The elements of astronomy; a textbook . Fig. 138. — Atmosphere of Venus as seen 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 precisio

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The elements of astronomy; a textbook . Fig. 138. — Atmosphere of Venus as seen 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 ;

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