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why is earth spherical in shape?
The surface gets much more intense sunlight, hence heat, at the equator than at the poles. On the equinox, the Sun passes directly overhead at noon on the equator and a square centimeter of ground receives about 1 calorie of heat energy. On the same day, at 60°N, the latitude of Anchorage, Alaska, or Oslo, Norway, or St. Petersburg, Russia, the Sun rises no higher than 30° above the horizon at noon and heats a given parcel of ground with only a half the intensity as at the equator. At the poles, the Sun appears to sit on the horizon for periods upwards of 24 hours, and its rays skim horizontally over the surface.

The 23.5° tilt of the axis means a substantial annual shift of sun angle. The June sun in San Francisco (or Wichita or Norfolk) is as high as the March sun in Guatemala City, while the December sun in San Francisco is no higher than the March sun in Anchorage, Alaska. During the course of a year, a temperate-zone location can be said to enjoy tropical heat in the summer and to suffer arctic cold in winter. The distribution of heat around the globe, and through the year, coupled with the physical properties of air, produce a distinctive pattern of climatic zones.


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