At the other side of the orbit, position (c), when the north pole is tilted away from the Sun and the south pole is tilted toward the Sun, defines the northern hemisphere’s winter and the southern hemisphere’s summer. In this period the south pole is tilted away from the Sun, so that the southern hemisphere receives less sunlight and less thermal radiation, defining the southern hemisphere’s winter. The northern hemisphere receives its maximum hours of sunlight and hence thermal radiation from the Sun, when the north pole is tilted toward the Sun, position (a), defining the northern hemisphere’s summer. As the Earth moves with its axis tilted at an angle from the vertical, during the year the Earth’s north pole is sometimes tilting toward the Sun and sometimes tilting away from it. The reason for seasonal changes can be described by reference to Figure 8.7. The variation of the distance between the Earth and the Sun and the Earth’s angle of inclination results in the longer term variations associated with the seasons.
A comparatively short-term variation occurs by day and night due to the Earth’s angular velocity.
The combination of the Earth’s tilt and elliptical path results in a variation of thermal radiation received by the Earth’s surface. Seasonal variations occur as a result of the Earth’s orbit around the Sun, which is illustrated schematically in Figure 8.6. The declination angle varies between +23.5° on June 21 and -23.5° on December 21. Because the two are parallel at the Spring and Fall Equinox, the declination angle is 0 σ on these two dates. The angle measured at any point on the earth's surface between the sun-earth line and the plane defined by the earth's equator is the “solar de-climation”. From the sun's position, an observer would see the earth's tilt at 23.5° with respect to the ecliptic. From the earth's point of view on these two dates, the sun rises and sets due east and west, respectively. On two days of the year, March 21 and September 21 (Spring and Fall Equinox), the sun's light beams are parallel to the earth's equitorial plane. This plane, geometrically described by the Sun-Earth line, is called by astronomers the solar “ecliptic.” It is useful to visualize the Sun-Earth line as a cluster of parallel light beams. The earth's polar axis is tilted 23☂7’ (assume 23.5° for practical purposes) with respect to the plane of the earth's orbit around the sun. The earth's position, tilted with respect to its orbital plane around the sun, provides the geometric basis for the annual variation in solar energy received on the earth's surface ( Fig.