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ed by the unlearned, with much expression, the old Moon in the new Moon's arms. Probably it is an optical illusion. Any object bright appears larger than when dark, or faintly illuminated. View from a distance a house painted partly white, the remainder unpainted or dark. The bright part will appear to rise above the unpainted or dark part.

It is a singular phenomenon, that the Sun or Moon near the horizon, appears larger than when seen in the meridian. It is the more singular, as the disk of either, particularly the Moon, must, by calculation, subtend at the Earth an angle increasing with the height of the luminary. To an observer on the Earth's surface, the Sun or Moon must be about 4000 miles nearer at the zenith than at the horizon. The principle is corroborated by actual admeasurement. When subjected to this, either luminary appears largest on the meridian. The apparent increase of magnitude at the horizon must be an illusion of the observer's sight. Objects at a distance appear smaller or larger as our imagination assigns them a situation more or less remote. Without doubt, the difference in appearance must be affected by the medium of vision. Objects often appear magnified in a mist or fog; and, in the dusk of the evening, a small cottage. has been mistaken for a distant church. Near the horizon, our vision is rendered more or less obscure by water and other particles floating in the atmosphere. But the view of remote objects may be most affected by the intervention of other objects. Bodies on the opposite side of a plain, a wide-spread meadow, or lake, appear small, because, by the imagination, they are placed at a distance far less than the reality. A ball or vane on the top of a lofty spire appears far less than the same object at an equal distance on the ground. But the error of vision seems corrected by intervening objects, which make us conceive those beyond at a greater distance.

Let the principle be applied to the Sun or Moon,

No objects in

seen on the meridian or at the horizon. tervene between us and the meridian luminary. Imagination, therefore, places it near the observer. But, at the horizon, a distant hamlet or village, hill, mountain, or other object, seems to throw back the heavenly body, and, making it seem at a greater distance, enlarges its apparent magnitude.

The uniformity of the Moon's visage, or its exhibiting always the same face, is subject to some alteration. Spots on the east and the west, on the north and the south of the Moon, appear and disappear in rotation. The phenomena are produced by the Moon's librations. These are of four kinds. The diurnal motion of the Earth on its axis, carrying the spectator farther north or south, causes the daily libration of the Moon.

The libration of the Moon in longitude is caused by her uniform motion on her axis, and the irregularity of her motion round the Earth.

The libration of the Moon in latitude is caused by the inclination of her orbit to the plane of the ecliptic.

The other is a small libration, caused by the attractive force of the Earth on the spheroidical figure of the Moon.

How far is the Moon distant from the Earth? Why is the Moon interesting to us? In what time does the Moon perform a revolution from a point in the ecliptic to the same again? What is her sidereal revolution? What is a lunation? Why does the Moon always present the same face to us? What may cause the Moon to turn on its axis? What has been the error of authors respecting the time of the Moon's revolution on its axis? What is the diameter of the Moon? How must the Earth appear to lunarians? What difference is there in the hemispheres of the Moon respecting light? Why does the Moon shine? What is the cause of the Moon's exhibiting different phases? Have the lunarians difference of seasons? Has the Moon an atmosphere? Why does the Moon exhibit dark spots, called the Man in the Moon? Are the mountains in the Moon similar to those of the Earth? Why are the irregularities in the Moon's surface useful to us? If the Moon's surface were covered with water, how would it reflect the Sun's image? What proportion does the light of the Moon bear to that of the Sun, according to different authors? How far distant from the Earth does the Moon appear to a spectator at the Sun? What is the apogee

and perigee of the Moon's orbit? What is syzygy? What did Dr. Herschel think he saw in the Moon? When the Moon is about three or four days old, what does she exhibit? Why do the Sun and Moon appear larger in the horizon than on the meridian ? What are the librations of the Moon.

SECTION IX. Of Mars.

Mars, in distance from the Sun, is next to the Earth in the solar system. The red, fiery color of this planet attracted the attention of the ancients. Hence they gave it the name of their god of war. Hence also it "is usually represented by this character, &, which is said to be rudely formed from a man holding a spear protruded, representing the god of war."

