i62 SEVENTEENTH CENTURY. pt. hi. 



tance is io8 times his diameter ; therefore 846,000 x 108, 

 or 91,368,000 miles would, by these measurements, be the 

 distance of the sun from the earth; and this is as near as we 

 can arrive at the truth when taking whole numbers. 



You will perhaps ask, if the measurement of the transit is 

 such a simple process, why it takes months to make the 

 proper calculations. But you must remember that in our 

 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. Tiierefore 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 

 of 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 last year from Europe and America. 

 Halley made many other valuable astronomical observa- 



