But I see another, and much slower movement, very clearly defined. The axis of rotation of the _photo-adumbrated sphere_ does not remain parallel with the terrestrial axis of rotation; it retires from it little by little, so as to form with it an angle which attains its maximum at the summer solstice (21-22 June); afterwards, returning upon itself, it coincides anew with the terrestrial axis of rotation (the autumn equinox), to make an angle in the contrary direction, whose maximum, of the same value as the former, corresponds to the winter solstice (21-22 December). This movement, which is annual, complicates itself with the diurnal. It is rendered visible by the displacement of the polar shroud of snow: the portion which, at the moment of the spring equinox, belonged, in the northern hemisphere, to the obscured hemisphere, moves onward, as a result of the inclination of its axis, to become an integral part of the illuminated hemisphere; while, at the same time, the portion which, in the southern hemisphere, belonged to the illuminated hemisphere, moves onward to become an integral part of the darkened hemisphere. Owing to this displacement, the sun shows itself for six consecutive months above the horizon, for the North Pole; at the spring equinox it begins to rise, at the summer solstice it attains its maximum elevation, and from the summer solstice it begins to decline. Thus, then, we have in reality _a day six months long_, of which the morning and the evening are the two equinoxes, its noon the summer solstice. In the same periods an exactly opposite order of things prevails in the southern hemisphere. Everything, even to the minutest detail, is in this way very clearly explained. Two facts--like the touches of a painter's brush--suffice to impress the whole upon the mind, namely:-- That, first, one half of the terrestrial surface is constantly illuminated by the sun, while the other half remains in darkness; That, secondly, the rotation of the earth upon its axis produces day and night, by carrying from east to west the illuminated hemisphere, always diametrically opposite to the darkened hemisphere.