 Season Designations for the Northern Hemisphere When the northern hemisphere is tilted towards the sun, the sun's rays strike the earth at a steeper angle compared to a similar latitude in the southern hemisphere. As a result, the radiation is distributed over an area which is less in the northern hemisphere than in the southern hemisphere, as indicated by the red line. This means that there is more radiation per unit area to be absorbed. Thus, there is summer in the northern hemisphere and winter in the southern hemisphere. This situation reaches a maximum on June 21st. Autumn In this configuration, the earth is not tilted with respect to the sun's rays. The earth in this picture is actually tilted towards you, as indicated by the fact that you can see the north pole or the green dot. Therefore, radiation strikes similar latitudes at the same angle in both hemispheres. The result is that the radiation per unit area is the same in both hemispheres. Since this situation occurs after summer in the northern hemisphere, we call it autumn, while in the southern hemisphere it is spring. This occurs on September 21st. Winter When the northern hemisphere is tilted away from the sun, the sun's rays strike the earth at a shallower angle compared to the smaller latitude in the southern hemisphere. As a result, the radiation is distributed over an area which is greater in the northern hemisphere than in the southern hemisphere, as indicated by the red line. This means that there is less radiation per unit area to be absorbed. Thus, there is winter in the northern hemisphere and summer in the southern hemisphere. This situation reaches a maximum on December 21st. Spring In this configuration, the earth is not tilted with respect to the sun's rays. The earth in this picture is actually tilted towards you, as indicated by the fact that you can see the north pole or the green dot. Therefore, radiation strikes similar latitudes at the same angle in both hemispheres. The result is that the radiation per unit area is the same on both hemispheres. Since this situation occurs after winter in the northern hemisphere, we call it spring, while in the southern hemisphere it is autumn. This occurs on March 21st.