Fig. 42.

APPEARANCE OF THE EARTH TO LUNARIANS.-If there be any lunar inhabitants on the side toward us, the earth must present to them all the phases which their world exhibits to us, only in a reverse order. When we have a new moon, they have a full earth, a bright full-orbed moon fourteen times as large as ours. The lunar inhabitants upon the side opposite to us of course never see our earth, unless they take a journey to the regions from whence it is visible, to behold this wonderful spectacle. Those living

near the limbs of the disk might, however, on account of the librations, get occasional glimpses of it near their horizon.

THE EARTH-SHINE.-For a few days before and after new moon, we may distinguish the outline of the unillumined portion of the moon. In England, it is popularly known as "the old moon in the new moon's arms." This reflection of the earth's rays must serve to keep the lunar nights quite light, even in new earth.

PHASES OF THE MOON.-The phases of the moon show conclusively that it is a dark body, which shines only by reflecting the light it receives from

the sun. Let us compare its various appearances with the positions indicated in the figure.

We see it (1) as a delicate crescent in the west just after sunset, as it first emerges from the sun's

rays at conjunction. It soon sets below the horizon Half of its surface is illumined, but only a slender edge with its horns turned from the sun is visible to us. Each night the crescent broadens, the moon recedes about 13° further from the sun, and sets correspondingly later, until at quadrature half of the enlightened hemisphere is turned toward us, and the moon is said to be in her first quarter. Continuing her eastern progress round the earth, the moon (2) becomes gibbous* in form, and, about the fifteenth day from new moon, reaches the point in the heavens directly opposite to that which the sun occupies. She is then in opposition, the whole of the illumined side is turned toward us, and we have a full moon. She is on the meridian at midnight, and so rises in the east as the sun sets in the west, and vice versa.

The moon (3) passing on in her orbit from opposition, presents phases reversed from those of the second quarter. The proportion of the illumined side visible to us gradually decreases; she becomes gibbous again; rises nearly an hour later each evening, and in the morning lingers high in the western sky after sunrise. She now comes into quadrature, and is in her third quarter.

From the third quarter the moon (4) turns her enlightened side from us and decreases to the crescent form again; as, however, the bright hemisphere

* Gibbous means less than a half and more than a quarter circle.

constantly faces the sun, the horns are pointed toward the west. She is now seen as a bright crescent in the eastern sky just before sunrise. At last the illumined side is completely turned from us, and the moon herself, coming into conjunction with the sun, is lost in his rays. To accomplish this journey through her orbit from new moon to new moon, has required 29 days—a lunar month.

Moon runs high or low.-All have, doubtless, noticed that, in the long nights of winter, the full moon is high in the heavens, and continues a long time above the horizon; while in midsummer it is low, and remains a much shorter time above the horizon. This is a wise provision of Providence, which is seen yet more clearly in the arctic regions. There the moon, during the long summer day of six months, is above the horizon only for her first and fourth quarters, when her light is least; but during the tedious winter night of equal length, she is continually above the horizon for her second and third quarters. Thus in polar regions the moon is never full by day, but is always full every month in the night. We can easily understand these phenomena when we remember that the new moon is in the same quarter and the full moon in the opposite quarter of the heavens from the sun. Consequently, the moon always becomes full in the other solstice from that in which the sun is. When, therefore, the sun sinks very low in the southern sky the full moon rises high, and when the sun rises high the full moon sinks low.

HARVEST MOON.-While the moon rises on the average 50 m. later each night, the exact time varies from less than half an hour to a full hour. Near the time of autumnal equinox the moon, at her full, rises about sunset a number of nights in succession. This gives rise to a series of brilliant moonlight evenings. It is the time of harvest in England, and hence has received the name of the Harvest Moon. Its return is celebrated as a festival among the peasantry. In the following month (October) the same occurrence takes place, and it is then termed the Hunter's Moon. The cause of this phenomenon lies in the fact that the moon's path is variously inclined to the horizon at different seasons of the year. When the equinoxes are in the horizon, it makes a very small angle with the horizon; whereas, when the solstitial points are in the horizon, the angle is far greater. In the former case, the moon moving eastward each day about 13°, will descend but little below the horizon, and so for several successive evenings will rise at about the same hour. In the latter, she will descend much further each day and thus will rise much later each night. The least possible variation in the hour of rising is 17 minutes-the greatest is 1 hour, 16 minutes.

In the figure, S represents the sun, E the earth, M the moon; CF the moon's path around the earth when the solstitial points are in the horizon-ED when the equinoxes are in the horizon; AMBS the