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In Summer, they rise about midnight, when the Moon is in her third quarter. On account of her rising so late, and giving but little light, her rising passes unobserved.

To the inhabitants at the equator, the north and south poles appear in the horizon; and therefore the ecliptic makes the same angle southward with the horizon when Aries rises, as it does northward when Libra rises; consequently the Moon rises and sets not only with angles nearly equal, but at equal intervals of time, all the year round: Hence, there is no harvest Moon at the equator. The farther any place is from the equator, if it be not beyond the polar circles, the angle which the ecliptic makes with the horizon gradually diminishes when Pisces and Aries rise.

Although in northern latitudes, the autumnal full Moons are in Pisces and Aries; yet in southern latitudes it is just the reverse, because the seasons are so :-for Virgo and Libra rise at as small angles with the horizon in southern latitudes, as Pisces and Aries do in the northern; and therefore the harvest Moons are just as regular on one side of the equator as on the other.

At the polar circles, the full Moon neither rises in summer, nor sets in winter. For the winter full Moon being as high in the ecliptic as the summer Sun, she must continue, while passing through the northern signs, above the horizon; and the summer full Moon being as low in the ecliptic as the winter Sun, can no more rise, when passing through the southern signs, than he does.

THE HORIZONTAL MOON.-The great apparent magnitude of the Moon, and indeed of the Sun, at rising and setting, is a phenomenon which has greatly embarrassed almost all who have endeavoured to account for it. According to the ordinary laws of vision, they should appear to be least when nearest the horizon, being then farthest from the eye; and yet the reverse of this is found to be true. The apparent diameter of the Moon, when viewed in the horizon by the naked eye, is two or three times larger than when at the altitude of thirty or forty degrees; and yet when measured by an instrument her diameter is not increased at all.

Both the Sun and the Moon subtend a greater angle when on the meridi. an, than they de in the horizon, because they are then actually nearer the place of the spectator, by the whole seini-diameter of the Earth.

Explain why there is no Harvest Moon at the equator. The farther any place is from the equator, how is the angle between the ecliptic and the horizon, when Pisces and Aries rise? Do the Harvest Moons happen as regularly, and in the same months, on the south side of the equator, as on the north? Why does not the full Moon rise in summer, nor set in winter, to the inhabitants of the polar circles? According to the ordinary laws of vision, how ought the magnitudes of the Sun and Moon to appear, when they are nearest the horizon? What is the fact? How much larger does the Moon appear to the naked eye, when in the horizon, than when at the altitude of thirty or forty degrees? Where, in reality, do the Sun and Moon subtend the largest angle? Why is it 803

This apparent increase of magnitude in the horizontal Moon, is chiefly an optical illusion, produced by the concavity of the heavens appearing to the eye to be a less portion of a spherical surface than a hemisphere. The eye is accustomed to estimate the distance between any two objects in the heavens by the quantity of sky that appears to lie between them; as upon the Earth we estimate it by the quantity of ground that lies between them. Now when the Sun or Moon is just emerging above the eastern horizon, or sinking beneath the western, the distance of the intervening | landscape over which they are seen, contributes, together with the refraction of the atmosphere, to exaggerate our estimate of their real magnitudes.

CHAPTER XXV.

REFRACTION-TWILIGHT.

The rays of light in passing out of one medium into another of a different density, deviate from a straight course; and if the density of the latter medium continually increase, the rays of light in passing through it, will deviate more and more from a right line towards a curve, in passing to the eye of an observer. From this cause all the heavenly bodies, except when in the zenith, appear higher than they really This bending of the rays of light, giving to the heavenly bodies an apparent elevation above their true places, is called Refraction.

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It is in consequence of the refracting power of the atmosphere that all heavenly bodies are seen for a short time before they rise in the horizon, and also after they have sunk below it. At some periods of the year the Sun appears 5 minutes longer, morning and evening, and about 34 minutes longer every day, at a mean rate, than he would do were there no refraction. The average amount of refraction for an object half way between the horizon and the zenith, or at an apparent altitude of 45°, is but one sixtieth of a degree, a quantity hardly sensible to the naked eye; but at the visible horizon it amounts to 33' of a degree, which is rather

How is the apparent increase of magnitude in the horizontal Moon, accounted for? How are the rays of light affected in passing out of one medium into another, of a different density? How, if the density of the latter medium continually increase? What astronomical phenomenon results from this cause? What is this bending of the rays of light out of their course called? What effect does refraction have upon the apparent rising and setting of the heavenly bodies? How much longer do we see the Sun, morning and evening, than we should, if there were no refraction? What is the average amount of refraction for an object half way between the horizon and the zenith 7 What is it at the horizon?

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more than the greatest apparent diameter of either the Sun or the Moon.

Hence it follows, that when we see the lower edge of the Sun or Moon just apparently resting on the horizon, their whole disc is in reality below it, and would be entirely out of sight and concealed by the convexity of the Earth, but for the bending, which the rays of light have undergone in their passage through the air to the observer's eye.

The following general notions of its amount, and law of variations, should be borne in mind :

1. In the zenith there is no refraction; a celestial object, situated directly over head, is seen in its true position, as if there were no atmosphere.

2. In descending from the zenith to the horizon, the refraction continually increases; objects near the horizon appearing more elevated by it than those of a higher altitude.

3. The rate of its increase is nearly in proportion to the apparent angular distance of the object from the zenith. But this rule, which is not far from the truth, at moderate zenith distances, ceases to give correct results in the vicinity of the horizon, where the law becomes much more complicated in its expression.

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The effects of refraction must be familiar to every person who has seen a walking stick partially plunged into a river, or other collection of water. While the stick is held upright, it appears straight, as usual, because there is no refraction in this position; but if it be ever so little inclined, the refraction takes place, and the stick appears bent; if the inclination be increased, the refraction is also increased.

