Opposition of Mars. 51 



branch of physical astronomy will be wholly lost to the 

 public. 



The ensuing opposition of Mars, on the 24th of March, is 

 one of this class : a phenomenon which occurs once only in a 

 period of about 780 days. It is well known that correspond- 

 ing observations of this planet, in the two hemispheres, as 

 compared with stars situated near its path, about the period 

 of its opposition, will serve to determine its parallax. And 

 the parallax of Mars being known, that of the sun may thence 

 be deduced. This was the plan adopted by Lacaille, when 

 he was at the Cape of Good Hope, in the year 1751 : since 

 which period, the method has fallen into disuse, for want of 

 an observatory in the southern hemisphere, with instruments 

 fit to be compared with those in Europe. 



The present period seems extremely favourable (for the 

 reasons above mentioned) for the revival of this method. At 

 the time of the last opposition in 1822, I ventured to draw 

 the public attention to the subject, by pointing out certain 

 stars, near which the planet would pass ; and with the posi- 

 tions of which it might be compared. Several valuable ob- 

 servations were made both in the southern and in the northern 

 hemisphere, which are published in various periodical works : 

 and which, being thus recorded, may be referred to with 

 advantage, by those who devote themselves to this branch of 

 physical astronomy. 



At the present opposition, there are but few stars, and those 

 of inferior magnitude, with which Mars can be advantageously 

 compared. For ten days preceding and subsequent to the 

 date of its opposition, Mars will not approach near to any 

 star given in the large catalogues of Bradley or Piazzi. There 

 are, however, five stars given in the catalogues of Lalande, 

 inserted in the Connaissance des Terns for the years VIII. and 

 XIII. with which die comparisons may be made. The mean 

 places of these stars, on January 1st of the present year, are 

 given in the following little table; together with the dates 

 when Mars will be in conjunction with them. 



G2 



Wlie 



