July 10, 1884] 



NA TURE 



2 55 



representing the orbit of the earth, and we have P, P 1 , P 2 , l'\ P 4 , 

 and P 5 representing different positions of the earth at different 

 times of the year, the distance between these points p and P 1 , v- 

 and P 3 , and P 4 and P 5 representing the portions of the orbit 

 passed over by the earth in equal intervals of time. This law 

 is known as the Law of Areas. It states that in equal intervals 

 of time the radius vector or line joining the earth and sun passes 

 over equal areas in its revolution. Thus the area of the triangle 

 s P P 1 is equal to the area of the triangle S r" p 3 , and is also 

 equal to the area of the triangle S P 4 P'\ and these areas of the 

 orbit are passed over by the radius vector in equal intervals of 

 time. When the earth is nearest to the sun she travels most 

 quickly ; when she is at her greatest distance from the sun she 



travels most slowly ; and thus she keeps the figures inclosed by 

 the radii vectores always of equal area during equal times. Let 

 us be quite clear on this point : the law is not that the earth 

 moves through equal distances in equal times, but that the areas 

 of the spaces swept over by the radius vector are the same for 

 equal intervals of time. 



We come then to this : that the earth moves round the sun ; 

 that she moves in an ellipse ; that she moves unequally, that is 

 to say, with different velocities at different times, but that these 

 different velocities are bound together by a well-defined and well- 

 recognised law. 



Now comes another question connected with this movement 

 of the earth round the sun. When the movement of the earth 

 on its axis was being discussed, it was pointed out that observa- 

 tions made by the transit instrument gave ample evidence that 

 the movement was a perfectly equable one, and of such a nature 

 that the axis of movement remained always practically parallel 

 to itself. Attention must now again be turned to this axis of 

 rotation. Let us take the earth in any part of its orbit, then 

 the question is this : Is the plane of the earth's motion round 

 the sun, or, as it must now be called, the plane of the ecliptic, 

 identical with the plane of the earth's motion of rotation ? That 

 is to say, if the earth were half immersed in an ocean of infinite 

 extent, whilst it was performing its orbital motion, would its axis 

 of rotation be at right angles to the surface of the ocean in wdiich 

 it swam. Suppose we had a globe to represent the earth, and 

 on it a model of a transit instrument were placed in the direction 

 it is pointed at Greenwich when the sun is being observed. 

 Then if the axis of the earth were really vertical the instrument 

 would always be at right angles to it, or practically so, for sun ob- 

 servations. Further, if the model were turned round to represent 

 one rotation of the earth, then if the axis on which it turned were 

 really perpendicular, the sun's declination would remain un- 

 changed, and its polar distance would always be 90 . Now let 

 us refer to the Greenwich observations of the north polar distance 

 of the sun. 



On March 16 N.P.D. was 91 -34 



June 22 



66 



That is to say, the observers at Greenwich in going from March 

 to June had to alter the inclination of their instrument, in conse- 

 quence of this variation in the N.P.D. of the sun, to the extent 

 of the difference between 90° and 66' 1 . On September 21 of the 

 same year the N.P.D. was 90 , but on December 17 instead 

 of being 90°, or 66°, it was H3°"24. How can these facts be 

 explained ? Suppose we had a lighted lantern to represent the 

 sun, and round it four globes were placed with their axes verti- 



cal to represent the earth in four different positions in its orbit. 

 It will be obvious that if we bring the light of the lamp 

 in succession upon the four globes with the axes in each thus 

 vertical, then the zenith distance of the sun, represented by 

 the lamp, would be the same in each case. In this position 

 of the globes we get the boundary of light and darkness at 

 the poles, and the line joining the centres of earth and sun 



;. 45. — Diagram showing the equality of the SI 

 two equinoxes. N, north pole of the earth ; 

 Greenwich. 



enith distance 

 uth pole ; z, zer 



will give us the zenith distance of the latter. Now assume that 

 the axis of the earth is not vertical but is inclined 23fj° to the 

 plane of the ecliptic. In that case its spin of course would not 

 be at right angles to thi^ plane. If the four globes were then 

 illuminated in succession, it would be found that the presenta- 

 tion of Greenwich to the sun would be vastly different at the Four 



,. 46. — Diagram show! 

 of Greenwich; 



he variation of the si 

 rth pole of the earth ; 



snith distance from 

 uth pole; z, zenith 



different positions. In the first, if it were placed in the proper 

 part of the orbit, we should get Greenwich, not turned fully to 

 the sun, but still well in his rays. In the second one we should 

 find the vertical at Greenwich pointing very much more to the 

 Min than before, when the axis was vertical. In the third globe 

 the conditions would be about the same as in the first, while in 



London). 



the fourth the line which points towards the zenith at Greenwich, 

 instead of being turned almost directly to the sun, would be turned 

 most away from it. This fourth position is that in which the 

 zenith distance, and therefore the X.P.D., was greatest, i.e. wdien 

 it was 1 13°. The second represents the position of the earth'when 

 it was the least possible, whilst the first and third positions would 



