Heavenly. 



SO long as we are dealing with measuremenls of 

 azimuth on the horizon, and altitude above the 

 horizon, we are considering only our position on the 

 earth — on that part of it which is bounded by our 

 horizon. We are not dealing with the true position 

 in the heavens of any body, whether sun or star, 

 which may rise or set in the directions defined by our 

 measures. 



We are onlv dealing, in fact, with what is termed 

 the sphere of observation. 



Flc. 4. — The sphere of observation, o, the position of an observer at the 

 centre of the sphere, surrounded by the horizon ; N. E. S. W. (the true 

 cardinal points), with z, the zenith, the point over the observer's head. 

 The hne N. S. is the true meridian line. The plane bounded by the line 

 N. E. S. W. is the plane of the horizon, or the horizontal plane. The 

 line o z, a vertical line, is at right angles to the plane of the horizon. 



The figure will illustrate what has already been 

 said about azimuth, and will enable us to define some 

 new technical terms which will be used later on. 

 For true azimuths the zero is at N., which represents 

 the N. point of the horizon, so that the azimuth of 

 the E. point is qo°, of the S. point i8o°, of the W. 

 point 270°. Next let us take some intermediate points, 

 A and T. The arc na is the azimuth of A, the arc 

 NT the azimuth of T. Sometimes it is convenient to 

 define the position of a point on the horizon, not 

 fiom the N. point (azimuth), but from the E. or W. 

 point; \\e speak of this measure as amplitude. In 

 any quadrant the one is the complement of the other, 

 that is, added together, they maice 90''. 



The points A, T, like the points N. E. S. W., are 

 represented as being on the norizon, so the distances 

 of all these points from z, called the zenith distance, 

 are the same. If we represented these points not 

 on, but above or belozi< the horizontal plane, it is 

 obvious that the zenith distances would not be the 

 same. The higher the point is above the true horizon, 

 as would happen if there were a hill there, the less 

 the zenith distance. 



The circle which we actually observe all round 

 us when the heavens seem to rest on the surface 

 whicli we see is termed the 7nsible horizon. We 

 imagine a plane parallel to the plane of the visible 

 horizon, but passing through the centre of the 

 earth ; this is called the rational or true plane of the 

 horizon. 



.So much for the horizon as a part of the earth's 

 surface. 



In the astronomical survey of ancient monuments, 

 the determination of the azimuth of the various sight- 



1 Continue 1 from p. 393. 



lines, and the altitude of that part of the horizon 

 which bounds them, is for the purpose of studying 

 the sight-lines in relation to the rising or setting 

 places of sun or star. 



What we have to do, therefore, is to study the 

 relation of the sphere of observation to what is called 

 the celestial sphere, the sphere on which in old 

 time the stars were supposed to be fixed by golden 

 nails. 



To do this we must pass from the consideration of 

 the sphere of observation at any place to a study of 

 the earth as a whole, and its movements, or at all 

 events of some of them. 



We have the earth in space with the universe of 

 stars, almost infinitely removed, all round it, and we 

 now know that the apparent movements of the stars 

 from east to west, their daily risings, passing over 

 the meridian and setting, in the sphere of observation 

 at any place, are onlv the reflections of the earth's 

 daily ■ movement, or spin, on its axis from west jto 

 east. 



The points at which this axis cuts the earth's surface 

 are called the \. and .S. poles, and half-way between 

 these the earth is bounded by a circle called the 

 equator. Now, as the daily motion of the earth is 

 reflected in the apparent daily motion of the stars, so 

 is the system of defining positions on the earth re- 

 flected in the system employed by astronomers in 

 defining po^^itions in the heavens. 



.As the earth is belted by parallels of latitude and 

 Meridians of longitude, so are the heavens belted to 

 the astronomer with parallels of declination and 

 meridians of right ascension. If we suppose the plane 

 in which our equator lies extended to the stars, it 

 will pass through all those which have no declination 

 fo°). Above and below we have north and south 

 declination, as on the earth's surface we have north 

 and south latitude, until we reach the poles of the 

 equator (90°). .As on the earth we start from the 

 meridian of Grcemvicli in the measure of longitude, 

 so do we start from a certain point in the celestial 

 equator occupied by the sun at the vernal equinox, 

 called the first point of .irics, in the measure of what 

 is termed right ascension. 



-So that we have terrestrial latitude, reckoned from 

 the terrestrial equator, corresponding with celestial 

 declination, recI<oned from the celestial equator, and 

 longitude corre>ponding with right ascension. 



It is the declination, that is, the distance from the 

 celestial equator, with which archaeologists chiefly 

 have to deal, for the reason that the rising and setting 

 places of celestial bodies depend upon their declina- 

 tion ; bodies ivith the same declination rise and set in 

 the same azintuths. 



Now the presentation of the plane of the horizon cf 

 a place to the surrounding stars which together con- 

 stitute the celestial sphere varies vastly with its posi- 

 tion on the earth's surface. Whether stars rise and 

 set at all, or if they do whether they rise and set 

 verticallv or obliquely, depends upon this position, or, 

 to be more precise, upon the latitude of the position. 

 It is a pitv that " calisthenics and the use of the 

 globes " no longer form part of a liberal education, 

 for a study of a terrestrial globe, which is a model 

 of the earth in relation to the celestial sphere, gives, 

 us help in the matters we are now considering. I 



.Such a globe is furnished with a wooden horizon, 

 which represents the true or rational horizon passing 

 through the centre of the earth as before defined. 

 The axis of the globe prolonged is fixed into a brass 

 ring: representing the meridian, and the axis can be 

 inclined at any angle in regard to the wooden 

 horizon. 



Now, wherever the archa;ologist is working, his 



NO. 2028, VOL. 78] 



