[§ 17.] What motion does the Moon make whilst the Earth is travelling round the Sun? How many days does the Moon need for a complete revolution round the Earth? Does the Moon during its revolution round the Earth always exhibit the same shape? What changes then does she gradually undergo? What do you call the successive changes in the Moon’s appearance? What do you call the time during which the Moon is not seen? What, that, during which she exhibits her full shape? What, the different periods of her waxing and waning? When does the First Quarter take place? When the last? [The teacher might now require the explanation of Plate V; the elder pupils may draw the Diagram.] [§ 18.] What does frequently occur during the Moon’s motion round the Earth? What is the consequence of the Moon’s position in a direct line between the Sun and ourselves? What is such a diminution of light in consequence of the Moon being placed between us and the Sun, called? What takes place when the Earth is placed between the Sun and the Moon? What is such an obscuration of the Moon, in consequence of the Earth’s position between her and the Sun, called? [The younger pupils ought now to explain Plate VIII; the elder pupils ought to draw the Diagram on a slate or paper.] [§ 19.] Is the number of Comets belonging to our Solar System precisely ascertained? How many are there _supposed_ to belong to our System? In what manner do they move round the Sun? What remarkable property do they possess? FOOTNOTES: [2] If the teacher has an orrery at hand, if it be even of the most simple construction, he may exhibit it now. But it is the author’s belief that a considerable portion of the pupil’s interest is lost, if he be acquainted with it, at the beginning of the study; or, as it is the custom in some schools, if an orrery is hung up among the charts and maps of the school room. The pupil ought not to see the orrery, until he knows that it is but a _faint illustration_ of the infinite grandeur of the heavens. Nothing detracts so much from our estimation of things as a too familiar acquaintance with them, before we know their real value. [3] The exact numbers are given in Table II, at the end of the book. LESSON IV. ROTATION OF THE SUN, THE EARTH, AND THE REST OF THE PLANETS ON THEIR AXES.—DAY AND NIGHT.—INCLINATION OF THE EARTH’S AXIS.—SEASONS. § 20. Besides the progressive motion of the Planets, of which we have spoken in the last Lesson, they have yet a peculiar motion like a wheel turning on its Axle-tree. This motion is called the _rotation of the Planets on their Axes_; because each of them seems to move round a straight line passing through its centre; like a ball turning round a piece of wire run through the middle of it.—Moreover it is customary to call such a straight line imagined to be drawn through the centre of a Planet—_the Axis of that Planet_.