LECTURE V.

Of the Earth's companion-the Moon.

WE proceed now to some account of the companion of the Earth in its annual course round the Sun, for so we may justly designate the Moon; which, next to the Sun, is the most conspicuous of the heavenly bodies. The changes she undergoes in appearance, are more striking and more obvious than those of any other of the planetary bodies, and her apparent motions are more rapid. Hence the motions and changes of the Moon were attended to before much was known of the Sun; and hence the earlier inhabitants of the Earth reckoned their time by the apparent motions of the Moon, and followed the lunar, not the solar year.

The united observations of many astronomers concur in determining the distance of the Moon from the Earth to be about 240,000 miles, being about 1-400th part the distance of the Earth from the Sun. Her diameter is computed to be 2144 English miles. That the Moon is a circular body, we have very frequent opportunity of seeing; as well as that she is subject to constant, regularly-repeated variations of light and shadow and as the enlightened side of the Moon is always turned towards the Sun, it amounts to demonstration that her light is borrowed from the Sun, and reflected to us.

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The regular variation of light, called the phases of the Moon, is extremely curious. If we follow the apparent changes of the Moon's situation in regard to the Sun and Earth, it will become obvious that her light is derived from the Sun, and that the proportion of her light reflected to us, depends on her situation in respect of the Sun and the Earth. The late eclipse of a large proportion of the Sun's face or disk, arose, as we all know, from the Moon's being betwixt us and the Sun-when, as it is termed by the astronomer, the Sun and Moon were in conjunction. The circumstance of her hiding from our sight a part of the Sun's disk, proves that the Moon is an opake body. She was traversing the starry sphere as has been said, between the Sun and Earth, and was then crossing the plane of the Earth's orbit, or the plane of the ecliptic; her illuminated side being towards the Sun, and her dark side towards us. During two or three succeeding days, the Moon was hidden from our sight by the splendour of the Sun's rays; but as she rises in the east about 50 minutes later every day, her apparent motion is not so quick as the apparent motion of the Sun: each day therefore the Sun appeared to us to approach more nearly to the west than the Moon; which, in two or three days after the eclipse, appeared, after the Sun had set in the west, a little to the east of him*. -a little above the horizon in the west,-shewing a small portion of the illumined part of her disk, turned towards the Sun. A few evenings after, the Moon appeared in the south, when the Sun was in the west,

*That the real motion of the Moon is from west to east is readily proved by observation. For if she be seen near a particular fixed star one evening, she will be seen about 13 degrees eastward f the same star the next night, and so on.

one half the enlightened part of her disk was then visible: and at the end of several days more, she appeared in the east when the Sun was in the west, with a round illuminated orb, which we term the full Moon. She was then in the situation which the astronomer terms, in opposition; the Earth was between, though not immediately between the Sun and the Moon; for if the Earth had been in a direct line between them, the Moon would have been eclipsed; the Earth would have intercepted the Sun's rays. After the full, her enlightened side, gradually, night by night, appeared less to us, until her next conjunction; until she was again between the Earth and the Sun, though not in a direct line. The reason why an eclipse did not then happen, I shall endeavour to point out by and by. Being then nearly between the Earth and the Sun, she was again hidden from us by his rays.

The variations of light exhibited by the Moon may be represented pretty correctly, by placing a lighted candle on a table to represent the Sun; a ball at some distance from it to represent the Earth, and then carrying a smaller white ball round it, as the Moon revolving about the Earth.

In this little experiment, the observer's eye is supposed to be just above the ball representing the Earth, and directed towards that substituted for the Moon. When the latter is between the Sun and Earth, or in conjunction, its dark side will be towards the Earth. Having advanced 1-8th part of the circle round the Earth, the Moon will appear horned; she is then said to be in her first octant. When advanced 1-4th of the circle, one half of her illuminated side is turned towards the Earth; the Moon is then in her first quarter. When advanced

3-8ths of the circle, she appears gibbous, and is then in her second octant. When half the circle is made, the Earth is between the Sun and Moon, and then her whole enlightened side is turned towards the Earth; she appears full, and is said to be in opposition-she is then in her second quarter. Advanced 5-8ths of the circle, she appears again gibbous, and is in her third octant. When 3-4ths of the circle are made, one half of her illuminated side is again turned towards the Earth; she is then in her third quarter. When 7-8ths are completed, she appears again horned, and is in her fourth octant. Advancing 1-8th more, she completes the circle, and is then between the Earth and Sun again, or in conjunction. It is then new Moon again.

But though this experiment shews correctly the variation of light, or phases of the Moon, we cannot fail to observe that there must be at least one false representation in it; for in it, the Earth, which we are certain moves continually, is fixed in one place: this, therefore, which is the common mode of explaining the phenomenon, is one instance in which the deceptive appearances belonging to the motions of the heavenly bodies may be represented with great apparent faithfulness, by means still more deceptive. So that in explaining delusive natural appearances (for in this science we must not believe our eyes) we accumulate error;-in attempting to account for one deception, we practise another.

The Earth is thought to be a moon to the Moon, exhibiting to it the same variations of light as the Moon does to us; but appearing 13 times as big, and afford ing 13 times more light to her than she does to us; but this is only supposition. It has however been proved by a set of curious experiments, that the light emitted by

the full Moon is 300,000 times less intense than that of the Sun. The Moon's light, even when concentrated by the most powerful glasses, produces no effect on the thermometer: so that she does not reflect to us any of the heat she derives from the Sun.

If it be true that the Earth is a moon to the Moon, then, at the conjunction or new Moon, one half of the Moon will be enlightened by the Sun, and the other by the Earth; and when in opposition or at full, one half of the Moon will be in darkness.

There is a remarkable difference in the apparent size of the Moon, during the same evening, according to her place in the sphere. When at her greatest elevation, she appears much less than when on the horizon. This is the more remarkable, since we may readily convince ourselves that the Moon when on the horizon, is about 4000 miles further from us, and therefore ought to appear less, than when at her greatest height above it.

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Let the largest of the above circles be the Earth, of which the spot in the middle represents the centre, and on the surface of which we reside at a, viewing the full Moon nearly in the zenith at M. The centre of the Moon is distant from that of the Earth 240,000 miles; at a therefore we are more than half the diameter of the earth (nearly 4000 miles) nearer to the Moon at her greatest elevation at M, than when she is in the horizon