كوكب الأرض هو وطننا الكبير الذي نعيش فيه ، ونحن فوق الأرض أشبه ما نكون بركاب سفينة فضائية ، سقفها الغلاف الجوي الذي تتعدد وظائفه وخدماته ، وجو دنيانا مكيف بحيث ترسل السفينة وسقفها إلى الفضاء ، نفس الطاقة تقريباً التي تكتسبها من الشمس ، فتظل محتفظة بنفس معدلات درجة الحرارة على مر السنين ، وهذا هو السبب في استمرار الحياة فوق سطحها.
تحيط بكوكب الأرض أغلفة مرئية مثل المحيطين اليابس والمائي ، ويدور كوكب الأرض بجميع أغلفته بسرعة كبيرة كوحدة لا تتجزأ، حول نفسه وحول الشمس ( بسرعة تبلغ حوالي ثلاثين كيلومتراً في الثانية ) مع باقي الكواكب ، ثم مع الشمس حول مركز مجرتنا ثم مع المجرة التي تتحرك هي الأخرى مع آلاف الملايين من المجرات التي يتكون منها الكون.
ويبعد كوكب الأرض في المتوسط عن الشمس بمسافة 149,6 مليون كيلومتر ، ويبلغ قطره نحو 12756 كيلومتراً ، وتتراوح درجة الحرارة في المناطق المختلفة ما بين 55 درجة مئوية و70 درجة تحت الصفر.
ولكوكب الأرض مجال مغناطيسي جبار ، ناتج عن الدوران السريع للطبقة المركزية ( النواة ) المكونة من الحديد والنيكل. ويمتد هذا المجال المغناطيسي بعيداً إلى الفضاء ، ليحمي الأرض من الإشعاعات الضارة التي تصدر عن الأجسام الفضائية الأخرى.
ويتكون باطن الأرض من ثلاث طبقات رئيسية :
القشرة ويبلغ سمكها حوالي مائة كيلومتر ،
والطبقة الوسطى وعمقها نحو 2900 كيلومتر ، ونحصل على معلومات عنها عن طريق ما تلفظه البراكين من مواد،
أما الطبقة المركزية ( النواة ) فهي مكونة من مواد ذات كثافة عالية جداً ، في حالة انصهار وسيولة ، ويبلغ سمكها حوالي 3300 كيلومتر ،
أما الغلاف الجوي فهو يتكون من خمس طبقات ( التروبوسفير ، الستراتوسفير ، الأوزونوسفير ، الأيونوسفير ، الإكسوسفير ) ، ويحتوي على غاز النتروجين 78% ، والأوكسجين 21% وبخار ماء و ثاني أكسيد الكربون و غازات أخرى 1%.
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