The orbit of the Earth around the Sun is called an Earth revolution. This celestial motion takes 365.26 days to complete one cycle. Further, the Earth's orbit around the Sun is not circular, but oval or elliptical (see Figure 6h-2). An elliptical orbit causes the Earth's distance from the Sun to vary over a year. Yet, this phenomenon is not responsible for the Earth’s seasons! This variation in the distance from the Sun causes the amount of solar radiation received by the Earth to annually vary by about 6%. Figure 6h-2 illustrates the positions in the Earth’s revolution where it is closest and farthest from the Sun. On January 3, perihelion, the Earth is closest to the Sun (147.3 million km). The Earth is farthest from the Sun on July 4, or aphelion (152.1 million km). The average distance of the Earth from the Sun over a one-year period is about 149.6 million km.

Tilt of the Earth's Axis

The ecliptic plane can be defined as a two-dimensional flat surface that geometrically intersects the Earth's orbital path around the Sun. On this plane, the Earth's axis is not at right angles to this surface, but inclined at an angle of about 23.5° from the perpendicular. Figure 6h-3 shows a side view of the Earth in its orbit about the Sun on four important dates: June solstice, September equinox, December solstice, and March equinox. Note that the angle of the Earth's axis in relation to the ecliptic plane and the North Star on these four dates remains unchanged. Yet, the relative position of the Earth's axis to the Sun does change during this cycle. This circumstance is responsible for the annual changes in the height of the Sun above the horizon. It also causes the seasons, by controlling the intensity and duration of sunlight received by locations on the Earth. Figure 6h-4 shows an overhead view of this same phenomenon. In this view, we can see how the circle of illumination changes its position on the Earth’s surface. During the two equinoxes, the circle of illumination cuts through the North Pole and the South Pole. On the June solstice, the circle of illumination is tangent to the Arctic Circle (66.5° N) and the region above this latitude receives 24 hours of daylight. The Arctic Circle is in 24 hours of darkness during the December solstice.

On June 21 or 22 the Earth is positioned in its orbit so that the North Pole is leaning 23.5° toward the Sun (Figures 6h-3, 6h-4, 6h-5 and see animation - Figure 6h-7). During the June solstice (also called the summer solstice in the Northern Hemisphere), all locations north of the equator have day lengths greater than twelve hours, while all locations south of the equator have day lengths less than twelve hours (see Table 6h-2). On December 21 or 22 the Earth is positioned so that the South Pole is leaning 23.5 degrees toward the Sun (Figures 6h-3, 6h-4, 6h-5 and see animation - Figure 6h-8). During the December solstice (also called the winter solstice in the Northern Hemisphere), all locations north of the equator have day lengths less than twelve hours, while all locations south of the equator have day lengths exceeding twelve hours (see Table 6h-2).

Earth Rotation and Revolution

The term Earth rotation refers to the spinning of our planet on its axis. Because of rotation, the Earth's surface moves at the equator at a speed of about 467 m per second or slightly over 1675 km per hour. If you could look down at the Earth's North Pole from space you would notice that the direction of rotation is counter-clockwise (Figure 6h-1). The opposite is true if the Earth is viewed from the South Pole. One rotation takes exactly twenty-four hours and is called a mean solar day. The Earth’s rotation is responsible for the daily cycles of day and night. At any one moment in time, one half of the Earth is in sunlight, while the other half is in darkness. The edge dividing the daylight from night is called the circle of illumination. The Earth’s rotation also creates the apparent movement of the Sun across the horizon.

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.