CH. xix. TRANSITS OF MERCURY AND VENUS. 153 



CHAPTER XIX 



SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 



Transits of Mercury and Venus Gassendi Horrocks Halley 

 Explanation of Halley's Method. 



First Transits ever observed of Mercury and Venus, 

 1631-1639. We must now pause for a moment before 

 passing on to Newton's discoveries in Optics, in order to 

 mention a remarkable astronomical suggestion made about 

 this time by the astronomer Halley (born 1656, died 1742), 

 who was the friend and disciple of Newton. 



You cannot fail to have heard and read something about 

 the expeditions sent in December 1874 to all parts of the 

 world to observe the Transit (or Passage') of Venus across 

 the sun. The object of these observations was to measure 

 the sun's distance from the earth ; and Halley was the first 

 to propose this method of measurement, in 1691, and to 

 show how it might be accomplished. 



It is well known that as the two planets Mercury and 

 Venus are nearer to the sun than our earth is, they are con- 

 stantly passing between us and it. But usually they pass 

 either below or above the sun, and it is only rarely that they 

 cross over the bright disc, so as to be seen through the 

 telescope as a round black spot upon the sun's face. With 

 Mercury this happens at intervals of from seven to thirteen 

 years ; but with Venus it is much more rare, for though two 

 transits generally come together with an interval of oniy 



