The elements of astronomy; a textbook . wo stations) and that of thesnn itself. This relative displacement is more than 2\ timesthe parallax of the sun, or, more exactly, -J|-f- as great. In other words, if two observersare situated so far apart that thedistance between them would sub-tend an angle of 8, as seen fromthe sun, then the apparent dis-placement of Venus on the sunsdisc, as seen from their two sta-tions, would be times 8, ornearly 21, a quantity quite meas-urable. To determine the solar parallaxthen, by means of a transit of Ve-nus, we must find the means of somehow measuring t
The elements of astronomy; a textbook . wo stations) and that of thesnn itself. This relative displacement is more than 2\ timesthe parallax of the sun, or, more exactly, -J|-f- as great. In other words, if two observersare situated so far apart that thedistance between them would sub-tend an angle of 8, as seen fromthe sun, then the apparent dis-placement of Venus on the sunsdisc, as seen from their two sta-tions, would be times 8, ornearly 21, a quantity quite meas-urable. To determine the solar parallaxthen, by means of a transit of Ve-nus, we must find the means of somehow measuring theangular distance between the two positions which Venus occu-pies on the suns disc, as seen simultaneously from two widelydistant stations of known latitude and longitude. The meth-ods earliest proposed and executed depend upon observationsof the times of contact between the planet and the edge of thesuns disc. There are four of these contacts, as indicated inFig. 136, the first and fourth being external, the second andthird Fig. in a Transit of Venus. 517. Halleys Method, or the Method of Durations. — The method suggested by Halley, who first noticed, in 1679, thepeculiar advantages that would be presented by a transit ofVenus as a means of finding the suns distance, consists inobserving the duration of the transit at two stations differinggreatly in latitude, and so chosen that the difference of dura- § 517] halleys method. 389 tions will be as large as possible. It is not necessary to knowthe longitude of the stations very accurately, since absolutetime does not come into the question. All that is necessaryis to know the latitudes accurately (which were easily obtainedeven in Halleys time), and the clock-rates for the four or fivehours between the beginning and end of the transit. Halleyexpected to depend mainly on the second and third contacts,which he supposed could be observed within a single so, the suns parallax could easily be dete
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