178 SCIENTIFIC APPARATUS. 



This instrument shows, in a very clear way, the great advances 

 made by Hipparchus, who introduced it. 



As the sun is in that part of the ecliptic, nearest to the north 

 pole, in summer, its position is represented by the point F on 

 the ecliptic and by N at the winter solstice; so, knowing the 

 time of the year, the sight Q can be placed the same number of 

 degrees from F as the sun is from the solstice, or in a similar 

 position on the circle o p to that which the sun occupies in the 

 ecliptic. The circle can then be turned round the axis c D till the 

 sight Q and the sight opposite to N Q' are in a line with the sun, 

 the circle o R will then be in the plane of the ecliptic, or of the path 

 of the earth round the sun. The circle R M is then turned on its 

 axis i K, and the sights R R moved until they point to the moon. 



The distance Q L, measured along o p, will then be the 

 longitude of the moon from the sun, and its latitude L R, 

 measuring along the circle R M. But why should Hipparchus use 

 the moon? His object was to determine the longitude of the 

 stars, but the only method available to him was by referring to 

 the motion of the sun, which was laid down in tables, so that 

 its longitude or distance from the vernal equinox was always 

 known. But we do not see the stars and the sun at the same 

 time ; therefore, in the daytime, while the moon was above the 

 horizon, he determined the difference of longitude between the sun 

 .and the moon ; that of the sun, or its distance from the vernal 

 equinox, being known by the time of the year : and after the sun 

 had set he determined the difference of longitude between the moon 

 and any particular star; and so he got a fair representation of the 

 longitude of the stars, and succeeded in tabulating the position of 

 1,022 of them. In Tycho's hands a planet was used instead of the 

 moon, and hence the greater accuracy of his work, which is repre- 

 sented in the Collection by a Quadrant, one of the most interesting 

 relics of past times in the building. 



In the vast and admirable collection of instruments brought 

 together by Tycho Brahe were to be found everything the 



