When Earth is in its summer solstice, as shown for June in figure 1-2-4, the Northern Hemisphere

Figure 1-2-4.Revolution of Earth around the Sun.

is inclined 23 1/2 TOWARD the Sun. This inclination results in more of the Suns rays reaching the Northern Hemisphere than the Southern Hemisphere. On or about June 21, direct sunlight covers the area from the North Pole down to latitude 66 1/2N (the ARCTIC CIRCLE). The area between the Arctic Circle and the North Pole is receiving the Suns rays for 24 hours each day. During this time the most perpendicular rays of the Sun are received at 23 1/2N latitude (the TROPIC OF CANCER). Because the Southern Hemisphere is tilted AWAY from the Sun at this time, the indirect rays of the Sun reach only to 66 1/2S latitude (the ANTARCTIC CIRCLE). Therefore, the area between the Antarctic Circle and the South Pole is in complete darkness. Note carefully the shaded and unshaded area of Earth in figure 1-2-4 for all four positions.

At the time of the equinox in March and again in September, the tilt of Earths axis is neither toward nor away from the Sun. For these reasons Earth receives an equal amount of the Suns energy in both the Northern Hemisphere and the Southern Hemisphere. During this time the Suns rays shine most perpendicularly at the equator. In December, the situation is exactly reversed from that in June. The Southern Hemisphere now receives more of the Suns direct rays. The most perpendicular rays of the Sun are received at 23 1/2S latitude (the TROPIC OF CAPRI-CORN).

The southern polar region is now completely in sunshine; the northern polar region is completely in darkness.

Since the revolution of Earth around the Sun is a gradual process, the changes in the area receiving the Suns rays and the changes in seasons are gradual. However, it is customary and convenient to mark these changes by specific dates and to identify them by specific names. These dates are as follows:

1. March 21. The VERNAL EQUINOX, when Earths axis is perpendicular to the Suns rays. Spring begins in the Northern Hemisphere and fall begins in the Southern Hemisphere.

2. June 21. The SUMMER SOLSTICE, when Earths axis is inclined 23 1/2 toward the Sun and the Sun has reached its northernmost zenith at the Tropic of Cancer. Summer officially commences in the Northern Hemisphere; winter begins in the Southern Hemisphere.

3. September 22. The AUTUMNAL EQUI-NOX, when Earths axis is again perpendicular to the Suns rays. This date marks the beginning of fall in the Northern Hemisphere and spring in the Southern Hemisphere. It is also the date, along with March 21, when the Sun reaches its highest position (zenith) directly over the equator.

4. December 22. The WINTER SOLSTICE, when the Sun has reached its southernmost zenith position at the Tropic of Capricorn. It marks the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere.

In some years, the actual dates of the solstices and the equinoxes vary by a day from the dates given here. This is because the period of revolution is 365 1/4 days and the calendar year is 365 days except for leap year when it is 366 days.

Because of its 23 1/2 tilt and its revolution around the Sun, Earth is marked by five natural light (or heat) zones according to the zones relative position to the Suns rays. Since the Sun is ALWAYS at its zenith between the Tropic of Cancer and the Tropic of Capricorn, this is the hottest zone. It is called the Equatorial Zone, the Torrid Zone, the Tropical Zone, or simply the Tropics.

The zones between the Tropic of Cancer and the Arctic Circle and between the Tropic of Capricorn and the Antarctic Circle are the Temperate Zones. These zones receive sunshine all year, but less of it in their respective winters and more of it in their respective summers. The zones between the Arctic Circle and the North Pole and between the Antarctic Circle and the South Pole receive the Suns rays only for parts of the year. (Directly at the poles there are 6 months of darkness and 6 months of sunshine.) This, naturally, makes them the coldest zones. They are therefore known as the Frigid or Polar Zones.

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