MOON

MOON, Influence of, on the Earth's Atmosphere.—It has

Moon. has been the opinion of the vulgar in almost all ages and countries, that the changes which take place in the state of our atmosphere, or the changes of the weather, depend in a great measure on certain situations of the moon. This particular opinion is alluded to by Vigil (A), and is applied in the shepherd of Banbury's rules for judging of the weather (B). We have, under METEOROLOGICAL, N^o 90 to 92, given the result of some observations on the connection between the changes of the moon and those of the weather.

It can scarcely be doubted that an opinion so generally received must be founded on something more than fancy or prejudice and; indeed the observations of several eminent meteorologists within the last thirty years have contributed materially to favour this opinion. Independent of actual observation, it appears reasonable to infer, that a body so large, and so near the earth, as the moon, whose gravitating influence on the earth's surface in producing the flux and reflux of the sea, cannot be altogether inactive with respect to the air, a fluid much more susceptible of changes than the sea.

We have already noticed (METEOROLOGICAL, N^o 14.) the theory of Mr Luke Howard, on the moon's influence on the mercury of the barometer, and we are now to give a short account of what has been advanced on her general influence by the philosophers of the continent. Among these, Signor Toaldo may be said to have led the way.

From observations made at Padua, during fifty years, on the state of the weather that corresponded to certain changes of the moon, he found that these changes were always accompanied by good or bad weather; and he at length became enabled to foretell with some degree of certainty what would be the state of the atmosphere that should follow any situation of the moon. There are ten situations of the moon, which, according to Toaldo, are capable of producing a sensible effect on the earth's atmosphere. These are the syzygies or new and full moon; the quadratures; the apodes, or apogee and perigee; the lunifices, or these points when the moon is nearest to our zenith and at the greatest distance from it; and the moon's equinoxes. There are three different relations of the moon's motion producing a corresponding number of revolutions, each having a certain duration, and each corresponding to some of the above ten situations, as it will be seen by the following table.

3. Periodical, in regard to the moon's passing the equator; continues 27 days, 7 hours, 43 minutes.

Ascending equinoxes.
Northern lunifices.
Descending equinoxes.
Southern lunifices.

Sig. Toaldo has calculated a series of probabilities that a change of weather will take place on the approach of any one of these ten situations, and these he has expressed in a tabular form as follows.

In general, each of the ten situations changes the weather that prevailed under the preceding situation, and it seldom happens that a change of weather takes place without a corresponding change in the lunar situations. From the inequality of their revolutions, these situations are often combined, and by this union their effect in producing changes of the atmosphere is greatly increased, especially when a union takes place between the syzygies and apodes. Thus,

These combined situations are generally accompanied or followed by storms and tempests, especially when they take place near the moon's passage over the equator. This is more particularly the case in the months of March and September, and we find that at the new and full moon in these months, the weather takes a certain character, by which it is distinguished for the succeeding three or six months. The same takes place at the solstices, especially at the winter solstice. The new moon does not always, however, produce a change of weather; and this want of effect is most likely to happen at those new moons which are most distant from the apodes.

Though Toaldo considers it as perfectly ascertained that each succeeding situation of the moon alters that state of the atmosphere which had been produced by the preceding situation; it must, however, be observed that some situations of the moon favour good and others bad weather. Thus the perigee, the new and full moon, the passage over the equator, and the northern lunifice are favourable to bad weather, while the apogee

Moon. gee, quadratures, and southern lunislice, are more favourable to good weather.

The changes produced by the influence of the lunar situations, seldom take place on the exact days on which these situations happen, but either precede or follow them; and Toaldo has found that, in the six winter months, the changes of weather commonly precede the lunar situations, whereas in the six summer months they more commonly follow them.

There are certain days before and after new and full moon, which deserve particular attention in forming our judgements of the weather, especially the octants or the fourth day before new and full moon, as at these times the weather is inclined to change, and it may be pretty certainly predicted, that a change will follow at the next lunar situation. Virgil has particularly noticed this fourth day as a sure mark of the succeeding weather (c). If the weather continues unchanged on the fourth, fifth, and sixth day of the moon, it proves that the lunar influence is at that time very weak, and we are to expect no change till the full moon, or perhaps till the next new moon.

