CH. xix. H ALLEY'S COMET. 159 



description we have neglected all the difficulties which really 

 occur. Our earth is not standing still as we have supposed 

 it to be. It is not only moving along in its orbit, but it is 

 turning round on its axis all the time, and this has to be 

 very carefully considered in choosing stations for observing 

 the transit, and allowed for in the results. Then, since our 

 earth moves in an ellipse, we are not always at the same 

 distance from the sun ; this also has to be allowed for. 

 Such simple difficulties as these you can understand, but 

 there are a great number of others which make the calcula- 

 tions very complicated indeed. Therefore you must not 

 imagine that you know all about the transit of Venus when 

 you have read this description of Halley's method. If you 

 have some general idea of the way by which the sun's 

 distance is found out, you will have learnt more than many 

 people ; and you must wait till you have studied mathe- 

 matics before you can expect to have a thorough knowledge 

 cf the matter. 



You will be glad to hear that Halley's advice was not 

 neglected. Several transit expeditions were sent out in 1761, 

 and again in 1769, when the celebrated Captain Cook made a 

 voyage to the Pacific Ocean for this purpose ; and it is to 

 correct these observations that no less than forty-six expe- 

 ditions were sent out in 1874 from Europe and America, 

 and again in 1882 all nations sent observers to favourable 

 spots. Now that these two transits are over, there will not 

 be another opportunity until December 2007. Halley 

 made many other valuable astronomical observations. He 

 was the first astronomer who foretold the return of a come.t. 

 Before his time it was thought that they went away and 

 never came back again; but when the comet of 1682 

 appeared, Halley began to search for former records of 

 comets and found that one had been seen about every 



