Page images
PDF
EPUB

the next evening, we find her about 13° farther east of the Sun than on the preceding evening, and her crescent of light sensibly augmented. Repeating these observations, we perceive that she departs farther and farther from the Sun, as her enlightened surface comes more and more into view, until she arrives at her first quarter, and comes to the meridian at sun-set. She has then finished half her course from the new to the full, and half her enlightened hemisphere is turned towards the Earth.

After her first quarter, she appears more and more gibbous, as she recedes farther and farther from the Sun, until she has completed just half her revolution around the Earth, and is seen rising in the east when the Sun is setting in the west. She then presents her enlightened orb full to our view, and is said to be in opposition; because she is then un the opposite side of the Earth with respect to the Sun.

In the first half of her orbit she appears to pass over our heads through the upper hemisphere; she now descends below the eastern horizon to pass through that part of her orbit which lies in the lower hemisphere.

After her full she wanes through the same changes of appearance as before, but in an inverted order; and we see her in the morning like a fine thread of light, a little west of the rising-sun. For the next two or three days she is lost to our view, rising and setting in conjunction with the Sun; after which, she passes over, by reason of her daily motion, to the east side of the Sun, and we behold her again a new Moon, as before. In changing sides with the Sun, she changes also the direction of her crescent. Before her conjunction, it was turned to the east; it is now turned towards the west. These different appearances of the Moon are called her phases. They prove that she shines not by any light of her own; if she did, being globular, we should always see her a round full orb like the Sun.

The Moon is a satellite to the Earth, about which she revolves in an elliptical orbit, in 29 days, 12 hours, 44 minutes, and 3 seconds: the time which elapses between one new moon and another. This is called her synodic revolution. Her revolution from any fixed star to the same star again, is called her periodic or siderial revolution. It is accomplished in 27 days, 7 hours, 43 minutes, and 111⁄2 seconds; but in this time, the Earth has advanced nearly as many degrees in her orbit; consequently the Moon, at the

How is it known that the Moon does not shine by her own light? About what does the Moon revolve, and what is the figure of her orbit? What is the time of her revolution from one new Moon to another? What is this revolution denominated? What is her pe riodic or sidereal revolution? In what time is this accomplished?

end of one complete revolution, must go as many degrees farther, before she will come again into the same position with respect to the Sun and the Earth.

The Moon is the nearest of all the heavenly bodies, being about 30 times the diameter of the Earth, or 240,000 miles, distant from us. Her mean daily motion, in her orbit, is nearly 14 times as great as the Earth's; since she not only accompanies the Earth around the Sun every year, but, in the meantime, performs nearly 13 revolutions about the Earth.

although the apparent motion of the Moon, in her orbit, is greater than hat of any other heavenly body, since she passes over, at a mean rate, ne less than 13° 10′ 35′′ in a day; yet this is to be understood as angular motion -motion in a small orbit, and therefore embracing a great number of degrees, and but comparatively few miles.

As the Moon, while revolving about the Earth, is carried with it at the same time around the Sun, her path is extremely irregular, and very different from what it seems to be. Like a point in the wheel of a carriage, moving over a convex road, the Moon will describe a succession of epicycloidal curves, which are always concave towards the Sun; not very unlike their presentation in the following figure.

[merged small][merged small][graphic]
[graphic]

To what is the difference of time in these two revolutions owing? How great is the distance of the Moon from the Earth, compared with that of the other heavenly bodies! What is her distance from us? What is her motion in her orbit, compared with the Earth's? How many times does she revolve around the Earth, every year? The apparent motion of the Moon is greater in her orbit than that of any other heavenly body; is it to be understood that she passes through a correspondent space? Describe the Moon's path.

Let Adb B represent a portion of the Earth's orbit; and a bede the lunar orbit. When the Earth is at b, the new Moon is at a; and while the Earth is moving from 6 to its position as represented in the figure, the Moon has moved through half her orbit, from a to c, where she is full; so while the Earth is moving from its présent position to d, the Moon describes the other half of her orbit from c to e; where she is again in conjunction.

The Moon, though apparently as large as the Sun, is the smallest of all the heavenly bodies that are visible to the naked eye. Her diameter is but 2162 miles; consequently her surface is 13 times less than that of the Earth, and her bulk 49 times less. It would require 70 millions of such bodies to equal the volume of the Sun. The reason why she appears as large as the Sun, when, in truth, she is so much less, is because she is 400 times nearer to us than the Sun.

The Moon revolves once on her axis exactly in the time that she performs her revolution around the Earth. This is evident from her always presenting the same side to the Earth; for if she had no rotation upon an axis, every part of her surface would be presented to a spectator on the Earth, in the course of her synodical revolution. It follows, then, that there is but one day and night in her year, containing, both together, 29 days, 12 hours, 44 minutes, and 3 seconds.

As the Moon turns on her axis only as she moves around the Earth, it is plain that the inhabitants of one half of the lunar world are totally deprived of the sight of the Earth, unless they travel to the opposite hemisphere. This we may presume they will do, were it only to view so sublime a spectacle; for it is certain that from the Moon the Earth appears ten times larger than any other body in the universe.

