The illustration above shows how equal sized rays from the sun have to cover different sized areas depending upon the Angle of Insolation.This Angle of Insolation is the angle at which the sun's rays strike the earth. You will observe that the smallest area (represented by a red cross section in the illustration) is at the location where the sun's rays strike perpendicular to the earth's surface. NOTE: Since the same amount of sunlight is covering a smaller area, this will tend to be the hottest area on the earth (the tropics). In the illustration above it would be summer in the northern hemisphere (assuming that north is up).Notice that the North Polar Region is tipped toward the sun and receives rather direct solar radiation. At the same time the South Polar area is tipped away from the sun and receives less direct or even no solar radiation at all. This means that it would be winter in the southern hemisphere. After carefully examining the illustration above, please study the movies below.
(notice the angle of inclination of the earth in the movies)
The movie on the left (or top- depending on your browser and screen size) illustrates a small globe as it would be illuminated by the sun during a summer in the northern hemisphere. |
The movie on the right (or bottom- depending on your browser and screen size) illustrates a small globe as it would be illuminated by the sun during a winter in the northern hemisphere. |
In the Insolation image (at the top of this page) you will observe that the rays from the sun strike the earth more directly (closer to perpendicular) in the northern hemisphere than they do in the southern hemisphere. When the sun's rays strike more directly, they tend to warm up the earth more than when they strike at a sharp angle. Since the North Pole area receives light (and warmth) all day long, the North Polar Ice Cap will melt back during the summer. While the Northern Hemisphere is experiencing summer, the Southern Hemisphere is having a winter season. At this time the South Pole Area is without sun (and warmth) all day long, therefore, the ice around Antarctica builds up due to the lack of heat. By examining the movies carefully, it is possible to determine where on the earth there will be 24 hour "days" and 24 hour "nights". On the movie on the right, the Northern Hemisphere is receiving less direct solar rays (or none at all) while the Southern Hemisphere is receiving more direct solar radiation. By examining the movie carefully, it is possible to determine where on the earth there will be 24 hour "days" and 24 hour "nights". This will be the opposite to the situation observed in the movie on the left. Therefore, in the movie on the right, the Southern Hemisphere gets warmer (summer) and the Northern Hemisphere gets colder (winter). Thus, you can see how the tilt of the earth's axis creates a situation in which the directness of the sun's rays changes throughout the year as the earth orbits the sun. This change to the Angle of Insolation (the angle at which the sun's rays strike the earth) is what makes certain parts of the earth either warm up more or cool down - creating the seasons. Fall and spring seasons are the transition periods between the left and right movies.
If you look at the movie, you will observe that the earth is orbiting the sun. The shape of the earth's orbit is an ellipse. This ellipse lies in a plane (flat surface) referred to as the Plane of The Ecliptic. You will also observe that the axis of the earth is not perpendicular to this plane. It is tipped slightly. When measured, it turns out that the earth's axis is tipped approximately 23 1/2 degrees from a perpendicular to the Plane of The Ecliptic. It is, also, important to observe that this angle remains constant as the earth orbits the sun. This tipped position of the earth as it orbits the sun allows different portions of the earth to receive more direct sunlight at various times of the year. It is important to know that around June 21 of each year the earth is approximately 152 million kilometers from the sun and around December 21 of each year the earth is approximately 147 million kilometers from the sun. These differences in distance are due to the elliptical shape of the earth's orbit.