Sig. Toaldo compared a diary which he had kept for many years of the state of the barometer with the ten situations of the moon, and from the comparison deduced the following conclusions, viz.

In this case Toaldo thinks that we must be guided, in our judgement of the weather, rather by the moon than by the barometer.

The case is similar during the coincidence of the equinoctial points with the perigee, at which time the mercury is unusually high; but this coincidence is a sign of great irregularity.

According to Toaldo, the rising and setting of the moon, as well as its superior and inferior passage of the meridian, all which situations he calls the moon's angles, may serve for foretelling rain. The seasons most exposed to rain, are the rising and setting of the moon; while its passage over the meridian is most favourable to good weather. It has ever been observed that during rainy days, the sky always clears a little while the moon is passing the meridian. An exception to this rule must, however, be made when the moon's angle does not coincide with that of the sun.

VOL. XIV. Part I.

Bad years take place when the apides of the moon fall in the four cardinal points of the zodiac. Their intervals, therefore, are as four to five, eight to nine, &c. or as the intervals of the passage of the apides through the four cardinal points of the zodiac. Thus the year 1777 was, in general, a bad year; and in that year the apides of the moon were in the equinoctial signs; and it is probable that the years in which the apides fall in the signs Taurus, Leo, Virgo and Aquarius, will be good and moderate years, as the year 1776 really was; and in that year the apides of the moon were in Taurus and Virgo.

Every 18th year must be similar. We, however, cannot depend upon a return altogether the same, on account of the three different revolutions of the moon; and therefore it may happen, that the epoch of this extraordinary year may be retarded a year or perhaps two. Though approximations only are here given, this does not prevent their being useful to farmers, if they only pay attention to circumstances. Besides, various exceptions must be made for different parts of the earth; and it is difficult to determine these before-hand, as what regards this system is applicable to the whole globe; but when the result of the system has been improved by local observations, the conjectures for each country will be attended with more certainty.

The 54th year must have a greater similarity to the first than to all the rest; because, at this period, the situations of the moon, in regard to the sun and the earth, are again found in the same points.

The quantity of the rain which falls in nine successive years, is almost equal to that which falls in the next following nine. But this is not the case when we compare in like manner the quantity of rain which falls in six, eight, or ten years *.

The observations of M. Lamarck, though they confirm the opinion of the moon's general influence on the atmosphere, do not agree with those of Toaldo, as to the situations of that luminary which correspond to the changes of the weather. He could not find that agreement between the syzgies and quadratures of the moon and a change of weather, which has been so much dwelt on by Toaldo; but he is of opinion, that we are to consider the declination of the moon as the principal cause of her influence on the atmosphere.

Lalande had conceived the idea that when the moon entered the northern hemisphere, or had north declination, the weather was most likely to be cold and dry, and that when she passed to the south of the equator, it was likely to be rainy. The observations of Lamarck, however, tend to establish the contrary opinion.

Lamarck considers the two following principles as established by his observations; viz.

1. That it is in the elevation of the moon above, and her depression below, the equator that we are to search for those regularly varied effects which she produces on our atmosphere.

2. That the determinable circumstances, which con-

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spire

Moon. spire to increase or diminish the moon's influence in her different declinations, are her apogees and perigees, her conjunctions with and oppositions to the sun; and lastly, the solar solstices and equinoxes.

Considering that every lunar month, or every revolution of the moon in the zodiac, may be divided into two distinct portions, each containing about fourteen days, and each giving occasion to a particular atmospheric constitution, we may assume these as two circumstances of importance in meteorology, and we may call one the boreal or northern constitution, viz. that in which the moon passes through the six northern signs of the zodiac, and the other, the austral or southern constitution, viz. that in which she traverses the six southern signs.

