Page:Popular Science Monthly Volume 10.djvu/428

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at most of them. If we add those obtained by the French, Germans, and English, the total number available reaches nearly 1,200, according to the best estimates.

PSM V10 D428 Correlating venus and sun positions.jpg
Fig. 5.

After the pictures are made and safely brought home, they have next to be measured—i. e., the distance (and in the American pictures the direction also) between the centre of Venus and the centre of the sun must be determined in each picture. This is an exceedingly delicate and tedious operation, rendered more difficult by the fact that the image of the sun is never truly circular; but, even supposing the instrument to be perfect in all its adjustments, is somewhat distorted by the effect of atmospheric refraction; so that the true position of the sun's centre with reference to the squares of the reticle is determined only by an intricate calculation from measurements made with a microscopic apparatus on a great number of points suitably chosen on the circumference of the image. The final result of the measurement would come out something in this form: Peking, No. 32, time, 14h 08’ 20.2” (Greenwich mean time); Venus north of sun's centre, 735.32”; east of centre, 441.63”; distance from centre of sun, 857.75”. (The numbers given are only imaginary.) It is this process of measurement which has required so long a time since the transit, and is not yet completed. When it is finished, and the results published in the form indicated, then will come the work of combining all the data thus obtained at all the stations, and from them deducing the true value of the solar parallax. Since, however, another transit is to occur so soon (in 1882), it is not unlikely that astronomers may defer the final grand combination until the observations of that transit also are ready to be included. It is very confidently hoped by most of those who have studied the subject that the remaining uncertainty in the