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Solstices and Equinoxes
Figure 1-5.Electromagnetic spectrum

Aerographers Mate, Module 05-Basic Meteorology
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In December, the situation is exactly reversed from that in June. The Southern Hemisphere now receives more of the Sun’s direct rays. The most perpendicular rays of the Sun are received at 23 1/2°S latitude (the Tropic Of Capricorn). The southern polar region is now completely in sunshine and 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 Sun’s  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  Earth’s axis is perpendicular to the Sun’s rays. Spring begins in the   Northern   Hemisphere   and   fall   begins   in   the Southern Hemisphere. 2. June  21.  The  summer  solstice,  when  Earth’s 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  equinox,  when Earth’s  axis  is  again  perpendicular  to  the  Sun’s  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, five natural light (or heat) zones according to the zone's relative position to the Sun's rays mark Earth. 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. 1-7 AGF0104 MARCH 21 SEPTEMBER 22 JUNE 21 SUN DECEMBER 22 Figure 1-4.—Revolution of Earth around the sun.







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