8

THE SUN.

otherwise, upon the occurrence of a transit, we might by thatmeans calculate our distance from Venus , and a simple sumin proportion would give our distance from the Sun .

It will, however, be a very different matter if, instead ofobserving the angles corresponding to tw r o such directions ofobservation, we cau use, as the foundation of our calculations,an interval of time which may amount to twenty or thirtyminutes. Such an interval may either be the difference of thewhole duration of a transit, or the difference of the momentwhen it begins or ends, as seen from two places upon theEarth .

If only its beginning or its end is observed at each station,the method is termed that of Delisle. This method, however,is found to involve the disadvantage, that, in order to deducethe Sun ’s distance, the longitudes of the stations must be deter-mined with extreme accuracy. And this is often a matter ofvery considerable difficulty.

If, on the other hand, the difference of the wffiole duration ofthe transit, as seen from two selected localities, is used, themethod is called that of Halley. In this case both placesmust see its beginning and its end. They will, in general, seeit throughout its continuance, although it may happen inplaces not far from the poles, that the beginning may occurbefore sunset, and the end after the next sunrise, a short Arcticnight having intervened.

The Halleyan method very much restricts the choice oflocalities, but it does not need the accurate determination oftheir longitudes. Its efficient application involves the use ofstations so far apart in north and south latitude, that access tosome of them is usually very inconvenient, if not impossible.The difference of latitude helps to separate the paths bywhich the planet is seen to cross the Sun , and consequentlyto increase the difference between the durations of its transit.It must not, however, be forgotten that this difference ofduration will also be much affected (and sometimes to a still

the Sun , which is itself correspondingly displaced. The above 3§ times is.therefore reduced to times.