. The transit of Venus. ion veryprevalent that a transit of Venus affords the bestmeans of determining this distance. So far as ourpresent knowledge goes we are hardly justified in such I-] THE TRANSIT OF VENUS. a statement until after the observations that shall bemade in the present year. Before entering upon the method by which wemeasure the suns distance, let us devote a few linesto explaining what is meant by the word parallax,which is continually employed in such a man stand in a street exactly north of a lamp-post. The lamp-post will seem to be south of let him c
. The transit of Venus. ion veryprevalent that a transit of Venus affords the bestmeans of determining this distance. So far as ourpresent knowledge goes we are hardly justified in such I-] THE TRANSIT OF VENUS. a statement until after the observations that shall bemade in the present year. Before entering upon the method by which wemeasure the suns distance, let us devote a few linesto explaining what is meant by the word parallax,which is continually employed in such a man stand in a street exactly north of a lamp-post. The lamp-post will seem to be south of let him cross over to the other side of the lamp-post will now be in some other direction,such as south-west. This movement of the directionof the lamp-post is the effect of parallax. Now letus suppose, by a stretch of imagination, that a manobserves the moon from the centre of the earth. Hewill see it in the direction C M (Fig. 7). If now hegoes to A he will see it in the direction A M. The r/G. 7. THE MOOfJ THEEARTh. angle AMC through which the moon appears to havebeen moved is the parallax of the moon as observedfrom A. It will be noticed that the parallax is anerror introduced into the observed position of themoon, and which must be allowed for if we wish toget the position as seen from C. Moreover, the paral-lax at B is different from what it is at A. But at no 14 THE TRANSIT OF VENUS. [CHAP- point on the surface of the earth can the parallax begreater than at A. And if we know the parallax ofthe moon at A, we can deduce that at B from aknowledge of the relative positions of A, B, and it is useful to have a distinct name for theparallax at A. Now it will be noticed that a linedrawn from C to A is the vertical line at A ; hencethe moon M will appear to be on the horizon to anobserver at A ; and hence the moon has its greatestparallax when on the horizon. For this reason theparallax at A is called the moons horizontal , since the equatorial diameter
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