AND DISTANCE FROM THE EARTH 



ever made to measure the sun's distance. It occasionly 

 happens that Venus is situated exactly between us and the 

 sun, and appears to move across the solar disc as a round 

 black spot. This phenomenon is called a transit of Venus, 

 and is visible at most twice in a life-time. These remark- 

 able events take place at regular intervals of 8 years, 105^ 

 years, 8 years and 1215^ years respectively, and the}' 

 always happen duritio^ June and December. We thus 

 find there was a transit of Venus iii June, 1761, followed 

 by one 8 years later in June, 1769. Another did not 

 happen for 105 5^ years, being in December, 1874, while 

 the last took place 8 years afterward in December, 1882. 

 Then follows a great gap of 121 >^ years, as not a transit of 

 Venus will occur until June, 2004, to be succeeded by 

 another in June, 2012, and so on. 



It became certain to Halley in the year 1716 that 

 advantage might be taken of these occurrences to deter- 

 mine the distance of the sun, and knowing that he could 

 not possibly live until 1761, the date of the next transit, 

 in order to verify his conclusions, he bequeathed the grand 

 problem of finding the sun's distance by this method to 

 posterity. Enough has been said to comprehend that as 

 viewed from two widely separated stations on the earth, 

 Venus appears at such times to travel over the sun in 

 different parallels of latitude, .so to speak, owing to the 

 effects of displacement caused by the postion of the 

 observers ; hence the problem here is to note the exact 

 time the planet takes to transit the solar disc as seen from 

 two stations Vx^hose distance is already kiiown. The 

 angular diameter or apparent size of the sun being easy to 

 measure each day the sun shines, it becomes possible, dur- 

 ing a transit, to calculate the angular distance between the 

 two observed paths of Venus as it moves acro.ss the sun. 

 This minute quantity bears a certain proportion to the 

 whole diameter of the sun which, on being worked out, 

 yields the .solar parallax and is, in other words, the angle 