As the Moon enlightens the Earth, by reflecting the light of the Sun, so likewise the Earth illuminates the Moon, exhibiting to her the same phases that she does to us, only in a contrary order. And, as the surface of the Earth is 13 times as large as the surface of the Moon, the Earth, when full to the Moon, will appear 13 times as large as the full moon does to us. That side of the Moon, therefore, which is towards the Earth, may be said to have no darkness at all, the Earth constantly shining upon it with extraordinary splendour when the Sun is absent; it therefore enjoys successively two weeks of illumination from the Sun, and two

What is her magnitude, compared with that of the other heavenly bodies? What is her diameter? How great are her surface and her bulk, compared with those of the Earth? How many such bodies would it require to equal the volume of the Sun? Why does she appear as large as the Sun, when in reality she is so much less? What is the time of her revolution on her axis, compared with that of her revolution around the Earth? How is this proved? How many days and nights then has she in the course of her synodical revolution? What is the length of both united? Describe the phenomena of the Earth as ween by the inhabitants of the Moon.

[graphic]
[graphic]
[graphic]

weeks of earth-light from the Earth. The other side of the Moon has alternately a fortnight's light, and a fortnight's darkness.

As the Earth revolves on its axis, the several continents, seas, and islands, appear to the lunar inhabitants like so many spots, of different forms and brightness, alternately moving over its surface, being more or less brilliant, as they are seen through intervening clouds. By these spots, the lunarians can not only determine the period of the Earth's rotation, just as we do that of the Sun, but they may also find the longitude of their places, as we find the latitude of

ours.

As the full Moon always happens when the Moon is directly opposite the Sun, all the full Moons in our winter, must happen when the Moon is on the north side of the equinoctial, because then the Sun is on the south side of it; consequently, at the north pole of the Earth, there will be a fortnight's moon-light and a fortnight's darkness by turns, for a period of six months, and the same will be the fact during the Sun's absence the other six months, at the south nole.

The Moon's axis being inclined only about 11° to her orbit, she can have no sensible diversity of seasons; from which we may infer, that her atmosphere is mild and uniform. The quantity of light which we derive from the Moon when full, is at least 300 thousand times less than that of the Sun.*

When viewed through a good telescope, the Moon presents a most wonderful and interesting aspect. Besides the large dark spots, which are visible to the naked eye, we perceive extensive valleys, shelving rocks, and long ridges of elevated mountains, projecting their shadows on the plains below. Single mountains occasionally rise to a great height, while circular hollows, more than three miles deep, seem excavated in the plains.

Her mountain scenery bears a striking resemblance to the towering sublimity and terrific ruggedness of the Alpine re

*This is Mons. Bouquer's inference, from his experiments, as stated by La Place, in his work, p. 42. The result of Dr. Wollaston's computations was different. Professor Leslie makes the light of the Moon 150,000 times less than that of the Sun: it was former'y reckoned 100,000 times less.

[graphic]

As the Earth revolves on its axis, how do its continents, seas, and islands, appear to the lunar inhabitants? For what purposes may these spots serve to the lunarians? What are the periods of the Moon's presence and absence to the polar inhabitants 3 Explain this. Why cannot the Moon have any sensible diversity of seasons? What then may we infer to be the character of her atmosphere? What is the quantity of light which she affords when full, compared with that of the Sun? Describe the appearance of the Moon when seen through a good telescope. What mountains of the Earth does her mountain scenery resemble ?

[graphic]

gions, or of the Appenines, after which some of her mountains have been named, and of the Cordilleras of our own continent. Huge masses of rock rising precipitously from the plains, lift their peaked summits to an immense height in the air, while shapeless crags hang over their projecting sides, and seem on the eve of being precipitated into the tremendous chasm below.

Around the base of these frightful eminences, are strewed numerous loose and unconnected fragments, which time seems to have detached from their parent mass; and when we examine the rents and ravines which accompany the overhanging cliffs, the beholder expects every moment that they are to be torn from their base, and that the process of destructive separation which he had only contemplated in its effects, is about to be exhibited before him in all its reality.

The range of mountains called the Appenines, which traverses a portion of the Moon's disc from north-east to southwest, and of which some parts are visible to the naked eye, rise with a precipitous and craggy front from the level of the Mare Imbrium, or Sea of showers.* In this extensive range are several ridges whose summits have a perpendicular elevation of four miles, and more; and though they often descend to a much lower level, they present an inaccessible barrier on the north-east, while on the south-west they sink in gentle declivity to the plains.

There is one remarkable feature in the Moon's surface which bears no analogy to any thing observable on the Earth. This is the circular cavities which appear in every part of her disc. Some of these immense caverns are nearly four miles deep, and forty miles in diameter. They are most numerous in the south-western part. As they reflect the Sun's rays more copiously, they render this part of her surface more brilliant than any other. They present to us nearly the same appearance as our Earth might be supposed to present to the Moon, if all our great lakes and seas were dried up.

The number of remarkable spots on the Moon, where latitude and longitude have been accurately determined, exceeds 200. The number of seas and lakes, as they were formerly considered, whose length and breadth are known,

* The name of a lunar spot.

[graphic]

Describe the appearance of her mountains. On what part of her disc is that range of mountains called the Appenines, situated? Describe it. What remarkable feature in the Moon's surface, bears no analogy to any thing observable on the Earth's surface? Describe their appearance. What is the number of remarkable spots in the Moon's surface, whose latitude and longitude have been accurately determined? What is the number of seas and lakes, as they were formerly considered, whose dimensions are known?

« PreviousContinue »