In the previous section, we have seen the sun as the source for our weather through the transfer of heat energy to the earth. The equatorial region receives the bulk of the heat energy but not always directly. Relative to the sun, the earth's axis is tilted approximately 23½°. The amount of radiation any one place receives each year varies throughout the year.
In the northern hemispheric winter, the southern hemisphere received the majority of the solar radiation. The day which the daylight hours are the shortest in the northern hemisphere is December 22. Conversely, the southern hemisphere daylight hours are the longest. Six months later, on June 22, the earth has completed one half of its orbit with the northern hemisphere receiving the majority of the radiation.
Twice a year, March 21 and September 23, both hemispheres receive the same amount of radiation. The days are called the equinox meaning equal night. Both hemispheres have 12 hours of daylight and darkness.