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SCIENCE PROGRESS 



at M when the earth is at P we have a transit across the centre 

 of the sun ; this is a unique event, for Venus is too far from M 

 eight years earlier and eight years later for the passage to take 

 place. The condition for the double occurrence is that Venus 

 is not too close to M. The paths are then displaced from the 

 centre of the sun and are seen near the upper and lower edges 

 in turn. This is the case with the transits of our own time. 



Although the connection between the thirteen Venusian 

 and the eight terrestrial year periods, to which reference has 

 been made, is accurate enough to predict a pair of transits or at 

 least to warn us when to look for them, yet a much more exact 

 relation can be found. A more accurate value of the Venusian 

 year is 224-7 days. Multiplying this by 395 we get 88756-5. 

 A more exact value of our own year is 365-256 days. Multi- 

 plying this by 243 we have 88757-2. Hence 243 of our years 

 are almost exactly equal to 395 years of Venus, the difference 

 being less than -008 per cent. We may, therefore, look with 

 confidence for transits separated by this interval of 243 years. 



The following table gives the dates of transits of Venus, and 

 also shows the two facts that have already been emphasised, 

 that the phenomenon occurs early in June or early in December, 

 and that successive passages at the same node are separated by 

 either 8 or 243 years. Fortunately for observers the transits at 

 the other node occur near the middle of this 243 year period. 



A winter transit occurred on 



Add 



The next transit occurred on 



To the former date . . Add 



The next winter transit was on . 



Add 



The next occurred on 



To the former date . . Add 



The next winter transit will occur 

 on . 



Add 



The following one will be on 



A summer transit occurred on 



Add 



The next occurred on 



To the former date . . Add 



The next will occur on 



Add 



And the next on . . . 



1 63 1 December 7 

 8 years 



1639 December 4 

 243 years 



1874 December 9 

 8 years 



1882 December 6 

 243 years 



2 1 1 7 December 1 1 

 8 years 



2125 December 8 



1 761 June 5 

 8 years 



1769 June 3 

 243 years 



2004 June 8 

 8 years 



2012 June 6 



