To show how this may be. suppose a ball, illuminated by another ball at a great distance in the direction ES, to be carried round the specta• tor at E. This ball may be always subdivided into a visible and in It appears then not only that the lunar month varies, but that there is no yearly cycle of variation. Before however we make any remark on the preceding, we shall place by its side materials for confidence in the almanac from whence the preceding times were quoted. Taking at hazard a volume of astronomical observations, and opening the part where the results of the lunar observations are found, we took the first right ascensions [AscEssioNs] of the moon which we came to, opposite to which, for comparison, were written the predicted right ascensions of the moon for the same times. The dates matter nothing, since it is only the accordance of prediction with observation which is to be noticed. (' (jamb. Ohs.; 1835.) The lunar theory then, resting upon the Newtonian doctrine, enables aatronorndrs to find the position of the moon within a part of the heavens answering to a second of diurnal revolution, while the rough obstrvations with which astronomy must always commence would not give the length of a lunation within an hour. It is also confidently expected that the lunar tables recently computed by Professor Hansen of Gotha, in accordance with his lunar theory, and printed at the ex pense of the British government, will, from their superiority, ensure a degree of accuracy of prediction hitherto unprecedented.
Taking he lunar phenomena in the order of discovery, we next notice that this planet writes its mark on the earth in terms which have been understood from the earliest ages of astronomical inquiry. The alternate rise and fall of the waters, called the tides, is found to follow its motions, so that high water is always fonnd to succeed the time when the moon comes on the meridian, whether on the visible or invisible side of it. At first sight it would appear that there is high water twice a-day (that is, in the common solar day), but it is found on farther examination that the interval between high water and high water is a little more than twelve hours ; no that in the year 1858 that visible half, since one-half must hide the other in all positions. But it may also be divided into an illuminated and unilliuninated half. At A the visible half is all unilluminated, and though we have called it the visible half (meaning in a position to be seen, if tilers were light), it will not be seen. But when the ball arrives at B, a small portion of the illuminated half is in the visible half, as much as is intercepted between the arrows. At n a larger portion of the illuminated part is visible, and at F a full half of the visible surface is illuminated. • A little consideration of this scheme (which is moreover explained in all popular works) will show not only the occurrence of phases precisely similar to those of the moon, but also that the circular boundary of the enlightened part is towards the illuminating body. We copy from Riccioli his collection of the Latiu and Greek terms used with respect to the different phases :— to move in an ellipse which itself moves in space, yet wo may better explain the subject by arriving at that ellipse from the real motion than by beginning with it.
When the motion of the moon is watched in the heavens with instruments fitted to measure her apparent diameter, it is soon found that mho changes her distance from the earth, becoming alternately larger and smaller. Her path la not very much inclined to the ecliptic, so that she is never 51' from some one of the positions which the sun If the moon moved in the plane of the ecliptic, or of the sun's motion, as in the figure, there would be an eclipse of the sun at every new moon (Al, and of the moon at every full moon (it); since in the former Case the moon would bide the sun, and in the latter the earth would intercept the sun's light. The moon however is generally a
little on one side or the other of the ecliptic, not enough to introduce any sensible error into the preceding explanation of the phases, but enough to hinder the eclipses from taking place, except now and then : we shall see more of this presently. Again, if the sun remained in the line x s, the lunation, or complete cycle of phases, would be of the same duration as the actual revolution of the moon round the heavens. Since however the sun moves slowly forward in the same direction as the moon, the latter does not alter its phases so rapidly as in the figure, nor is the cycle of phases complete until the moon has overtaken the sun.
It is usual to divide the whole lunation into four quarters, the first from new moon to increasing half moon, the second from half moon to full moon, the third from full moon towaning half moon,the fourth from half moon to new moon. Each of these is called the change of the moon, and it is a very common belief that a change of weather and wind is to be expected, if not at every change of tho moon, at least muoh oftener at the changes than in the intervals. This opinion, when not absolutely received as true, is usually treated as the extreme of absurdity. It is in truth neither one thing nor the other, as the following considerations will show.
The atmosphere is continually undergoing a slight alteration from the effects of the tide. At new and full moon (or rather a little after these phenomena) there are those great tides called the spring-tides, arising from the action of both luminaries; at the two quarters the same luminaries oppose each other, and the quarters are followed by the smaller floods, called neap-tides. What effect may be produced by this succession of smaller and greater oscillations of the sea, which must produce oscillations of the atmosphere, it is impossible to say beforehand. Again, we know nothing of the electric action of either luminary upon the earth, or whether any and what electric state mny depend upon their relative position. We have therefore abundant grounds a priori to abstain from forming any opinion upon the effect of the heavenly bodies upon the weather; and we shall now state the results of such facts as observation has furnished. M. Arago collected the evidence on this subject in an article published in the Annuaire' for 1833 ; the general conclusion derived from them is, that upon the whole there is a little more rain during the second quarter than during either of the others ; but that there is no evidence to confirm the common notion that a change of the moon is actomjemie1 by change of weather. It has also been observed that the height of the barometer is, one time with another, less in the middle of the second quarter than in that of either of the others; and that It is somewhat greater at new and full moon than at the quarters. 1Vith regard to a great many other asserted effects of the moon upon animal and vegetable life, it can only be said that there is no conclusive evidence for or against them ; nothing but a long series of observations can settle such points. and this is not likely to be made (or it made, to be made fairly) by those who have predetermined the questions in oue wey or the other.