THE STORY OF TRANSITS 433 



The thirteen-year period is very well defined, but the seven- 

 year period holds twice only in the century. The forty-six 

 year period is also well defined, for if we start from 1907 and 

 add 46 we have 1953, and adding 46 again we have 1999. The 

 periods seem to fall in the order three thirteens and a seven, 

 giving rise to the forty-six. 



There are only three May transits in the century. 



The first is on . . . . 1924 May 7 



Add 13 years 



The next is on . . . . 1937 May 10 



If now \ve add 46 to 1924 we have 1970, in which year there 

 is a transit on May 9. 



The first transit of Mercury ever seen was observed by 

 Gassendi on November 7, 1631, the year in which he had 

 looked in vain for the like event for Venus. 



The only importance of transits of Mercury from the astro- 

 nomical point of view is that they give us very accurate deter- 

 minations of the planet's place, and from these data we can 

 discuss its orbit. The great American astronomer, Simon 

 Newcomb, at one time made an investigation of all recorded 

 transits in order to test the regularity of the earth's rotation. 

 He concluded that it was not quite uniform. 



Transits of Venus, however, are important, because they 

 give us one of the famous historical methods for determining 

 the distance of the sun. In giving this distance for astronomical 

 purposes we do not usually express it in miles but as the magni- 

 tude of a certain angle which we call the solar parallax. This 

 is a term we are always using ; two or three words will make it 

 perfectly clear. 



Let us suppose we are looking at two vertical rods. When 

 we place our eye in line with them they appear to be together, if 

 neither is too near. But when the eye is moved, the rods ap- 

 pear to separate and are no longer seen together. Their 

 apparent movement relatively to one another is called their 

 parallax. Now apply this to the heavens. Suppose we are 

 standing at a certain point on the surface of the earth. We see 

 a star in a certain direction. If we were able to get down to the 

 centre of the earth and look at the same star, we should see it in 

 a different direction. The difference between these two direc- 

 tions is called the parallax of the star. In the diagram (Fig. 2) 

 let S be the star, C the centre of the earth, and O the observer 

 on the surface of the earth. The observer O sees the star in 

 the direction OS, but if he went to the centre of the earth he 

 would see it in the direction CS. The difference between these 



