Elements of astronomy ..with explanatory notes, and questions for examination . earth fromthe sun could not be determined,1 The manner inwhich the parallax is obtained, it is not difficult to un-derstand, but the various mathematical processes which 1. In order to ascertain the solar distances and the magnitudes of theplanets, we have seen that the distance of the earth from the sun must befirst known. It is also needed in order to determine the solar distances ofsome of the fixed stars, as will be shown in Part III. The accurate deter-mination of the suns parallax, is therefore of the utmost
Elements of astronomy ..with explanatory notes, and questions for examination . earth fromthe sun could not be determined,1 The manner inwhich the parallax is obtained, it is not difficult to un-derstand, but the various mathematical processes which 1. In order to ascertain the solar distances and the magnitudes of theplanets, we have seen that the distance of the earth from the sun must befirst known. It is also needed in order to determine the solar distances ofsome of the fixed stars, as will be shown in Part III. The accurate deter-mination of the suns parallax, is therefore of the utmost importance in as-tronomical researches. What is the mass of Venus 1 What her density ? What appearances have led astrono-mers to suspect that this planet is possessed of an atmosphere 1 What is said in regard tothe transits of Venus ? TRANSIT OF VENUS. 215 conduct to the result are too complicated and abstruse tobe admitted into an elementary work like this. 460. Let E, V, and S, Fig. 71, represent the relativepositions of the earth, Venus, and the sun when a tran- FIG. sit takes place, HVI a portion of the orbit of Yenus,and FG, a diameter of the earth perpendicular to theecliptic. If at the time of the transit two spectatorswere respectively placed at A and B, the diameter ofthe earth apart, the observer at F would see Yenusin the direction FY, projected as a dark spot on the sunat/ and, at the same instant, the person at G would inlike manner behold the planet in the direction GV, pro-jected on the sun at e. 461. That such would be the case is evident from va-rious familiar examples. Thus for instance if a tree isstanding in the middle of a square field, and one personviews it from the south-east corner of the lot, while asecond beholds it from the north-east corner; the firstsees the tree against the north-west portion of the sky,while the second observes it in the south-west quarter ofthe heavens. 462. Now Yenus in the above position is nearly68,000,000 miles from the
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