Axial Tilt
The plane of the ecliptic 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 is tilted 23 1/2 degrees from being perpendicular to the plane of the ecliptic. The axis of rotation remains pointing in the same direction as it 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". 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. As the sun angle decreases, the beam of light is spread over a larger area and decreases in intensity. 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. The tilt of the Earth and its impact on sun angle is the reason the Northern and Southern Hemisphere have opposite seasons. Summer occurs when a hemisphere is tipped toward the Sun and winter when it is tipped away from the Sun.
Path length and Insolation The distance that a beam of light travels greatly affects the amount of solar energy that ultimately reaches the Earth. The Earth-Sun distance only varies by about 3 million miles compared to an average distance of about 93 million miles over the year. A more significant impact on insolation is the thickness of the atmosphere on depletion of a beam of light. As the amount of atmosphere through which the beam passes increases, the greater the chance for reflection and scattering of light, thus reducing insolation at the surface. Due to the curvature of the Earth, a beam of light striking the Equator passes through less atmosphere than one at a higher latitude.
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