THE STORY OF TRANSITS 427 



We may regard the sun as being at the centre of the solar 

 system, and the planets as going round it in paths that are very 

 nearly circles. The nearest to the sun is Mercury, then comes 

 Venus, and then our earth. The paths of Mercury and Venus, 

 therefore, lie between us and the sun ; at some points these 

 paths will cross the plane that contains the earth and the sun, 

 and if either of the planets happens to be at one of these points 

 at the right time, the earth, the planet, and the sun will be in 

 the same straight line, with the earth and the sun at either end. 

 At such a time the planet will be seen from the earth as a black 

 dot against the brilliant background of the sun. To this sight 

 we give the name of a transit of Mercury, or of Venus, as the case 

 may be. 



Whenever such a thing occurs the planet seems to cross the 

 sun from left to right, or, as we say in the language of science, 

 from east to west. The planets go round the sun in a counter- 

 clockwise sense ; that is a fundamental fact that cannot be 

 stated too often ; on a diagram the planets always move in the 

 direction opposite to that in which the hands of a clock go round. 

 Venus goes along its path at about twenty-two miles a second ; 

 we move more slowly, but still very fast, for we go along our path 

 at something more than eighteen miles a second. So, just before 

 the positions for a transit are brought about, Venus is always 

 overtaking the earth, and as this goes on in the direction already 

 stated, the planet moves across the sun from left to right. 

 This may appear to contradict the previous statement that the 

 movement is from east to west, for on the maps of all countries 

 the west is found on the left, the east on the right. Exactly the 

 opposite is the case with maps of the sky and charts of the 

 heavenly bodies ; the east is on the left, the west on the right, so 

 that if the south be faced and the map held against the sky the 

 cardinal points are seen just where they would be expected. 



Such is the fundamental conception of a transit. In practice, 

 however, a slight modification becomes necessary. We have 

 tacitly made but little reference to the planes which contain 

 the orbits of the planets. We will first of all confine ourselves 

 to the case of Venus, and to make the points quite clear let us 

 take a simple illustration. 



Let us imagine that a big clock face is painted on a horizontal 

 table. The edge of its circle may represent the path of the 

 earth round the sun, the latter being regarded as a small globe 

 placed at the centre. The earth travels round this circle, this 

 clock face, once a year, so that it will be at some definite point 

 on any day we care to select. The most important day of 

 the year from the mathematical point of view is the vernal 

 equinox, which comes on or about March 22. This is the day 

 on which the sun rises due east and sets due west, the time 



