510 ^ KANSAS CITY REVIEW OF SCIENCE. 



value of the separation of the two chords, in miles, will be the same. It is evi- 

 dent, for instance, that if the Sun were exactly as far from Venus on one side- 

 as we are on the other, and the observers occupied the two poles of the Earth, 

 the separation would be equal to the Earth's diameter ; but as the Sun is farther 

 from Venus than we are, in the proportion of 72 to 28, if the transit were ob- 

 served from two stations, each chosen not too far from the Earth's poles, the sepa- 

 ration of the two chords on the Sun would amount to 18,000 miles; and this pro- 

 portion holds good whatever the distance. 



If it were possible to photograph the Sun at the same moment at the twa 

 stations, the thing would be done; we could at once measure the amount of sepa- 

 ration, determine its proportion to the whole diameter of the Sun, and deter- 

 mine the size of the Sun, whence its distance would at once follow, as we could 

 at once determine how great an angle the Earth's semi-diameter would subtend 

 at that same distance, which, in fact, would be the Sun's parallax. 



Simultaneous observations, however, are out of the question ; so the obser- 

 vations will take this form; the moments of ingress and egress are carefully notedt 

 at both stations, and the differences between the two chords will show us on what 

 part of the Sun they lie; this known, it is easy to determine the separation. 



As the difference between the observed times of transit at the two stations is- 

 the quantity which determines the amount of separation, it is important to make 

 this difference as great as possible, as then any error bears a smaller proportion to 

 the observed amoupt. 



This is accomplished by carefully choosing the stations, bearing the Earth's 

 rotations well in mind. Let us introduce this consideration, and see not only 

 how it modifies the result, but also with what anxious foresight astronomers pre- 

 pare for such phenomena, and why it was requisite in 1874, and will be again 

 necessary this year to go far from home to observe them. We already know the 

 instant and place (true, perhaps, to a second of time and arc), at which the planet 

 will enter and leave the solar disc, in other words we know exactly how the 

 Earth will be hanging in space as seen from the Sun — how much the south pole 

 will be tipped up — how the axis will lie — how the Earth will be situated at the- 

 moments of ingress and egress. 



Now if we suppose two planes cutting the centre of the Earth and those 

 points of the Sun's limb at which the planet will enter and leave the solar disc,, 

 •we shall see in a moment that some parts of the Earth will see the planet enter oa 

 the disc sooner than others. Some parts, on the other hand, will see it leave the 

 disc later ; in other words, according to the position of a place with reference to 

 the place of which we have spoken, both ingress of the planet and its egress will 

 appear earlier or later, as the case may be. 



