ERA OF NEWTON, HALLEY, AND IIERSCHEL. 43 



the zenith, and a the pole. The ellipse is the annual apparent movement of the star, 

 the transverse diameter of which is about forty seconds. 



Aberration, or wandering, is the name given to this phenomenon. The term is not 

 strictly accurate, as the apparent movements thus denominated are not irregular, but 

 uniform. To discover the physical cause became an object of intense interest to Bradley, 

 but it long baffled his researches and reasonings, and was at length developed by an acci- 

 dental circumstance. He was accompanying a pleasure-party in a sail on the river 

 Thames. The boat in which they were was provided with a mast which had a vane on the 

 top of it ; it blew a moderate wind, and the party sailed up and down the river for a con- 

 siderable time. Bradley remarked, that every time the boat put about, the vane at the top 

 of the mast shifted a little, as if there had been a slight change in the direction of the 

 wind. He observed this three or four times without speaking ; at last he mentioned it to 

 the sailors, and expressed his surprise that the wind should shift so regularly every time they 

 put about. The sailors told him that the wind had not shifted, but that the apparent 

 change was owing to the change in the direction of the boat, and assured him that the same 

 thing invariably happened in all cases. From that moment he conjectured that all the 

 phenomena of aberration he had observed, arose from the progressive motion of light com- 

 bined with the earth's motion in its orbit. This sagacious conjecture satisfactorily explains 

 the apparent movement of the stars. Suppose a body to pass from 

 A to B in the same time that a ray of light passes from c to B. 

 Owing to the two motions, the impression of the ray of light 

 meeting the eye of a spectator at B will be exactly similar to 

 what it would have been if the eye had been at rest at B, and 

 A the molecule of light had come to it in the direction D, B. The 

 star, therefore, whose real place is at c, will appear at D to the spectator at B. This effect 

 is precisely analagous to what takes place when a person moves or travels rapidly through 

 a shower of rain or snow in a perfectly calm state of the atmosphere. Without locomo- 

 tion the rain-drops or snow-flakes will fall upon his hat, or upon the head of the carriage 

 that conveys him, and not beat in his face, or against the front windows of the carriage. 

 But if he isjjassing along swiftly, in any direction, east, west, north, or south, the rain or 

 snow will come in contact with his face, or enter the front windows of the carriage if they 

 are open, as though the drops or flakes fell obliquely, and not from the zenith. Now as 

 an object appears to us in the direction in which the rays of light strike the eye, it is 

 easy to understand that a star in the zenith will appear at a little distance from it, to a 

 spectator carried along with the earth in its orbit. This discovery established the fame 

 of Bradley, who was exonerated from all future payments to the Royal Society on account 

 of it ; and it is of great importance, as the only sensible evidence we have of the earth's 

 annual motion. Soon after his appointment to the Greenwich observatory, he effected his 

 second great discovery, that of the nutation of the earth's axis, a slight oscillation of 

 the pole of the equator about its mean place, describing an ellipse in the period of 

 eighteen years. He determined likewise its cause, which theory had previously inferred to 

 be the action of the moon upon the equatorial regions of the earth. Some idea of his in- 

 dustry may be formed from the fact, that in conjunction with his nephew, he made no 

 less than eighteen thousand observations in a single year while astronomer royal ; and the 

 number from the year 1750 to 1 762 amounted to upwards of sixty thousand. The death 

 of Bradley was interpreted as a Divine judgment by the populace. He had taken an active 

 part with the Earl of Macclesfield and others, in urging on and assimilating the British 

 calendar to that of other nations. This rendered it necessary to throw eleven days out 

 of the current year in the month of September 1752 a measure which the ignorance of 

 great numbers of the people led them to regard as an impious intermeddling with the 



