ASTRONOMY OF EIGHTEENTH CENTURY. 251 



this effect is in no way dependent on the use of a telescope, which we 

 have supposed only in order to make plainer the first idea of the aber- 

 ration of light. It is, in fact, simply the result of the composition 

 of two motions. Looking again at the figure, it is plain that we have 

 = zSlif'; but the ratio of these two lines is the tangent 

 of angle c, i, D (p. 61), for angle c, i, D = angle A X , i, 3. 



The angle of aberration was estimated by Bradley to be 21*2", but 

 subsequent astronomers have estimated it at very nearly 2o % 5". This 

 discovery is not only one of great importance in practical astronomy, 

 but, in the absence of any observed sensible parallax of the stars, it 

 amounted to a direct proof of the earth's motion, which established 

 the Copernican theory beyond any possibility of doubt, and at the 

 same time confirmed Romer's estimate of the velocity of light. This 

 discovery ranks next in importance only to the laws of Kepler and 

 the precession of the equinoxes. But Bradley, continuing his obser- 

 vations with the sector, discovered an apparent motion of the stars, 

 which he traced to an oscillation of the earth's axis caused by the 

 attraction of the moon on the earth's equatorial protuberance, just as 

 the precession of the equinoxes discovered by Hipparchus is caused by 

 the attraction between the same mass of matter and the sun. Hence 

 there is a certain propriety in the title of the " English Hipparchus " 

 which has been bestowed upon Bradley. This motion of the axis was 

 announced by Bradley in 1745, and is known as the nutation of the 

 earth's axis. 



The middle of the eighteenth century is an important period in 

 the history of astronomy on account of the great improvements in the 

 arts of observing which were then introduced by Bradley, La Caille, 

 Mayer, and others. It was now that the transit instrument began 

 to be extensively employed, and the mural circle, now fitted with a 

 suitable telescope and graduated with great precision, was rendered 

 capable of giving very accurate results. The mural circle is so called 

 because it is attached very solidly to a wall, and is fixed in the plane 

 of the meridian. From this period dates the formation of trustworthy 

 catalogues of the fixed stars and other celestial objects. Bradley, 

 when he was appointed Astronomer Royal, had a. new transit instru- 

 ment erected, and also a mural circle of 8 feet radius. With these 

 instruments and an excellent clock, Bradley made, from 1750 to 1762, 

 one of the largest and most valuable series of observations which have 

 ever been the work of a single astronomer. La Caille was a French 

 astronomer, who went to the Cape of Good Hope to form a more com- 

 plete catalogue of the stars of the southern hemisphere than had been 

 made by Halley. La Caille in ten months observed the position of 

 ten thousand stars. On returning to Paris he engaged in researches 

 on atmospheric refraction and other subjects. Tobias Mayer also 

 investigated the laws of refraction, and published tables for the calcu- 

 lation of the positions of the sun and the moon. " 



