Now it is clear that we can hold our knitting-needle straight out, horizontally, or we can hold it straight up and down, perpendicularly, as it moves around the sun-lamp. But neither of these straight positions would represent the direction of the axis of the earth; we must _tip_ the needle a little, so as to hold it in a slanting position. Now if we held the needle exactly up and down, or exactly straight out, the sun-lamp would shine on just half of the orange, as divided from pole to pole. But when we tip the needle you see we bring our orange-world in such a position that, as it passes around the sun-lamp, the light falls now more around one pole, now more around the other; this tipped position is “inclination to the plane of orbit.” LESSON VIII. PLAN AND PROGRESSION. “They rise in joy, the starry myriads burning-- The shepherd greets them from his mountains free; And from the silvery sea To them the sailor’s wakeful eye is turning-- Unchanged they rise--” --HEMANS. We have now found that our earth is one among several planets revolving around a great central star which we name the sun. The sun and his attendant planets and their satellites form what is called the solar system. We have also found that the heavens are filled with such systems, and that these are gathered together in nebulæ or clusters. Prominent among these clusters is the Milky Way, near the centre of which our system is placed. The bodies which in our system revolve about the sun, are called planets. With respect to the sun and other great stars the planets are small, but considered in themselves they are enormous bodies. They are all dark spheres,[17] having no light in themselves. Is it true that the moon with her silvery radiance and Venus and Mars with their steadfast beams, are dull and dark as burnt-out cinders? This is indeed true; the planets receive their light and heat from the sun, and it is as they reflect back this light that they shine so brilliantly. As an example of such reflection take a new tin plate; it has no light in itself; shut it in a dark room, it gives out no light; but hold it where a broad ray of sunshine strikes it, and at once the light is reflected back from the tin with such burning splendor that the eyes can scarcely look upon it. So we have observed windows struck by the light of the setting sun, and at once blazing forth like fires.