§ 15. The time which our Earth needs to travel once completely round the Sun, (to finish one revolution) is called a Year. Such a year has three hundred and Sixtyfive days;—and each of these days has again Twentyfour hours.
You see from this that the revolution of the Earth round the Sun gives us a means of measuring time, by which we are able to bring order and regularity into our business and transactions of life. For the making of clocks and watches is but a late invention, and we should be left entirely in the dark as regards the history of former ages, and a great many people would, at this present moment, be incapable of forming a correct estimate of time, if Providence had not given to all this appropriate means of measuring it.
§ 16. While the Earth needs a whole year for one revolution round the Sun; Mercury requires but Eightyone days, and Venus only about two thirds of One of our years. Mars, on the contrary, needs for one of his revolutions almost Two years; Vesta almost Four; Juno, Ceres and Pallas over Four years; Jupiter almost Twelve, Saturn over Twentynine, and Herschel nearly Eightyfour of our years![3]—And if these Planets, as we have reason to believe, are inhabited by beings endowed with human understanding and faculties, numbering their years as we do ours—by the revolution of their Planets round the Sun—how different from ours must be the Period of their existence!!
§ 17. During the time that the Earth is performing her journey round the Sun, the Moon, our constant attendant, is continually moving round the Earth, and completes one of these revolutions in little more than Twentyseven days.—Very important and interesting to us are the changes in appearance which she exhibits during each of these revolutions.—You probably will know, that the Moon does not appear to us, at all times, the same. Sometimes she is hardly at all visible, (at least not with the naked eye); at other times only a small rim of her is seen, which by degrees becomes larger and larger, until finally she appears in her full round form. After this she begins again to diminish, changes again into a small luminous rim, and finally disappears entirely from our sight. These successive changes in the Moon’s appearance are called the Moon’s Phases, or the waxing and waning of the Moon. The time during which the Moon is not seen is called New Moon; the time during which she exhibits her full shape is called the Full Moon; and the different periods of her waxing and waning (when she appears to us in the form of a crescent) are called Quarters. Thus we speak of the First and of the Last Quarter of the Moon. The First Quarter takes place after New Moon; the last Quarter after Full Moon.
The following diagram, Plate No. [VII], may serve to represent to you the Moon’s phases as seen from our Earth.
When the Moon is in a, then the light of the Sun falls just on that side of it which is turned from the Earth. It is then, we have New Moon. When in b, a small brim of the Moon is seen, because a small portion of its lighted surface is then turned towards the Earth.—When in c half of her lighted surface is turned towards us, and we have the First Quarter. In d a still greater portion of the Moon’s lighted surface is visible, and in e, we have Full Moon, because her whole lighted surface is then turned towards the Earth. In f the moon commences to wane (to grow smaller,) and in g the last quarter commences; finally, when passed through the point h, we have in a, again New Moon. For familiar illustration you may also take a white ivory ball, holding it before a lighted candle, which may take the place of the Sun. When the ball is in a straight line between your eye and the candle it will appear to you all dark; because the lighted part is then entirely turned toward the candle (away from you), and you have the same case which is represented to you in the diagram, when the Moon is in a. But if you move the ball a little to the right, you will perceive a streak of light, similar to the First Quarter represented in the Diagram, when the Moon is in c. If moved still farther to the right, so that the whole lighted part of the ball is seen, it will resemble the Full Moon; represented in the Diagram, when the Moon is in e.
No. VII.
No. VIII.