The Physical Environment
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Earth System

plane of ecliptic

 Figure 2.10 Plane of the Ecliptic

The plane of the ecliptic (also known as the "Earth-Sun plane") is a plane that cuts through the center of the Earth and the Sun in which the Earth revolves around the Sun. The Earth's axis of rotation (called the "axial tilt") is tilted 23 1/2 degrees from being perpendicular to the plane of the ecliptic. The axis remains pointing in the same direction as the Earth revolves around the Sun, pointing toward the star Polaris. As a result, the Earth's axis of rotation remains parallel to its previous position as it orbits the sun, a property called "parallelism".


earth orientation

Figure 2.11 Orientation of Earth to Sun 
Courtesy NASA (Source)

The constant tilt and parallelism causes changes in the angle that a beam of light makes with respect to a point on Earth during the year, called the "sun angle". The most intense incoming solar radiation occurs where the sun's rays strike the Earth at the highest angle. During the summer months the Earth is inclined toward the Sun yielding high sun angles. During the winter, the Earth is oriented away from the Sun creating low sun angles. 

earth tilt effect on sun angleFigure 2.12  Effect of axial tilt on incoming  solar radiation

Figure 2.24 shows the orientation of Earth with respect to the Sun during the northern hemisphere summer and its affect on sun angle. The dashed lines are the Tropic of Cancer (23.5oN) and Tropic of Capricorn 23.5oS). The northern hemisphere is tilted into the sun yielding a higher angle (A) and warmer temperatures than in the southern hemisphere were low angle sun angles are striking (B).


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For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography.
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Michael Ritter (
Last revised 6/5/12

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