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LECTURE VII.

Of the Harvest Moon.-Of Equation of Time.

I. Of the Harvest Moon.

It is said to have been remarked by the husbandman long before it was noticed by the astronomer, that at about the time of harvest, the full Moon rises for several successive evenings soon after sun set; and as the Moon is commonly believed to rise each evening 50 minutes later than on the preceding, this deviation about the time of harvest, so favorable to the husbandman, caused it to be termed the Harvest Moon. In tracing the causes of this phenomenon, the reader is advised to revert to some circumstances that have already been explained; for unless he retains clear notions of the terms plane of the ecliptic and plane of the horizon, it will be well to re-peruse what has been said on those subjects.

We cannot fail to have observed that the Sun and the Moon rise to us, at different times of the year, in different parts of our horizon; more or less to the north or south of the east. These variations are in great degree dependent on the situation of the Earth in its orbit. But the Moon is now our particular object. It has been shewn that her path with the Earth round the Sun, is not precisely on the plane of the ecliptic, but is inclined to that plane 5 degrees 9 minutes; that is to say, she is,

during one half of her monthly revolution about 5 degrees above, and the other half about 5 degrees below, that plane. This deviation forms so small a proportion of the whole circle, that it may be omitted in attempting to explain the phemonenon of the Harvest Moon, because it would not occasion any material difference.

Now, in regard to the Earth, the terms upper or lower side, north and south, east and west, are merely relative terms; they relate to the situations of other places, as compared with our own. For, as the Earth is nearly spherical, and as every inhabitant stands with his head towards the starry sphere, and his feet towards the centre of the Earth; every one is induced, by that circumstance, to fancy himself on the top of it; consequently, the plane of the horizon of every one, is at right angles with an imaginary line proceeding from him to the star immediately over his head. And as every

place on the Earth has an horizon of its own, it must be clear, that as there are as many horizons as places, the plane of each horizon will differ more or less from the plane of the ecliptic, which never varies; and it will presently be shewn, that the plane of the horizon of the same place varies, at different times of the year, with the plane of the ecliptic.

Then, in order that we may make such an horizon to a common globe, as ours appears to be, let us incline the north pole to the plane of the wood horizon, the same number of degrees as our latitude; which, as has been shewn, is reckoned from the equator, and is 51 degrees. If England be then brought to the meridian, it will be in its apparent situation; on the upper side of the Earth. As the bounder of our sight is one half the starry sphere, the wood horizon of the globe will then

represent the plane of our horizon; which, if it reached from the Earth to the sphere of the Fixed stars, would pass through the centre of each, dividing each into two equal parts.

Now it has been said that the Moon's true path differs so little from that of the Earth, that we may on this occasion consider them to be the same; and it has been shewn that the true path of the Earth is the apparent path of the Sun, transferred to the terrestrial globe, for reasons that have already been given. It will then be observed that the ecliptic, or Moon's path as we now reckon it, is inclined greatly to the plane of our horizon, -it forms a considerable angle with it.

The phenomenon of the Harvest Moon is always observed in September; on the 23d of which month, as has been shewn, the Autumnal equinox always takes place; for then the Sun is immediately opposite to that place on the Earth at which the ecliptic crosses the equator; when, as the Sun shines equally towards each pole, we have equal day and night. The Sun, as seen from the Earth, is then in the beginning of the Balance; the Moon, however, being on the opposite side of the Earth, is then in the first of the opposite sign, the Ram: but as the Harvest Moon is observed to rise for several successive evenings about the same time, it may be said she is then in the Fishes and Ram.

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Having, as it is termed, rectified the Globe for the latitude of London,' and thereby obtained our actual horizon, let us bring the first of the Ram, to it. We must now observe that the Moon travels through her orbit about 12 degrees every day. Then, if small black patches or wafers, be placed along the ecliptic at 12 degrees apart, by setting the index to any hour, say to

6, and turning the globe eastwards, the patches, as they rise one after the other, will indicate the actual difference in time of the Moon's rising, day by day. The consequence of this will be, that we shall find the Moon does not rise 50 minutes later every day to us; nor for some days more than 20 minutes: but that the difference in her time of rising, increases until we arrive at a certain point; and then decreases.

Now, we are of course desirous of having the cause of these considerable differences pointed out: more especially why, during our harvest, the Moon rises for several successive evenings so nearly at the same hour. Let us bring back the first point of the Ram to the horizon; and without attending to the black spots, remark the inclination to the horizon, of the ecliptic, which we now consider as the path of the Moon; and we shall observe that its obliquity to the horizon is least when the Moon appears to us in the Fishes and the Ram; and (attending to the black spots) we see them for several days, rising above the horizon nearly at the same time.

As the Moon is on every side of the Earth once a month, it follows that she must once each month appear in the signs called the Fishes and the Ram. It may be asked why this phenomenon is only observable in September. To this it may be replied, that the Moon is never full when in these signs, except in September, or about the time of the autumnal equinox; because she is never full in the Fishes and Ram but when the Sun appears to us in the opposite sign, which never happens but in September. But when we see the Sun in the Crab or the Goat, and the Moon's path is more oblique to the horizon, there is a greater difference in the times at which the black spots rise to it. When the Sun is

in the Balance, the index on the globe will denote a difference of 20 minutes; when in the Crab or the Goat, of more than 50 minutes.*

It is therefore clear that our having what we term the Harvest Moon, depends on the degree of obliquity of the Moon's path with our horizon. But the horizon of other places in north latitude, at a distance from us, do not enjoy the same privilege. The inhabitants near the equator for instance have it not; for to their horizon the path of the Moon would be at a very different angle; nor do they want it, for they have but little variety of

season.

In southern latitudes the Harvest Moon is just as regu lar as in northern, but at opposite times of the year.

It has been shewn that owing to the north pole of the Earth's axis being turned towards the Sun in summer, the inhabitants near the south pole, if there be any, do not derive any light and heat from the Sun for six months together; but then, it must be remarked that the full moon never sets to them, and never rises to them during the other six months in which they enjoy the Sun.

On the contrary in winter, when the north pole is in darkness, the full Moon never sets to them, and never rises during the other six months.

*Though the Ecliptic be taken as the Moon's path for the sake of easy illustration, yet it should be noticed that when the Moon's ascending node is in the first of the Ram, her orbit makes a less angle by 5 degrees with the horizon than the ecliptic does; and when the descending node is in the same point, the angle is greater; consequently every Harvest Moon is not the same every year;— that is, its times of rising vary less for several evenings in some years than others:—when the angle is least they vary least, and when greatest they vary most. In the years 1820 and 1889, the Harvest Moon will be most remarkable, and in 1830 least so.

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