Lamarck is convinced by observation, that in these climates, during a boreal constitution, there chiefly prevail southerly, south-westerly, and westerly winds, though sometimes, in the summer, the winds pass to the south-east. In general, during this constitution, the barometer exhibits only moderate elevations of the mercury; most commonly the season is rainy or moist, and the air loaded with clouds. And lastly, it is particularly during this constitution that we observe the effects of storms and tempests, when the causes which occasion them become active.

On the contrary, during an austral constitution, the winds which chiefly predominate blow from the north and north-west, and in the summer north-east, and even easterly winds. In general during this constitution, the barometer exhibits considerable elevations in the column of mercury, at least if the wind is not very strong; the weather is then most usually clear, cold and dry, and in the summer it is seldom (we might almost say never) during this constitution that storms are formed.

These atmospheric constitutions are not, however, so permanently characterised as to render it easy to distinguish them at all times by the state of the atmosphere. The atmospheric air is a moveable fluid, and so easily displaced, that it is not surprising that in the temperate zones, where the influence of the heavenly bodies acts less strongly than between the tropics, from various causes, that counteract very often the regular influence of the moon, and tend to mask and even change its effects.

The perturbations which these variable causes produce on the regular effects of the influence of the moon on the atmosphere, occasion in fact many variations in the two atmospheric constitutions which we have been describing; and this is doubtless the reason why they have been hitherto disregarded. M. Lamarck positively asserts, that these perturbations, though frequent, and sometimes very considerable, do not prevent the character of each of these constitutions from being remarked in the greatest number of cases.

The probability that he finds, according to his observations, is estimated at five out of eight; that is to say, out of 48 atmospheric constitutions comprehended in the lunar year, he estimates there will be found at least 30 agreeing with the principles pointed out in his memoir; and he adds, that among the disturbing causes which modify the before-mentioned effects, several may be foreseen, and perhaps even appreciated as to their quantity of effect.

He considers what is here pointed out as a fact; as an

order of things which any one may prove by observation*.

Lamarck has also endeavoured to ascertain what truth there may be in the periodical return of the variations of the atmosphere at the end of nineteen years; and he has found, by comparing meteorological observations, that this return is far from being so correct as is generally believed.

Astronomers also know well, that the cycle of nineteen years is not exact within an hour and a half; an error which amounts to a whole day in the course of 308 years†.

M. Cotte has also bestowed much attention on this subject of the moon's influence; but appears to think that our observations are not sufficiently numerous or accurate, to deduce any thing like a correct theory, and he is not disposed to go so far as M. Lamarck.

M. Cotte agrees in general with Mr Luke Howard's observations on the moon's influence. (See METEOROLOGICAL.) He noted, during the space of 34 years and five months, (from the 1st of January 1768, to the 22d of May 1802), the ascending and descending direction of the barometer in each of the syzgies and quarters of the moon which have occurred through that period of time. He states the total sum of the elevations and depressions of the mercury at each of the phases as follows.

These results, of nearly 35 years' observations, confirm, as will be seen, the conclusions drawn by Mr Howard, both from his observations for one year at Plaistow, and those made for 10 years in the Royal Society's apartments.

It is to be remarked, 1st, That the four numbers which express the differences between the elevations and depressions are nearly in an exact proportion, since $63\; :\; 67\; ::\; 80\; :\; 85\backslash frac\{1\}\{2\}$.

2dly, That the two latter phases, viz. the full moon and last quarter, have more effect than the two first.

3dly, He examined what phases of the moon corresponded to the greatest and least height of the mercury for each month during ten years, and obtained the following results.

The science may be therefore said to have advanced one step farther towards perfection on this occasion; and it is to be hoped that, by redoubling our diligence in multiplying observations, and combining them in various ways to obtain their results, its progress may be still accelerated. The useful purposes which may be thereby answered in philosophy, agriculture, and medicine, may be properly urged to observers as the means of

Moon
Moore.
*Phil. Mag.
vol. xiii.

of supporting their ardour, and indemnifying them for those sarcasms and rejections which even some learned men have been pleased to bestow upon observations of this sort, together with their authors.