62. _Day and Night when the Sun is at the Winter Solstice._--When the sun is at the winter solstice, it is 23-1/2° south of the celestial equator. The diurnal circle described by the sun is then 23-1/2° south of the celestial equator. More than half of this diurnal circle will therefore be above the horizon at all places south of the equator, and less than half of it at all places north of the equator: hence _the days will be longer than the nights south of the equator, and shorter than the nights at places north of the equator_. At places within 23-1/2° of the south pole, the diurnal circle described by the sun will be entirely above the horizon, and the sun will therefore not set. At places within 23-1/2° of the north pole, the diurnal circle described by the sun will be wholly below the horizon, and therefore the sun will not rise. The illumination of the earth at this time is shown in Figs. 74 and 75, and is seen to be the reverse of that shown in Figs. 72 and 73. [Illustration: Fig. 74.] [Illustration: Fig. 75.] 63. _Variation in the Length of Day and Night._--As long as the sun is north of the equinoctial, the nights will be longer than the days south of the equator, and shorter than the days north of the equator. It is just the reverse when the sun is south of the equator. The farther the sun is from the equator, the greater is the inequality of the days and nights. The farther the place is from the equator, the greater the inequality of its days and nights. When the distance of a place from the _north_ pole is less than the distance of the sun north of the equinoctial, it will have _continuous day without night_, since the whole of the sun's diurnal circle will be above the horizon. A place within the same distance of the _south_ pole will have _continuous night_. When the distance of a place from the _north_ pole is less than the distance of the sun south of the equinoctial, it will have _continuous night_, since the whole of the sun's diurnal circle will then be below the horizon. A place within the same distance of the _south_ pole will then have _continuous day_. At the _equator_ the _days and nights are always equal_; since, no matter where the sun is in the heavens, half of all the diurnal circles described by it will be above the horizon, and half of them below it. 64. _The Zones._--It will be seen, from what has been stated above, that the sun will at some time during the year be directly overhead at every place within 23-1/2° of the equator on either side. This belt of the earth is called the _torrid zone_. The torrid zone is bounded by circles called the _tropics_; that of _Cancer_ on the north, and that of _Capricorn_ on the south.