In either of the satellites of _Jupiter_ or _Saturn_, these different appearances in the neighbouring Worlds are much oftener seen than they are by us in the primary Planets. We never observe these different motions in the Moon, because she turns round the Earth as her center; neither do we observe them in the Sun, because he is the center of the Earth’s motion; whence the apparent motion of the Sun always appears the same way round the Earth. _Of the Annual and Diurnal Motion of the Earth, and of the increase and decrease of Days and Nights._ The Earth in her annual motion round the Sun, has her axis always in the same direction, or parallel to itself; that is, if a line be drawn parallel to the axis, while the Earth is in any point of her orbit, the axis in all other positions of the Earth will be parallel to the said line. This parallelism of the axis, and the simple motion of the Earth in the ecliptic, solves all the phænomena of different seasons. These things are very well illustrated by the _Orrery_. [Illustration: Plate 3.] If you put on the lamp in the place of the Sun, you will see how one half of our globe is always illuminated by the Sun, while the other hemisphere remains in darkness; how Day and Night are formed by the revolution of the Earth round her axis; for as she turns from West to East, the Sun appears to move from East to West. And while the Earth turns in her orbit, you may observe that her axis always points the same way, and the several seasons of the year continually change. To make these things plainer, we will take a view of the Earth in different parts of her orbit. When the Earth is in the first point of _Libra_ (which is found by extending a thread from the Sun, and over the Earth, to the ecliptic) we have the _Vernal Equinox_, and the Sun at that time appears in the first point of ♈. In this position of the Earth, two Poles of the world are in the line separating light and darkness; and as the Earth turns round her axis, just one half of the equator, and all its parallels, will be in the light, and the other half in the dark; and therefore the days and nights must be every where equal.