126 THE HAMILTON ASSOCIATION. 
another in 2012. Why is this? How can astronomers tell to 
such a nicety? ‘This is a question frequently asked, and one is of 
the opinion that if more pains were taken at our meetings to insure 
our members an acquaintance with some of the simple methods of 
ascertaining sizes of planets, distances and time measurements, the 
recurrences of eclipses and the appearance of comets, much needed 
information would be scattered abroad, much good would be accom- 
plished, and one would not be called upon to give a reason in this 
day for believing in the theory of the earth’s rotundity. I will try 
to make clear the reason for the recurrence of transit in pairs. The 
periodic time of Venus bears a remarkable relation to the periodic 
time of the earth. Venus accomplishes thirteen revolutions around 
the Sun in the same time that the earth requires for eight revolutions. 
If, therefore, Venus and the Earth were in line with the Sun in 1874, 
then, in eight years’ time, the Earth having made eight revolutions, 
and Venus thirteen, a transit having occurred on the first occasion, 
a transit must occur in the second. 
Now we said that transits occur in pairs at an interval of eight 
years. ‘This is only approximately true, that thirteen revolutions of 
Venus are coincident with eight revolutions of the earth. Each re- 
currence of conjunction takes place at a slightly different position of 
the planets, so that when the two planets came together in 1890, or 
eight years after 1882, the point of conjunction was so far removed 
from the critical point, that the line from the earth to Venus did 
not intersect the Sun, and thus, although Venus passed very near 
the Sun, no transit occurred. Although many transits must have 
taken place hundreds of years before the year 1631, it was only in 
that year that attention began to be directed to the matter. The 
attention and success of Gassendi in observing the transit of Mer- 
cury, lead him to believe that he would be equally fortunate in 
observing the transit of Venus, which Keppler had foretold. Gas- 
sendi looked at the sun on the 4th, 5th and 6th of December, but 
saw no signs of Venus, We know now the reason the transit took 
place between the 6th and 7th, during the night, and must therefore 
have been invisible to European observers. Kepler had not noticed 
that another transit would occur in 1639. This discovery was 
made by a young astronomer named Horrocks, and it is the one 
which the history of the subject may be said to commence, as it 
