reason why the latter is longer than the former is, that although the Moon might actually pass round the Earth in 27 days, if the Earth were still; yet, a longer time is consumed from one phase of the Moon to the same phase again, owing to the motion of the Earth in its orbit, in the same direction as the Moon's motion, from west to east; so that the extra 24 days are spent by the Moon in fetching up the overplus of the progress made in the mean time by the Earth.

The Moon revolves on an axis, and it is remarkable that the time of this revolution is just equal to the time which she takes to revolve round the Earth; a consequence of which is, that the Earth always has the same side of the Moon presented to it. The inhabitants, if any, on this side of the Moon have the Earth always before them, while those on the remote side of the Moon can never be blessed with the view of it. The disk, which the Earth must exhibit to the inhabitants of the Moon, having a diameter nearly four times as large as the Moon's, is more than twelve times as large as that which the Moon offers to the Earth. The Earth, must, of course, rise and set to the Moon, and go through the various phases of light, just as we see is done by our celestial attendant, owing to the motion of the Moon round the Earth; otherwise, the Earth will appear fixed in the heavens, relatively to the stars, because the Earth is the centre of the Moon's motion. The Earth too would turn on its axis nearly thirty times, while the Moon is moving once round,-a rapi dity which must seem astonishing to the Lunarians. The phases exhibited to the Moon by the Earth, must be always the reverse of those exhibited at the same time by the Moon to the Earth, as will be evident from

*

inspecting the subsequent diagram. (Fig. 18.) We may imagine what must be the feeling of intense curiosity with which those on the further side of the Moon seek for information concerning the splendid orb, visible to the other lunar hemisphere. Suppose now that we, in England, were to hear of a splendid Moon, of vast dimensions, being visible to the inhabitants of the Sandwich Islands or New-Zealand, and that we ourselves were without a Moon, how eagerly would our navigators and travellers look out for the brilliant visiter, when they arrived at the line on the Earth's surface at which this spectacle became visible! At all seasons of the year we see the Sun and also the Moon: this arises from the circumstance, that although the Earth revolves on her axis, yet the time which she takes in that rotation is neither that which she takes to revolve round the Sun, nor that which the Moon takes to revolve round the Earth. This is a striking instance of the great effects which often result from what we should, at first sight, deem slight causes. We know not whether any living beings inhabit the surface of the Moon; but if such be the case, they must be divided into two portions, very unequally favoured, as regards the view of the Earth, one portion having it always in sight, while to the other portion the Earth is altogether invisible.

From the foregoing statements it will be seen that one day and one night are completed to the inhabitants of the Moon during one of her revolutions round the Earth. Hence the lunar day and night must be each as long as 143 of our days. One side of the Moon must have the full glare of the Sun's light and heat, almost unmitigated: the lunar atmosphere, if there be

any, being very rare, and therefore not holding clouds suspended in it, while the contrary side of the Moon is enveloped in almost profound darkness and intense cold; this must especially be the case at that side of the Moon which the Earth never sees. From this will result, also, the lunar summer and winter, coincident with the lunar day and night; for such must be produced merely by the Moon's rotation on its axis, the inclination of this axis to the ecliptic being, as we said, so small as to produce no change of seasons.

Though, as was before said, we see only one side of the Moon, yet there is occasionally seen somewhat more than an exact hemisphere, which effect is termed libration, a word implying a poising, or oscillation, of the body of the Moon, owing to certain disturbing causes.

When a little of the further side of the Moon on either side is seen, owing to the irregularity of the Moon in her orbit, by reason of the attractions of the Sun and the Earth, this effect is termed the Moon's libration in longitude; and when, owing to the Moon's axis not being quite perpendicular to her orbit, a little of the further side of the Moon is seen at either of her poles, this effect is called the Moon's libration in latitude. Owing to the causes which produce these effects, the Moon's motion is the most irregular and complicated of any of the celestial bodies. Moreover, we do not exactly see the same face of the Moon when she is in the horizon as when she is in the zenith, which difference is termed diurnal libration.

The phenomenon of the HARVEST-MOON has naturally attracted the attention of mankind at all times; who, feeling the benefit of the Moon's light more especially in the autumnal months, have considered it a

token of the goodness of God, in giving them a greater share of natural light at a time, when it was so necessary to use diligence in getting in the fruits of har

vest.

But, as it has pleased the Almighty to ordain physical causes to act in His moral government of the world; so also in this instance ;-not, however, leaving us to be less grateful to His superintending Providence. Now, in the month of September, when the Sun is in the constellation Virgo, and the Moon in Pisces, the latter rises full every evening together, soon after sunset, with very little difference of time. The like occurs in October, when the Sun is in Libra, and the Moon in Aries. The former is called the Harvest-Moon, and the latter the Hunter's Moon.

The phenomenon of the Moon's rising with very little difference of time for several days together, occurs likewise in other months besides September and October; as the Moon is in the constellations Pisces and Aries every month. But, as at one season the Moon is new, and at others the Moon is in her quarters, her rising is then scarcely regarded. The fact of the Moon thus rising every evening with but little difference of time, is due to the smallness of the angle made in those months, by the ecliptic and horizon. This is easily and beautifully exemplified with the celestial globe.

The orbit of the Moon is inclined to that of the Earth, or the ecliptic, at an angle of about 5°; and the points, where these orbits cut each other, are called nodes, from the Latin nodus, a knot. This observation applies likewise to all the planets in their crossing the ecliptic, or orbit of the Earth. The Moon and the different planets are alore the orbit of the Earth, during

one-half of their orbital revolutions, and below the Earth's orbit during the other half. In figure 17, let A B C D represent the ecliptic, and a b c d the orbit of Mercury. The dotted line e f is called the line of the nodes, because it passes through those parts of the respective orbits which cut each other, or are on a level with each other, which level is e c a f. The planet, in moving round from west to east, goes in the direction a b c d; a is, therefore, called the ascending node, c the descending node.

Fig. 17.

B

THE NODES OF A PLANET.

The word disk, so frequently used in reference to the Sun and Moon, implies a flat circular appearance, such as is presented by the Sun, Moon, and other heavenly bodies when magnified; it is derived from the Greek word for hurling, the disk being a flat circular piece of metal, used in this exercise; we call it a quoit.

We shall now notice in order those phenomena presented by the Earth and Moon, which are dependent

I