Some have thought the color of Mars may arise from his being of a nature suited to reflect the red rays of light. But the prevailing opinion is, that it arises from the extended and dense atmosphere of the planet. The color of a beam of light, passing through a dense medium, inclines to red; the color always being brightened in proportion to the density of the medium, and the distance passed. The red, the least refrangible rays, seem more strong and vigorous than the violet, the most refrangible rays. The former will traverse an atmosphere, when the latter will be absorbed or diverted. Hence the rud

dy appearance of this planet and of the Moon eclipsed; and hence the beautiful tinge of the morning and evening clouds.

In 1665, Dr. Hooke discovered spots on Mars. From a motion perceived in these, he concluded this planet had a rotation on its axis. In 1666, Mr. Cassini observed spots on Mars. By diligent observation on these, at different times, he ascertained that Mars performed a revolution round his axis in 24 hours 40 minutes.

To the inhabitants of the Earth Mars appears sometimes gibbous; sometimes full; never horned Fig

ures 4, 5, and 6, of Plate iii, represent different telescopic appearances of Mars. At Fig. 5 he appears gibbous.

Besides the dark spots on Mars, Miraldi observed (a luminous zone round his south pole, which he asserts had been seen by astronomers 60 years before his time. This singular phenomenon is represented in Plate iii. Figs. 5 and 6. At Fig. 5, this spot presents a singular appearance, apparently projecting beyond the disk of the planet, and producing a breach which seems the greater on account of the gibbous appearance of the planet. Other astronomers have observed a peculiar splendor or brightness at both the poles of Mars. This is subject to much variation. Dr. Herschel supposes the splendor arises from the snow around the poles; and that the variation in the appearance is caused by the melting of the polar ice.

The greatest distance of Mars from the Earth is about 269,000,000 miles; the least, 49,000,000 miles. Hence this planet, when in conjunction with the Sun, appears 23 times less than when in opposition.

The figure of Mars is an oblate spheroid, the equatorial diameter being to the polar nearly as 16 to 15.

To an inhabitant of Mars the Earth must appear as a morning and evening star as Venus does to us, but of a less magnitude, and less brilliant. It must exhibit all the phases of the Moon, like Mercury and Venus, to the inhabitants of the Earth.

ELEMENTS OF MARS.

Mean diameter, 4189 miles.

Mean diameter, as seen from the Sun, 6".
Inclination of his orbit to the ecliptic, 1° 51' 4".
Tropical revolution, 686 d. 22 h. 57 m. 58 s.
Sidereal revolution, 686 d. 23 h. 30 m. 35 s.
Place of ascending node, Taurus, 18° 15′ 8′′.
Place of descending node, Scorpio, 18° 15′ 8′′.

Motion of the nodes in longitude for 100 years,

45' 33".

Retrograde motion of the nodes in 100 years, 37' 59". Place of the aphelion, 5 s. 2° 57′ 54′′.

Motion of the aphelion in longitude for 100 years, 1° 51' 40".

Diurnal rotation, 24 h. 40 m.

Mean distance from the Sun, 144,000,000 miles.
Eccentricity, 13,474,515 miles.

Why was Mars represented by the character ? Why does Mars appear of a fiery color? Who discovered spots on Mars? In what time does Mars perform a revolution on his axis? How does Mars sometimes appear to the inhabitants of the Earth? What did Miraldi observe in Mars? What have other astronomers observed in this planet? How much less does Mars appear to us in conjunction than in opposition? What is the form of Mars? How must the Earth appear to the inhabitants of Mars?

SECTION X. Of the Asteroids.

The term Asteroid is formed from two Greek words, asteer, a star, and eidos, appearance. The term is said to have been first applied by Dr. Herschel.

The Asteroids are four small planets, the orbits of which are between those of Mars and Jupiter

Prior to the discovery of the Asteroids, irregularities in the motions of the old planets) led some astronomers to suppose there must be a planet between Mars and Jupiter. (The first day of the present century is memorable for the discovery of an intervening planet: three others were afterwards discovered; so that the astronomer of the present century has the peculiar felicity to know, that not one planet only, but four, occupy the intervening space between Mars and Jupiter.

"These minor planets are also represented by symbols: Vesta, by a character resembling an ancient altar, with the sacred fire Juno, by the symbol of Mercury, with the superior curves turned the contrary way,

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