Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/840

774  receives his and. At a and e the axis is perpendicular to the plane of the, so that the s are situated in the boundaries of the illuminated hemisphere, and, the being directly over the , the s and s are equal at all places. In this figure Æ is the terrestrial equator, T the, the dotted circle the of , U the  or , and P the , where all the s or hour-circles meet. The spectator is supposed to be placed at the pole of the. It is also manifest that if the earth circles around S, as in fig. 14, the observed phenomena of apparent solar motion will be precisely the same as though the circled around the fixed. Let us follow the round from the position a, noting how the  would appear to move on the, and also how the length of  would be affected by the varying position of the 's axis with respect to the. When the is at a, the beginning of, about the 20th of , the , as seen from the , appears at the beginning of  in the opposite part of the , the  is just coming into light, and the  is vertical to the , which, with all its parallels, is divided into two equal parts by the circle which forms the boundary between the dark and illuminated hemispheres, and therefore the s and s are equal all over the. As the moves in the, according to the order of the letters A, B, C, D, &c., the  P comes more and more into the light, and the s increase in length at all places north of the  Æ. When the comes to the position between B and C, or the beginning of, the , as seen from the , appears at the beginning of  about the 21st of ; and the  of the  inclines towards the , so as to bring into light all the , and more of each of the northern parallels of  in proportion as they are farther from the. As the advances from  towards, and the  appears to move from  towards , the  recedes from the light, which causes the s to decrease and the s to increase in length till the  comes to the beginning of , and then they are equal as before, the boundary of light and darkness cutting the  and all its s equally. The then goes into the dark, and does not emerge till the  has completed a semi-revolution of its, or from the 22d of  till the 20th of. Similar changes occur, mutatis mutandis, in the southern hemisphere. It may be well to advise the reader not to allow his mind to be led astray by the proportions indicated in such pictures as fig. 14. It is absolutely impossible to illustrate the s, either by diagrams or by the use of a, without introducing incorrect relative dimensions; but by combining two sets of pictorial illustrations, the mental error apt to arise from the study of such pictures as fig. 14 may be got rid of. Thus, after carefully studying the relations illustrated in that figure, the reader should turn to Plate XXVII., and after noting that the figure of the there shown is the same as that in fig. 14 (held so as to have GgF uppermost), he should endeavour to picture to himself such a figure of the as is shown in the plate, travelling around the path EE′, but so reduced in dimensions that its whole disk would have a diameter less than the hundredth part of that of the small white disk, at the centre of the plate, which represents the. It will then be instructive to extend this method to, as figured in Plate XXVII., carrying this (after first imagining his disk reduced 6000 times) round his path MM′, with constant axial pose. The s pictured in Plate XXVIII. can be dealt with in like manner. From a careful study of the two plates, with special reference to the indicated scales, the general relations of the entire solar system can be inferred, and to some degree conceived. But for the purpose of actually picturing these relations to his mind, the reader may conveniently use Sir J. Herschel's illustration, as follows: Choose any well-levelled field. On it place a globe 2 feet in diameter to represent the ; will be represented by a grain of, on the circumference of a circle 164 feet in diameter for its ;  a , on a circle 284 feet in diameter; the  a [somewhat larger] , on a circle of 430 feet;  a rather large 's head, on a circle of 654 feet; the s grains of , in s of from 1000 to 1200 feet;  a moderate-sized , on a circle of half a mile;  a small , on a circle of 4/5ths of a mile;  a full-sized , on a circle more than 1½ miles;  an extra-sized , on a circle of 2½ miles in diameter.

—Apparent Motions of the Moon and Planets—Parallax.

We have seen that while the s remain fixed, to all appearance, on the celestial concave, the circuits around a great circle of the star-sphere, moving always in one direction, and at a rate which, though variable in different parts of the circuit, does not vary largely, and is constant for each part of the. Moreover, to ordinary observation, continued for periods of a few years, the 's path in the heavens appears to remain always the same, and to bear the same relation to the and  of the rotating star-sphere. But we have now to consider bodies which neither remain fixed like the s, nor move in a constant apparent path like the.

The is the most noteworthy of these bodies, because of her apparent size and brightness, and also because of the remarkable changes of appearance which she presents according to her varying position with reference to the. When she is seen near him in the, she appears always like a fine of light, with the horns turned away from him. When she is in the part of the directly opposite to the, she appears with a full orb. When she is exactly midway between the point occupied by the sun and that opposite to him, she appears as a semicircle of light, with the convexity towards the ; and in positions intermediate to these she appears with more or less of her circle illuminated, according as she is nearer to or further from the point directly opposite the. All this corresponds with what would happen if the moon were an opaque orb nearer to the than the, and illuminated by him. Now, when the moon is watched, even for a few hours only, she is found to be travelling on the star-sphere in the same direction as the (and, like him, on a path inclined to the ), but much more rapidly than the travels. It is impossible to watch the moon completely round the, because she is found to pass close to the once in each circuit, and when very near to him cannot be seen. But while she is visible, she travels continuously in one direction, and when she reappears, after having been for a day or two lost in the sunlight, she is seen to have shifted her place as though, during that interval, she had travelled continuously onwards.

The 's circuit of the star-sphere is found to be completed in about 27⅓ solar days. But her circuit, considered with reference to the, occupies a longer interval. Thus, suppose we observe her when she is opposite to the, or &quot;full.&quot; Then she is in the same (or very nearly the same) place among the s about 27⅓ s later. But in the meantime the has advanced along the  