ASTROGEOLOGY 2 (ADVANCED LEVEL)

The Earth’s Companions

            The Earth is the third planet amongst the nine planets orbiting the sun.  Its other companions that make up the solar system include the sun, the other planets and many other tiny rocks and matter left over from the Big Bang (currently accepted explanation of the beginning of the universe).

The Sun

The sun is a star.  A star is a huge sphere of glowing hot gases.  The sun appears so much larger and brighter than the other stars because of its relative nearness to the earth.  It is the main source of light and warmth for the other members of the solar system.  Thanks to the sun, photosynthesis takes place on Earth to support life.

-          The sun is about 149,591,000 km away from the earth.  

-          The sun is approximately 1,392,000 km in diameter.

-          The sun's surface gravity is almost 28 times that of the earth; that is a body on the surface of the sun would weigh about 28 times its weight on earth. 

-          The Sun is the largest object in the Solar System, accounting for 99.86% of the mass. Its mass is almost 700 times the total mass of all the bodies in the solar system and 332,000 times that of the earth.   It is this mass concentration that holds the solar system together by means of gravitational attraction.

-          The sun is made up mainly of hydrogen and helium and traces of other naturally occurring elements.  Hydrogen plays an important role in the sun because it is the fuel that keeps it shining. 

-          The sun has a surface temperature of approximately 6000^o C, but at its center, the sun has a temperature of 10 to 20 million degrees Celsius, and a pressure of over 1 billion atmospheres.  This extreme temperature leads to the fusion of hydrogen atoms to form helium (Fusion reactions).  This process releases the energy that flows to the sun’s surface and then into space to light and warm the other planets.

-          The bright surface of the sun is called the photosphere. Its temperature is about 6,000°C. The photosphere appears darker near the edge (limb) of the sun's disk because of greater absorption of light by the sun's atmosphere in this area; this phenomenon is called limb darkening. During an eclipse of the sun the chromosphere and the corona (the outer layers of the sun's atmosphere) are observed. Also of interest is the high-speed, flame-like rise of clouds of gas from the corona known as the solar wind.

-          A spot at the Sun’s equator takes 25 days to rotate completely, while a spot 15° from the poles takes 34 days to make a complete rotation. This phenomenon is known as differential rotation.