Page:Popular Science Monthly Volume 6.djvu/236

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cle (Fig. 3), the shorter one being her track to the southern observer, the longer to the northern. In Fig. 4, a b is her apparent path in the first case, c d in the second. This figure shows the direction of the planet's motion, and, with approximate truth, its apparent size as compared with the sun and the decree of actual displacement. Its first appearance, touching the outside of the sun as at a, is what is called "first external contact." This is shortly followed by "first internal contact," when the planet has moved wholly on to the sun's face, and is just quitting the edge. After some four hours it touches the edge again ("second internal contact"), crosses it and disappears ("second external contact"). The external contacts have not hitherto been much relied on, but, now that with the spectroscope we can see the planet a little way off the sun, they can be better observed. The internal contacts are the important ones, and these have heretofore been rendered more or less uncertain, by the phenomenon called the "black drop," already referred to, as consisting in an optical illusion, by which the planet seems to cling to the limb and pull out of shape, like a drop of ink just about falling from the pen. (Fig. 5.)

PSM V06 D236 Portion of the sun at first internal contact of venus.jpg
Fig. 5.—A Portion of the Sun at First Internal Contact of Venus, showing the "Black-Drop."

Since there is no actual track left to reckon the distance between the chords from, the northern and southern observers time the planet across, very accurately, and, from the times, the lengths of these chords, and hence the distance between them, may plainly be found, since we know just how long the planet would take to go over the sun's diameter. There is another way, by measuring the distance, from the sun's centre, of Venus at different stages of her progress, as seen by a pair or any number of pairs of observers; but probably best of all is photography, which is to be used by nearly every station, and which will give us almost any number of pictures (as many as 150 or 200 to a station), showing exactly how the planet looked from minute to minute to the photographer's lens—an observer which does not get flur-