Another easy and familiar illustration of the effect of refraction may be thus obtained:-Put any small object, as a piece of money, into an empty basin, as near the centre as possible, and retire to such a distance as just to lose sight of the object. Let an assistant then pour water in the basin, and the object will soon appear. Retire again till it is no longer seen; let more water be added, and it will again appear. The experiment may be repeated till the basin is full. The edge of the basin may be supposed to represent the horizon; the water, the atmosphere; and the piece of money, the Sun, or other object which is thus made to appear by the power of refraction, when otherwise it would be invisible.

It follows from this, that one obvious effect of refraction must be to shorten the duration of night and darkness, by prolonging the apparent stay of the Sun and Moon above the horizon. But even after they appear to have set, the influence of the atmosphere still continues to send us a portion of their light; not, indeed, by direct transmission, but by reflection:-for as long as the Sun continues to illuminate

What interesting facts result, from this truth? What is the first general law of atmospheric refraction? What is the second general law? What is the third? Mention a familiar instance of refraction often seen in water. Mention some familiar experiment, to illustrate refraction, and show its application to astronomy? How does this principle affect the duration of nocturnal darkness? By what principle is it that the atmosphere sends us a portion of the solar light, for a considerable time before the Sun rises, and after it has set?

any portion of the atmosphere which is above the horizon, the light from this portion is reflected to the Earth, and it is this that causes twilight.

In the morning, when the Sun arrives at 18o below the horizon, his rays pass over our heads into the higher region of the atmosphere, and are thence reflected, or as it were, bent down to the Earth. The day is then said to dawn, and the light gradually increases until the Sun appears above the horizon: this is called Morning Twilight, or Aurora, which the heathens personified as a goddess. They assigned to her the office of opening the Gates of the East, to introduce the chariot of Apollo or Phœbus.

In the evening, after sunset, the rays of the Sun continue to illuminate the atmosphere, till he sinks 18o below the horizon, and a similar effect, called the Evening Twilight, is produced, only in an inverse progression, for the twilight now gradually lly becomes fainter till it is lost in dark night.

The quantity of reflection and the duration of twilight are much influenced by the changes which are perpetually taking place with respect to the heat and cold, the dryness or moisture, &c. of the atmosphere. The height of the atmosphere, also, has an influence in determining the duration of twilight: Thus in winter, when the air is condensed with cold, and the atmosphere upon that account lower, the twilight will be shorter; and in summer, when the limits of the atmosphere are extended by the rarefaction and dilation of the air of which it consists, the duration of the twilight will be longer. And for the same reason, the morning twilight, (the air being at that time condensed and contracted by the cold of the preceding night,) will be shorter than the evening twilight, when the air is more dilated and expanded.

It is entirely owing to the reflecting power of the atmosphere that the heavens appear bright in the day time. For without such a power, only that part of the heavens would be luminous in which the Sun is placed; and, if we should turn our backs to the Sun, the whole heavens would appear as dark as in the night, and the stars, even at noon day, would be seen as clear as in the nocturnal sky.

In regions of the Earth situated towards the poles, the Sun, during their summer months, is never more than 18° below the horizon; consequently their twilight continues

What is Twilight? How is it occasioned? How is the Evening Twilight produced? By what are the quantity of reflection, and the duration of twilight, considerably influenced? Why is twilight shorter in winter? Why longer in summer? Why is the morning twilight shorter than the evening twilight? To what is it entirely owing, that the heavens appear bright in the day time? How would the heavens appear, if it were not for this power? What are the duration and advantages of twilight in high latitudes?

during the whole night. The same cause has a tendency to diminish the gloom of the long polar nights; for as far north as in lat. 84° 32+ the Sun even when at the winter solstice approaches to within 18° of the horizon, and affords a short twilight once in 24 hours, and the pole itself is left in total darkness not more than 80 days.

There is still another cause which has a tendency to diminish the length of the polar nights, the extraordinary | refraction occasioned by the extreme density of the air in | those regions. This is so great, as to bring the Sun above the horizon some days before it should appear, according to calculation.

A remarkable phenomenon of this kind was observed by the Dutch navigators who wintered in Nova Zembla, in the year 1596. After enduring a continual night of three months, they were agreeably surprised to find that the Sun began to rise seventeen days sooner than according to computation! The observed altitude of the pole, at the place, (says Dr. Smith,) being only 76°, it is impossible to account for the phenomenon, otherwise, than by supposing an extraordinary refraction of the Sun's rays. Kepler computes that the Sun was almost 50 below the horizon when he first appeared; and consequently, that the refraction of his rays was about 10 times greater than with us.

CHAPTER XXVI.

AURORA BOREALIS.

The sublime and beautiful phenomena presented by the Aurora Borealis, or Northern Lights, as they are called, have been in all ages a source of admiration and wonder alike to the peasant and the philosopher. In the regions of the north, they are regarded by the ignorant with superstitious dread, as harbingers of evil; while all agree in placing them among the unexplained wonders of nature.

These lights, or meteoric coruscations, are more brilliant in the arctic regions, appearing mostly in the winter season and in frosty weather. They commonly appear at twilight near the horizon, and sometimes continue in that state for several hours without any sensible motion; after which they send forth streams of stronger light, shooting with great velocity up to the zenith, emulating, not unfrequently, the lightning in vividness, and the rainbow in colouring; and again, silently rising in a compact majestic arch of steady

Relate a remarkable phenomenon of this kind. rora Borealis regarded by the ignorant? In what do are these appearances most frequent and brilliant? their appearance.

How are the phenomena of the Auall agree, respecting them? Where Describe the timos and manner of

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