Earth-Sun Relations

Solar radiation is one of many sources of energy, and probably one of the most important sources, that drive environmental processes acting at the surface of the Earth. The amount and intensity of solar radiation reaching the Earth is affected by the geometric relationship of the Earth with respect to the Sun.

The Solar Constant

Though the temperature of the air near the ground is primarily determined by the heat released by Earth's surface, the principal source for heating the Earth is solar radiation. The Earth is "constantly" bathed in solar radiation propagated through space ultimately reaching the surface of the earth. On average, the Earth receives 1368 W/m²  (1.96 ly/min) of solar radiation at the outer edge of the atmosphere, called the "solar constant". However, the actual amount received at the edge of the atmosphere or the Earth's surface varies from place to place and day to day on account of the orientation of the Earth to the Sun.  

Earth Revolution and Rotation

Earth, the third planet of our solar system 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.