The elements of astronomy; a textbook . during aTransit. (Vogel, 1882.) § 518] HELIOMETRIC OBSERVATIONS. 391 over, if the weather prevents the end from being visible afterthe beginning has been observed, the method fails. De lIsles method, on the other hand, employs pairs ofstations near the equator, and does not require that the ob-server should see both the beginning and end of the of either phase can be utilized, which is a greatadvantage. But it does require that the longitudes of thestations should be known with extreme precision, since it con-sists essentially in obs
The elements of astronomy; a textbook . during aTransit. (Vogel, 1882.) § 518] HELIOMETRIC OBSERVATIONS. 391 over, if the weather prevents the end from being visible afterthe beginning has been observed, the method fails. De lIsles method, on the other hand, employs pairs ofstations near the equator, and does not require that the ob-server should see both the beginning and end of the of either phase can be utilized, which is a greatadvantage. But it does require that the longitudes of thestations should be known with extreme precision, since it con-sists essentially in observing the absolute time of contact {,Greenwich or Paris time) at both stations. Suppose that an equatorial observer, E, Fig. 139, on oneside of the earth notes the moment of internal contact inGreenwich time, the planet being then at Fi; when W notesthe contact (also in Greenwich time), the planet will be atV2, and the angle ViDV2 is the earths apparent diameter asseen from the sun ; , twice the suns horizontal parallax. Now. Fig. 139. —De Ilales Method. the angle at D is at once determined by the time occupied byVenus in moving from V\ to V2. It is simply just the samefraction of 360°, that the time is of 584 days, the planets syn-odic period. If, for example, the time were 12 minutes, weshould find the angle at D to be about 18. 519. Heliometric and Photographic Observations.—Insteadof observing merely the four contacts and leaving the rest ofthe transit unutilized, we may either keep up a continuedseries of measurements of the planets position upon the sunsdisc with a heliometer, or we may take a series of photo-graphs to be measured up at leisure. Such heliometer meas- 392 APPENDIX. [§519 ures or photographs, taken in connection with the recordedGreenwich times at which they were made, furnish the meansof determining just where the planet appeared to be on thesuns disc at any given moment, as seen from the observersstation. A comparison of these positions wi
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