2.1.5: The Earth in Space
Earth revolves around the Sun once every 365 1/4 days. The elliptical orbit of the earth varies from 91.5 million miles on January 3 called " perihelion ", to 94.5 million miles on July 4 called " aphelion " for an average earth-sun distance of 93 million miles. The elliptical path causes only small variations in the amount of solar radiation reaching the earth.
The Earth rotates at a uniform rate on its axis once every 24 hours. Turning in an eastward direction the Sun "rises" in the east and seemingly "travels" toward the west during the day. The Sun isn't actually moving, it's the eastward rotation towards the morning Sun that makes it appear that way. The Earth then rotates in the opposite direction to the apparent path of the Sun. Looking down from the North Pole yields a counterclockwise direction. From over the South Pole a clockwise direction of rotation occurs. You can demonstrate this by looking down at the North Pole of a counterclockwise rotating globe. Lift the globe while keeping it spinning in a counterclockwise direction and look at it from below.
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 ".
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.
Figure \(\PageIndex{4}\) 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 (23.5 o N) and Tropic of Capricorn 23.5 o S). 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).