In December, the situation is exactly reversed fromthat in June. The Southern Hemisphere now receivesmore of the Sun’s direct rays. The most perpendicularrays of the Sun are received at 23 1/2°S latitude (theTropic Of Capricorn). The southern polar region is nowcompletely in sunshine and the northern polar region iscompletely in darkness.Since the revolution of Earth around the Sun is agradual process, the changes in the area receiving theSun’s rays and the changes in seasons are gradual.However, it is customary and convenient to mark thesechanges by specific dates and to identify them byspecific names. These dates are as follows:1.March 21. The vernal equinox, when Earth’saxis is perpendicular to the Sun’s rays. Spring begins inthe Northern Hemisphere and fall begins in theSouthern Hemisphere.2.June 21. The summer solstice, when Earth’saxis is inclined 23 1/2° toward the Sun and the Sun hasreached its northernmost zenith at the Tropic of Cancer.SummerofficiallycommencesintheNorthernHemisphere;winterbeginsintheSouthernHemisphere.3.September 22. The autumnal equinox, whenEarth’s axis is again perpendicular to the Sun’s rays.This date marks the beginning of fall in the NorthernHemisphere and spring in the Southern Hemisphere. Itis also the date, along with March 21, when the Sunreaches its highest position (zenith) directly over theequator.4.December 22. The winter solstice, when theSun has reached its southernmost zenith position at theTropic of Capricorn. It marks the beginning of winter inthe Northern Hemisphere and the beginning of summerin the Southern Hemisphere.In some years, the actual dates of the solstices andthe equinoxes vary by a day from the dates given here.This is because the period of revolution is 365 1/4 daysand the calendar year is 365 days except for leap yearwhen it is 366 days.Because of its 23 1/2° tilt and its revolution aroundthe Sun, five natural light (or heat) zones according tothe zone's relative position to the Sun's rays mark Earth.Since the Sun is ALWAYS at its zenith between theTropic of Cancer and the Tropic of Capricorn, this is thehottest zone. It is called the Equatorial Zone, the TorridZone, the Tropical Zone, or simply the Tropics.1-7AGF0104MARCH 21SEPTEMBER22JUNE 21SUNDECEMBER22Figure 1-4.—Revolution of Earth around the sun.
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