It will also be seen, that, at every place within 23-1/2° of either pole, there will be, some time during the year, a day during which the sun will not rise, or on which it will not set. These two belts of the earth's surface are called the _frigid zones_. These zones are bounded by the _arctic_ circles. The nearer a place is to the poles, the greater the number of days on which the sun does not rise or set. Between the frigid zones and the torrid zones, there are two belts on the earth which are called the _temperate zones_. The sun is never overhead at any place in these two zones, but it rises and sets every day at every place within their limits. 65. _The Width of the Zones._--The distance the frigid zones extend from the poles, and the torrid zones from the equator, is exactly equal to _the obliquity of the ecliptic_, or the deviation of the axis of the earth from the perpendicular to the plane of its orbit. Were this deviation forty-five degrees, the obliquity of the ecliptic would be forty-five degrees, the torrid zone would extend forty-five degrees from the equator, and the frigid zones forty-five degrees from the poles. In this case there would be no temperate zones. Were this deviation fifty degrees, the torrid and frigid zones would overlap ten degrees, and there would be two belts of ten degrees on the earth, which would experience alternately during the year a torrid and a frigid climate. Were the axis of the earth perpendicular to the plane of the earth's orbit, there would be no zones on the earth, and no variation in the length of day and night. 66. _Twilight._--Were it not for the atmosphere, the darkness of midnight would begin the moment the sun sank below the horizon, and would continue till he rose again above the horizon in the east, when the darkness of the night would be suddenly succeeded by the full light of day. The gradual transition from the light of day to the darkness of the night, and from the darkness of the night to the light of day, is called _twilight_, and is due to the _diffusion of light from the upper layers of the atmosphere_ after the sun has ceased to shine on the lower layers at night, or before it has begun to shine on them in the morning. [Illustration: Fig. 76.] Let _ABCD_ (Fig. 76) represent a portion of the earth, _A_ a point on its surface where the sun _S_ is setting; and let _SAH_ be a ray of light just grazing the earth at _A_, and leaving the atmosphere at the point _H_. The point _A_ is illuminated by the whole reflective atmosphere _HGFE_. The point _B_, to which the sun has set, receives no direct solar light, nor any reflected from that part of the atmosphere which is below _ALH_; but it receives a twilight from the portion _HLF_, which lies above the visible horizon _BF_. The point _C_ receives a twilight only from the small portion of the atmosphere; while at _D_ the twilight has ceased altogether.