Astronomy 579 



had been previously observed as a fixed star, it would be of 

 great importance to recover such an observation, and thus to 

 make a longer arc of the planet's orbit available. 



Professor Sears C. Walker, at that time one of the astron- 

 omers of the United States Naval Observatory, undertook 

 the investigation, utilizing the first four months during which 

 the planet had been under observation. 



He first computed a preliminary orbit, and traced the 

 motion of the planet backwards to determine in what portion 

 of the sky and at what time it might possibly have fallen 

 among the fixed stars of some catalogue and have been 

 observed as one of them. 



His methodical manner of procedure was sure to detect 

 such an observation if it had been made, so that it was by no 

 accident that he discovered an ancient determination of the 

 position of the planet among the zone observations of Lalande 

 in 1795. 



In this way a position of the planet was obtained fifty years 

 earlier than the date of its discovery, and a long arc of its 

 orbit was determined by observation instead of a very short 

 one. With the data thus available Walker calculated new 

 elements of the planet and prepared ephemerides of its 

 positions each year from 1846 onwards. Much of this 

 computation was done at the expense of the Smithsonian 

 Institution, and the results were duly printed in the " Con- 

 tributions to Knowledge " and they constitute a work of 

 sterling value. 



The orbit of Neptune was also investigated by Professor 

 Peirce, of Harvard College, and the principal perturbations 

 calculated, so that the pure elliptic orbit of Walker could 

 be corrected for the action of the disturbing planets, and 

 the ephemeris correspondingly improved. The discovery of 

 Lalande's early observation, and the prompt utilization of it 



